Introduction to Nanofiber Materials
Presenting the latest coverage of the fundamentals and applications of nanofibrous
materials and their structures for graduate students and researchers, this book bridges
the communication gap between fiber technologists and materials scientists and engineers.
Featuring intensive coverage of electroactive, bioactive and structural nanofibers, it
provides a comprehensive collection of processing conditions for electrospinning and
includes recent advances in nanoparticle-/nanotube-based nanofibers. The book also
covers mechanical properties of fibers and fibrous assemblies, as well as characterization
methods.
Frank K. Ko is Canada Research Chair Professor in Advanced Fibrous Materials and
Director of the Advanced Materials and Process Engineering Laboratory, University of
British Columbia.
Yuqin Wan is Research Associate in Advanced Fibrous Materials, Advanced Materials
and Process Engineering Laboratory, University of British Columbia, Canada, and
Associate Professor in the School of Textiles and Clothing, Jiangnan University, China.
Introduction to Nanofiber
Materials
F R AN K K. K O
and
YUQIN WAN
University of British Columbia, Vancouver
University Printing House, Cambridge CB2 8BS, United Kingdom
Published in the United States of America by Cambridge University Press, New York
Cambridge University Press is part of the University of Cambridge.
It furthers the University’s mission by disseminating knowledge in the pursuit of
education, learning and research at the highest international levels of excellence.
www.cambridge.org
Information on this title: www.cambridge.org/9780521879835
© Frank K. Ko and Yuqin Wan 2014
This publication is in copyright. Subject to statutory exception
and to the provisions of relevant collective licensing agreements,
no reproduction of any part may take place without the written
permission of Cambridge University Press.
First published 2014
Printed in the United Kingdom by . . .
A catalogue record for this publication is available from the British Library
Library of Congress Cataloguing in Publication data
ISBN 978-0-521-87983-5 Hardback
Cambridge University Press has no responsibility for the persistence or accuracy of
URLs for external or third-party internet websites referred to in this publication,
and does not guarantee that any content on such websites is, or will remain,
accurate or appropriate.
Contents
1
2
Introduction
page 1
1.1
1.2
1.3
1.4
1.5
1.6
How big is a nanometer?
What is nanotechnology?
Historical development of nanotechnology
Classification of nanomaterials
Nanofiber technology
Unique properties of nanofibers
1.6.1. Effect of fiber size on surface area
1.6.2. Effect of fiber size on bioactivity
1.6.3. Effect of fiber size on electroactivity
1.6.4. Effect of fiber size on strength
References
1
1
2
5
5
6
7
7
8
9
10
Fundamentals of polymers
13
2.1
2.2
13
14
14
15
16
16
17
20
21
23
23
24
24
25
25
25
26
27
27
2.3
Polymeric materials
Polymer flow, nonlinearity and heterogeneity
2.2.1 Linear kinetics
2.2.2 Nonlinear behavior
2.2.3 Viscoelastic models
2.2.3.1 The basic elements: spring and dashpot
2.2.3.2 Maxwell model
2.2.3.3 Voigt (Kelvin) model
2.2.3.4 Four-element model
Intrinsic structures of polymers
2.3.1 Molecular bondings
2.3.1.1 Van der Waals forces
2.3.1.2 Hydrogen bonding
2.3.2 Configuration and conformation
2.3.2.1 Configuration
2.3.2.2 Conformation
2.3.2.3 Other chain structures
2.3.3 Order and disorder
2.3.3.1 Amorphous and crystal structure
v
vi
3
Contents
2.3.3.2 Orientation
2.3.3.3 Measurement of order and disorder
2.3.4 Molecular weight and molecular weight distribution
2.4 Thermal behavior
2.5 Polymer solutions
2.5.1 Solubility parameter
2.5.2 Solution viscosity
2.5.2.1 Intrinsic viscosity
2.5.2.2 Intrinsic viscosity and molecular weight
2.5.2.3 Measurement of intrinsic viscosity
2.6 Fiber, plastic and elastomer
2.7 Fiber formation
2.7.1 Melt spinning
2.7.2 Wet spinning
2.7.3 Dry spinning
2.7.4 Fiber properties
2.7.4.1 Polymer structure and fiber mechanical properties
2.7.4.2 Processing and fiber properties
References
28
29
29
32
33
33
36
36
36
37
38
39
39
40
40
40
40
43
43
Nanofiber technology
45
3.1
45
45
45
46
47
47
47
47
49
49
50
50
52
52
52
53
53
54
55
55
55
56
3.2
3.3
Nanofibers forming technology
3.1.1 Conjugate spinning (island in the sea)
3.1.2 Chemical vapor deposition (CVD)
3.1.3 Phase separation (sol–gel process)
3.1.4 Drawing
3.1.5 Template synthesis
3.1.6 Self-assembly
3.1.7 Meltblown technology
3.1.8 Electrospinning
Electrospinning process
Processing parameters
3.3.1 Spinning dope concentration and viscosity
3.3.2 Applied voltage
3.3.3 Spinning dope temperature
3.3.4 Surface tension
3.3.5 Electrical conductivity
3.3.6 Molecular weight of polymer
3.3.7 Spinning distance
3.3.8 Spinning angle
3.3.9 Orifice diameter
3.3.10 Solvent boiling point
3.3.11 Humidity
Contents
4
5
vii
3.3.12 Dielectric constant
3.3.13 Feeding rate
3.4 Melt electrospinning
3.5 Applications of nanofibers
3.5.1 Reinforcement fibers in composites
3.5.2 Protective clothing
3.5.3 Filtration
3.5.4 Biomedical devices
3.5.4.1 Wound dressing
3.5.4.2 Medical prostheses
3.5.4.3 Tissue scaffolds
3.5.4.4 Controlled drug delivery
3.5.5 Electrical and optical applications
3.5.6 Nanosensors
References
57
57
58
59
59
59
60
61
61
61
61
61
62
62
62
Modeling and simulation
65
4.1
4.2
Electrospinning mechanism
Fundamentals of process modeling
4.2.1 Newton’s law
4.2.2 Conservation laws
4.3 Taylor cone
4.4 Jet profile
4.5 Models
4.5.1 One-dimensional model
4.5.2 Three-dimensional models
4.5.2.1 Spivak–Dzenis model
4.5.2.2 Rutledge’s model
4.5.2.3 Wan–Guo–Pan model
4.6 Application of models in parametric analysis
4.7 Computer simulation
References
65
65
66
67
67
68
69
70
72
72
72
73
74
75
79
Mechanical properties of fibers and fiber assemblies
81
5.1
5.2
5.3
5.4
81
81
82
84
84
86
87
88
90
5.5
Structure of hierarchy of textile materials
Size effect on mechanical properties
Theoretical modulus of a fiber
Mechanical properties of nonwovens
5.4.1 Geometry of nonwovens
5.4.2 Deformation of nonwovens
Mechanical properties of yarns
5.5.1 Yarn geometry
5.5.2 Mechanical properties of linear fiber assemblies
viii
6
7
Contents
5.5.2.1 Stress analysis
5.5.2.2 Strain analysis
5.5.3 Mechanical properties of staple yarns
5.6 Mechanical properties of woven fabrics
5.6.1 Woven fabric geometry
References
90
92
93
96
97
100
Characterization of nanofibers
102
6.1
Structural characterization of nanofibers
6.1.1 Optical microscopy (OM)
6.1.2 Scanning electron microscopy (SEM)
6.1.3 Transmission electron microscopy (TEM)
6.1.4 Atomic force microscopy (AFM)
6.1.5 Scanning tunneling microscopy (STM)
6.1.6 X-ray diffraction
6.1.6.1 Wide-angle X-ray diffraction
6.1.6.2 Small-angle X-ray scattering
6.1.7 Mercury porosimetry
6.2 Chemical characterization of nanofibers
6.2.1 Fourier transform infra-red spectroscopy (FTIR)
6.2.2 Raman spectroscopy (RS)
6.2.3 Nuclear magnetic resonance (NMR)
6.3 Mechanical characterization of nanofibers
6.3.1 Microtensile testing of nanofiber nonwoven fabric
6.3.2 Mechanical testing of a single nanofiber
6.4 Thermal analysis
6.4.1 Thermogravimetric analysis (TGA)
6.4.2 Differential scanning calorimetry (DSC)
6.5 Characterization of other properties
6.5.1 Wettability and contact angle
6.5.2 Electrical conductivity
6.5.3 Electrochemical properties
6.5.3.1 Linear-sweep voltammetry and cyclic voltammetry
6.5.3.2 Chronopotentiometry
6.5.4 Magnetic properties
References
102
102
103
106
108
110
112
114
114
115
116
116
119
121
122
123
125
128
129
131
134
134
136
137
137
139
140
143
Bioactive nanofibers
147
7.1
7.2
147
148
148
148
149
The development of biomaterials
Bioactive nanofibers
7.2.1 Nanofibers for tissue engineering
7.2.1.1 Extracellular matrices for tissue engineering
7.2.1.2 Nanofiber scaffolds for tissue engineering
Contents
7.2.2
8
9
ix
Nanofibers for drug delivery
7.2.2.1 Drug delivery systems
7.2.2.2 Nanofibers for drug delivery
7.2.3 Nanofibers for biosensors
7.2.3.1 Biosensors
7.2.3.2 Nanofiber biosensors
7.3 Assessment of nanofiber bioactivity
7.3.1 Assessment of tissue compatibility
7.3.2 Assessment of degradation
References
156
156
157
159
159
160
161
163
163
163
Electroactive nanofibers
167
8.1
8.2
Introduction
Conductive nanofibers
8.2.1 Conductive polymers and fibers
8.2.2 Fundamental principle for superior electrical conductivity
8.2.3 Electroactive nanofibers
8.3 Magnetic nanofibers
8.3.1 Supermagnetism
8.3.2 Supermagnetic nanofibers
8.4 Photonic nanofibers
8.4.1 Polymer photonics
8.4.2 Fluorescent nanofibers
8.4.3 Photo-catalytic nanofibers
References
167
167
167
169
170
179
179
180
182
182
184
186
189
Nanocomposite fibers
192
9.1
192
192
192
196
196
197
197
197
199
200
205
205
207
209
210
210
Carbon nanotubes
9.1.1 Structure and properties
9.1.1.1 Structure
9.1.1.2 Mechanical properties
9.1.1.3 Electrical properties
9.1.1.4 Thermal properties
9.1.2 Dispersion of carbaon nanotubes
9.1.2.1 Purification
9.1.2.2 Mechanical dispersion
9.1.2.3 Chemical dispersion
9.1.3 Alignment of carbon nanotubes
9.1.3.1 Alignment of carbon nanotubes in solution
9.1.3.2 Alignment of carbon nanotubes in matrix
9.1.4 Carbon nanotube nanocomposite fibers
9.1.4.1 Methods for producing carbon nanotube fibers
9.1.4.2 Chemical vapor deposition
x
10
Contents
9.1.4.3 Dry spinning
9.1.4.4 Liquid crystal spinning
9.1.4.5 Wet spinning
9.1.4.6 Traditional spinning
9.1.4.7 Electrospinning
9.2 Nanoclay
9.2.1 Structure and properties
9.2.2 Clay nanocomposites
9.2.3 Nanoclay nanocomposite fibers
9.3 Graphite graphenes
9.3.1 Structure and properties
9.3.2 Graphene nanocomposites
9.3.3 Graphene nanocomposite nanofibers
9.4 Carbon nanofibers
9.4.1 Vapor-grown carbon nanofibers
9.4.2 Electrospun carbon nanofibers
9.4.3 Carbon nanofiber composites
References
212
212
213
214
214
215
215
216
218
220
220
222
222
223
223
225
226
227
Future opportunities and challenges of electrospinning
240
10.1
10.2
Past, present and future of nanotechnology
Global challenges and nanotechnology
10.2.1 Nanofibers in energy
10.2.1.1 Electrolytes in fuel cells or batteries
10.2.1.2 Supercapacitors
10.2.1.3 Dye-sensitized solar cells
10.2.1.4 Power transmission lines
10.2.2 Nanofibers in filtration
10.2.3 Nanofibers in biomedical engineering
10.3 Challenges
10.3.1 Mechanism analysis
10.3.2 Quality control
10.3.3 Scale-up manufacturing
10.3.4 Structural property improvement
10.4 New frontiers
References
240
241
242
242
242
243
243
243
244
244
244
245
245
253
254
255
Appendix
Appendix
Appendix
Appendix
Index
259
261
263
264
266
I
II
III
IV
Terms and unit conversion
Abbreviation of polymers
Classification of fibers
Polymers and solvents for electrospinning
1
Introduction
1.1
How big is a nanometer?
By definition, a nanometer, abbreviated as nm, is a unit for length that measures one
billionth of a meter. (1 nm ¼ 103 μm ¼ 10–6 mm ¼ 107 cm ¼ 109 m.) Our hair is
visible to the naked eye. Using an optical microscope we can measure the diameter of
our hair, which is in the range of 20–50 microns(Mm) or 20 000–50 000 nm. Blood
cells are not visible to the naked eye, but they can be seen under the microscope,
revealing a diameter of about 10 microns or 10 000 nm. The diameter of hydrogen
atoms is 0.1 nm. In other words 10 hydrogen atoms can be placed side-by-side in 1 nm.
Figure 1.1 provides an excellent illustration of the relative scales in nature. The
discovery of nanomaterials ushered us to a new era of materials. We have progressed
from the microworld to the nanoworld.
1.2
What is nanotechnology?
According to the National Science Foundation in the United States nanotechnology is
defined as [1]:
Research and technology development at the atomic, molecular or macromolecular levels, in
the length scale of approximately 1–100 nanometer range, to provide a fundamental
understanding of phenomena and materials at the nanoscale and to create and use structures,
devices and systems that have novel properties and functions because of their small and/or
intermediate size. The novel and differentiating properties and functions are developed at a
critical length scale of matter typically under 100 nm. Nanotechnology research and
development includes manipulation under control of the nanoscale structures and their
integration into larger material components, systems and architectures. Within these larger scale
assemblies, the control and construction of their structures and components remains at the
nanometer scale. In some particular cases, the critical length scale for novel properties and
phenomena may be under 1 nm (e.g., manipulation of atoms at ~0.1 nm) or be larger than 100 nm.
(e.g., nanoparticle(nanoparticle) reinforced polymers have the unique feature at ~ 200–300 nm
as a function of the local bridges or bonds between the nano particles and the polymer).
Accordingly nanotechnology is the scientific field that is concerned with the study of
the phenomena and functions of matters within the dimensional range of 0.1–100 nm.
It is the study of the motion and changes of atoms, molecules, and of other forms of
1
2
Introduction
Fig. 1.1 Illustration of relative scale in nature. (The linear distance which each plate represents is
indicated on this logarithmic scale in meters.)
matter. Nanotechnology, building upon the foundation of nanoscience, is concerned
with the manufacturing of new materials, new devices and the development of research
methodology and techniques for new technology.
Nanotechnology can also be referred to as the technology for the formation of
nanomaterials and nanodevices, including the formation of nanostructural units
according to a specific methodology to form macroscopic treatment (processing) of
nanomaterials such as dispersion, forming technology as in the case of the formation of
nanofibers and their composites.
Nanotechnology can be organized into three levels. The first level is molecular
(atomic) nanotechnology wherein the molecules (atoms) are spatially organized in the
nanospace in a repetitive manner. This in turn will create internally ordered nanostructures. Self-assembly and mineralization in biological materials are examples
of molecular nanotechnology. The technology for controlling the morphology and
uniformity of nanostructures is called the second level of nanotechnology.
For example, in colloids and gels we do not concern ourselves with the order of
arrangement of the molecule itself at the nanoscale. They form only morphologies of
nanostructure of certain regularity. The third level of nanotechnology is concerned
with the technology of the formation of nanosacle structures but is unable to control
the degree of order of the molecules and atoms in the nanostructures. At the third level
of nanotechnology the morphology and uniformity of the nanostructure are also
uncontrolled [2].
1.3
Historical development of nanotechnology
Although the use of nanomaterials can arguably be traced back to over 1000 years ago
when the smoke from a candle was used in China as ink, the first scientific discussion of
nanotechnology is widely attributed to the 1959 Nobel Prize winning physicist Richard
Feyman in his well known “There’s Plenty of Room at the Bottom” lecture at the
California Institute of Technology (Caltech). In this lecture he boldly challenged his
audience in his now famous statement.
1.3 Historical development of nanotechnology
3
People tell me about miniaturization, and how far it has progressed today. They tell me about
electric motors that are the size of the nail on your small finger. And there is a device on the
market, they tell me, by which you can write the Lord’s Prayer on the head of a pin. But that’s
nothing; that’s the most primitive, halting step in the direction I intend to discuss. It is a
staggeringly small world that is below. In the year 2000, when they look back at this age, they
will wonder why it was not until the year 1960 that anybody began seriously to move in this
direction. Why cannot we write the entire 24 volumes of the Encyclopedia Brittanica on the
head of a pin?
It is of interest to note that, 40 years after the Feyman lecture, coincidentally in the
year 2000, US President Clinton announced the Notational Nanotechnology Initiative (NNI) that kicked-off the global gold rush in nanotechnology. Over a period of
10 years, developed countries have invested over $22.134 billion in nanotechnology
research with more than 1/3 ($8.918 billion) of that amount spent in the USA
alone [3].
Notwithstanding the foresight of Professor Feyman, the development of nanotechnology as a revolutionary/game-changing technology is the results of three ingredients:
(1) the availability of tools to see and manipulate matter at the nanoscale; (2) astute
observations and recognition of nanoscale matters and nanoeffects; and (3) sustained
financial support from government and industry.
More than a decade after the Feyman lecture, in 1974 Norio Taniguchi, University of
Tokyo, coined the word nanotechnology when he made the distinction between engineering of micrometer scale microtechnology and a new submicrometer level which he
dubbed ‘nanotechnology’ [3].
In 1981, IBM’s scanning tunneling microscope (STM) was developed at the IBM
Zurich Research Laboratory. STM and its offspring, including the atomic force microscope invented in 1986, provided researchers around the world with the basic tools they
needed to explore and manipulate materials at the atomic scale [3].
The atomic force microscope was invented in 1985 by ZRL researchers Gerd Binnig
and Christoph Gerber, together with Professor Calvin Quate of Stanford University.
Their invention earned them the Nobel Prize and expanded the scope of nanotechnology
research to nonconducting materials [3].
In 1989, IBM Fellow Don Eigler was the first to controllably manipulate individual
atoms on a surface, using STM to spell out ‘I-B-M’ by positioning 35 xenon atoms and,
in the process, perhaps creating the world’s smallest logo [3].
Armed with the new tools, several discoveries of fullerenes were made and they have
added considerable excitements to the rapidly growing field of nanotechnology. For
example, although there are conflicting opinions, Sumio Iijima from Japan is largely
credited for the discovery of fullerene-related carbon nanotubes in 1991 [3, 4]. The
tubes contained at least two layers, often many more, and ranged in outer diameter from
about 3 nm to 30 nm. They were invariably closed at both ends. In 1996, Smalley shared
the Nobel Prize in Chemistry with his Rice University colleague Robert Curl and the
British chemist Harold Kroto for their discovery of the buckyballs in 1985. A good
example of the application of nanoeffects is the nanoshells invented in 1998 by Naomi
Halas at Rice University. The nanoshells are a new class of multi-layered nanoscale
4
Introduction
Fig. 1.2 Nanotechnology historical timeline (adapted from Ref. [5] with modifications).
particles with unique optical properties controlled by the thickness and composition of
their constituent layers. By varying the relative size of the glass core and the gold shell
layer, researchers can ‘tune’ nanoshells to respond to different wavelengths of light. For
biomedical applications, nanoshells can be designed and fabricated to absorb near
infrared light. Near-infrared light, a region of the spectrum just beyond the visible
range, is optimal for medical imaging and treatment because it passes harmlessly
through soft tissue.
With the seed planted by Feyman, the tools invented by researchers in IBM, and the
innovative discoveries by scientists from Japan, Rice University and elsewhere in
the 1980s and 1990s, created the necessary but insufficient conditions for the coming
of the nanotechnology age until the establishment of the National Nanotechnology
Initiative thanks largely to dedicated effort of dedicated governmental scientific officers
such as Dr. Roco [3]. The 1999 IWGN workshop report prepared by the National
Science and Technology Council Committee on Technology Interagency Working
Group on Nanoscience, Engineering and Technology (IWGN) forms the basis for the
NNI announced by President Clinton in 2000. Figure 1.2 shows the Nanotechnology
historical timeline.
1.5 Nanofiber technology
5
Fig. 1.3 Dimensional classification of nanomaterials.
1.4
Classification of nanomaterials
The materials produced in nanotechnology can be classified according to dimension,
chemical composition, materials properties, material applications and manufacturing
technology. From the view of dimension, nanomateirals are classified as zerodimensional (0-D), one-dimensional (1-D) and two-dimensional (2-D) materials, as
shown in Fig. 1.3. A 0-D nanomaterial has three directions of nanosymmetry. Quantum
dots and nanoparticles are examples of 0-D nanomaterials. 1-D nanomaterials such as
nanowires and nanotubes have two directions of nanosymmetry or have dimensions less
than 100 nm. Examples of 2-D materials are nanoclays and graphene sheets wherein the
through thickness direction is less than 100 nm.
Based on chemical composition we have nanometals, nanoceramics, nanopolymers,
nanoglasses and nanoctystals. Regarding material properties we have nanomagnetic
materials, nonlinear nanophotonic materials, suerconducting nanomaterials, thermoelectric nanomaterials, semiconducting nanomaterials, etc. One can also classify nanomaterials based on their applications, these include nanoelectronic materials, optoelectronic
materials, energy storage nanomaterials, nanosensor materials, nanomedicines, etc.
1.5
Nanofiber technology
Although promising, most nanotechnology research is limited to dozens to a few hundred
particles or molecules [6, 7]. In order to realize massive assembly techniques, large-scale
devices and commercializable products need to be developed. Another challenge for
nanotechnology is the lack of effective and efficient ways for fabrication macroscale
structures. To be utilized in this macroworld, nanomaterials need to be converted to
micromaterials and macromaterials. Nanofibre technology is a technique involving the
synthesis, processing, manufacturing and application of fibers with nanoscale dimension.
As a technique of fabrication of continuous 1-D nanomaterials, nanofibre technology is a
6
Introduction
promising technique that can massively assemble 1-D and 2-D nanomaterials, realize
large-scale production of nanomaterials involved products and prepare continuous 1-D
fiberous elements that facilitate the fabrication of microsale and macroscale structures.
Fibers are solid state linear nanomaterials characterized by flexibility and an aspect
ratio greater than 1000:1. Nanofibres are defined as fibers with a diameter equal to or
less than 100 nm. But in general, all the fibers with a diameter below 1 μm (1000 nm)
are recognized as nanofibers. Materials in fiber form are of great practical and fundamental importance. The combination of high specific surface area, flexibility and
superior directional strength makes fibers a preferred material form for many applications varying from clothing to reinforcements for aerospace structures. Fibrous materials in the nanometer scale are the fundamental building blocks of living systems. For
instance, DNA molecules are double helix strands with a diameter of 1.5 nm, cytoskeleton filaments have a diameter around 30 nm, and even sensory cells such as hair cells
and rod cells of the eyes are structures with extra-cellular matrices or a multifunctional
structural backbone for tissues and organs formed with nanofibers.
Analogous to nature’s design, nanofibers of electronic polymers and their composites
can provide fundamental building blocks for the construction of devices and structures
that perform unique functions that serve the needs of mankind. Other areas impacted by
nanofiber technology include drug delivery systems and scaffolds for tissue engineering, wires, capacitors, transistors and diodes for information technology, systems for
energy transport, conversion and storage, such as batteries and fuel cells, and structural
composites for aerospace structures.
Considering the potential opportunities provided by nanofibers, there is an increasing
interest in nanofiber manufacturing technology. Amongst the technologies, including
the template method [8], vapour grown [9], phase separation [10] and electrospinning
[9, 11–27], electrospinning has attracted the most recent interest. Using the electrospinning process, Reneker and co-workers [11] demonstrated the ability to fabricate organic
nanofibers with diameters as small as 3 nm. These molecular bundles, self-assembled by
electrospinning, have only six or seven molecules across the diameter of the fiber! Half
of the 40 or so parallel molecules in the fiber are on the surface. Collaborative research
in MacDiarmid and Ko’s laboratory [12, 15] demonstrated that blends of nonconductive
polymers with conductive polyaniline polymers and nanofibers of pure conductive
polymers can be electrospun. Additionally, in situ methods can be used to deposit films
of 25 nm thickness of other conducting polymers, such as polypyrrole or polyaniline, on
preformed insulating nanofibers. Carbon nanotubes, nanoplatelets and ceramic nanoparticles can easily be embedded in nanofibers by being dispersed in polymer solutions
and consequent electrospinning of the solutions [28].
1.6
Unique properties of nanofibers
By reducing fiber diameters down to the nanoscale, an enormous increase in specific
surface area to the level of 1000 m2/g is possible. The reduction in dimension and
increase in surface area greatly affect the chemical, biological reactivity and
1.6 Unique properties of nanofibers
7
Fig. 1.4 Relation of surface area to fiber diameter [30].
electroactivity of polymeric fibers. By reducing the fiber diameter from 10 μm to 10 nm,
a million times increase in flexibility is expected. Recognizing the potential nanoeffect
that will be created when fibers are reduced to the nanoscale, there has been an
explosive growth in research efforts around the world [29]. Specifically, the role of
fiber size has been recognized in significant increase in surface area, bio-reactivity,
electronic properties and mechanical properties.
1.6.1
Effect of fiber size on surface area
For fibers having diameters from 5 nm to 500 nm, the surface area per unit mass is
around 10 000 to 1 000 000 square meters per kilogram, as shown in Fig. 1.4. In
nanofibres that are 3 nm in diameter, and which contain about 40 molecules; about half
of the molecules are on the surface. As seen in Fig. 1.4, the high surface area of
nanofibers provides a remarkable capacity for the attachment or release of functional
groups, absorbed molecules, ions, catalytic moieties and nanometer-scale particles of
many kinds. One of most significant characteristics of nanofibers is the enormous
availability of surface area per unit mass.
1.6.2
Effect of fiber size on bioactivity
Considering the importance of surfaces for cell adhesion and migration, experiments
were carried out in the Ko Laboatory (the Fibrous Materials Laboratory at Drexel
University) using osteoblasts isolated from neonatal rat calvarias and grown to
8
Introduction
Fig. 1.5 Fibroblast cell proliferation as indicated by the thymidine uptake of cell as a function
of time showing that a polylactic–glycolic acid nanofiber scaffold is most favorable for cell
growth [30].
confluence in Ham’s F-12 medium (GIBCO), supplemented with 12% Sigma foetal
bovine on PLAGA sintered spheres, 3-D braided filament bundles and nanofibrils [14].
Four matrices were fabricated for the cell culture experiments. These matrices include
(1) 150 300 µm PLAGA sintered spheres, (2) unidirectional bundles of 20 mm filaments, (3) 3-D braided structure consisting of 20 bundles of 20 µm filaments and (4)
nonwoven matrices consisting of nanofibrils. The most prolific cell growth was
observed for the nanofibrils scaffold as shown in the thymidine–time relationship
illustrated in Fig. 1.5. This can be attributed to the greater available surface for cell
adhesion as a result of the small fiber diameter which facilitates cell attachment.
1.6.3
Effect of fiber size on electroactivity
The size of the conductive fiber has an important effect on system response time to
electronic stimuli and the current carrying capability of the fibre over metal contacts.
In a doping–de-doping experiment, Norris et al. [15] found that polyaniline/PEO submicron fibrils had a response time an order of magnitude faster than that of bulk
polyaniline/PEO. There are three types of contact to a nanopolymeric wire: ohmic,
rectifying and tunneling. Each is modified due to nanoeffects. There exist critical
diameters for wires below which metal contact produces much higher barrier heights,
thus showing much better rectification properties. According to Nabet [31], by reducing the size of a wire we can expect to simultaneously achieve better rectification
properties as well as better transport in a nanowire. In a preliminary study [32], as
shown in Fig. 1.6, it was demonstrated, using sub-micron PEDT conductive fiber mat,
that a significant increase in conductivity was observed as the fiber diameter decreases.
This could be attributed to intrinsic fiber conductivity effects or to the geometric
surface and packing density effect, or both, as a result of the reduction in fibre
diameter.
1.6 Unique properties of nanofibers
9
Fig. 1.6 Effect of fiber diameter on electrical conductivity of PEDT nanofibers [30].
1.6.4
Effect of fiber size on strength
Materials in fiber form are unique in that they are stronger than bulk materials. As the
fiber diameter decreases, it has been well established in glass fiber science that the
strength of the fiber increases exponentially due to the reduction of the probability of
including flaws, as shown in Fig. 1.7a. As the diameter of matter gets even smaller, as in
the case of nanotubes, the strain energy per atom increases exponentially, contributing
to the enormous strength of over 30 GPa for carbon nanotube, as showm in Fig. 1.7b.
Although the effect of fiber diameter on the performance and processibility of fibrous
structures has long been recognized, the practical generation of fibers down to the
nanometer scale was not realized until the rediscovery and popularization of the
electrospinning technology by Professor Darrell Reneker almost a decade ago [16].
The ability to create nanoscale fibers from a broad range of polymeric materials in a
relatively simple manner using the electrospinning process coupled with the rapid
growth of nanotechnology in the recent years have greatly accelerated the growth of
nanofiber technology. Although there are several alternative methods for generating
fibers in a nanometer scale, none matches the popularity of the electrospinning technology due largely to the great simplicity of the electrospinning process. In this book we
will focus on the electrospinning technology. The relative importance of the various
processing parameters in solution electrospinning is discussed. The structure and
properties of the fibers produced by the electrospinning process are then examined with
particular attention paid to the mechanical and chemical properties. There is a gradual
recognition that the deceptively simple process of electrospinning requires a deeper
scientific understanding and engineering development in order to capitalize on the
benefits promised by the attainment of the nanoscale and to translate the technology
from a laboratory curiosity to a robust manufacturing process. To illustrate the method
10
Introduction
Fig. 1.7 (a) Dependence of glass fiber strength on fiber diameter [33], and (b) strain energy as a
function of nanotube diameter (adapted from Ref. [34]).
for connecting properties of materials in the nanoscale to macrostructures, the approach
of multi-scale modeling and a concept for the translation of carbon nanotubes to
composite fibrous assemblies is presented.
Nanotechnology is anticipated to have a tremendous impact on a broad range of
industries including the textile industry as is evident in the stain-resistant clothing and
precision filter media. The rapid growth of nanofiber technology in recent years can be
attributed to the rediscovery of the 70-year-old electrostatic spinning technology or the
electrospinning technology [8]. This technique has been used to produce highperformance filters [9, 10], wearable electronics [11] and for scaffolds in tissue engineering [35] that utilize the high surface area unique to these fibers.
Accordingly, it is the objective of this book to introduce the basic elements of
nanofiber technology. Through the electrospinning process, we will examine the parameters that affect the diameter of electrospun fibers. Examples of applications of
electrospun fibers will be presented to illustrate the opportunities and challenges of
nanofibers.
References
1. Subcommittee, N. Nanotechnology definition 2000 February 2000 [cited 2012 April 8];
available from: http://www.nsf.gov/crssprgm/nano/reports/omb_nifty50.jsp.
2. J. T. Bonner, The Scale of Nature. New York: Harper and Row, 1969.
3. M. C. Roco, National Nanotechnology Initiative-Past, Present, Future. Handbook on Nanoscience, Engineering and Technology, pp. 3.1–3.26, 2007.
4. S. Iijima, “Helical microtubules of graphitic carbon,” Nature, vol. 354(6348), pp. 56–58,
1991.
5. C. M. Shea, “Future management research directions in nanotechnology: a case study,”
Journal of Engineering and Technology Management, vol. 22(3), pp. 185–200, 2005.
References
11
6. M. S. Huda, et al., “Effect of fiber surface-treatments on the properties of laminated
biocomposites from poly(lactic acid) (PLA) and kenaf fibers,” Composites Science and
Technology, vol. 68(2), pp. 424–432, 2008.
7. Y. Wang. Nanomanufacturing technologies: advances and opportunities, in IAMOT 2009.
Orlando, Florida, USA, 2009.
8. H. Allcock, and F. Lampe, Contemporary Polymer Chemistry. Prentice Hall, 1981.
9. Y. Fan, et al., “The influence of preparation parameters on the mass production of vaporgrown carbon nanofibers,” Carbon, vol. 38(6), pp. 789–795, 2000.
10. T. Hongu and G. Philips, New Fibers. Woodhead Publ. Ltd., Cambridge, 1997.
11. D. H. Reneker, and I. Chun, “Nanometre diameter fibres of polymer, produced by electrospinning,” Nanotechnology, vol. 7(3), pp. 216–223, 1996.
12. A. MacDiarmid, et al., “Electrostatically-generated nanofibers of electronic polymers,”
Synthetic Metals, vol. 119(1–3), pp. 27–30, 2001.
13. F. Ko, et al., “Structure and properties of carbon nanotube reinforced nanocomposites,” 2002.
DETAILS?
14. F. Ko, et al., The Dynamics of Cell–Fiber Architecture Interaction. Society for Biomaterials,
1998. DETAILS?
15. I. D. Norris, et al., “Electrostatic fabrication of ultrafine conducting fibers: polyaniline/
polyethylene oxide blends,” Synthetic Metals, vol. 114(2), pp. 109–114, 2000.
16. J. Doshi and D. Reneker, “Electrospinning process and applications of electrospun fibers,”
Journal of Electrostatics, vol. 35(2), pp. 151–160, 1995.
17. J. Kim and D. Reneker, “Polybenzimidazole nanofiber produced by electrospinning,” Polymer Engineering and Science, vol. 39(5), pp. 849–854, 1999.
18. A. Formhals, “Process and apparatus for preparing artificial threads,” U.S. Patent, 1934.
19. G. Taylor, “Electrically driven jets,” Proceedings of the Royal Society of London. Series A,
Mathematical and Physical Sciences (1934–1990), vol. 313(1515), pp. 453–475, 1969.
20. C. J. Buchko, et al., “Processing and microstructural characterization of porous biocompatible protein polymer thin films,” Polymer, vol. 40, pp. 7397–7407, 1999.
21. P. Baumgarten, “Electrostatic spinning of acrylic microfibers,” Journal of Colloid and
Interface Science, vol. 36(1), 1971.
22. L. Larrondo and R. St John Manley, “Electrostatic fiber spinning from polymer melts.
I. Experimental observations on fiber formation and properties,” Journal of Polymer Science
Polymer Physics Edition, vol. 19(6), pp. 909–920, 1981.
23. I. Hayati, A. I. Bailey, and T. F. Tadros, “Mechanism of stable jet formation in electrohydrodynamic atomization,” Nature, vol. 319(6048), pp. 41–43, 1986.
24. I. Hayati, A. Bailey, and T. Tadros, “Investigations into the mechanism of electrohydrodynamic spraying of liquids. II: Mechanism of stable jet formation and electrical forces acting
on a liquid cone,” Journal of Colloid and Interface Science, vol. 117(1), pp. 222–230, 1987.
25. D. Smith, “The electrohydrodynamic atomization of liquids,” IEEE Transactions on Industry
Applications, pp. 527–535, 1986.
26. “World’s best in ultra-fine bicomponent microfibers,” [cited 2009 November 21]; Available
from: http://www.hillsinc.net/nanofiber.shtml.
27. J. Deitzel, et al., “Generation of polymer nanofibers through electrospinning,” Army
Research Lab Aberdeen Proving Ground Md, 1999.
28. M. Roco, R. Williams, and P. Alivisatos, Nanotechnology research directions: IWGN
Workshop report: vision for Nanotechnology R&D in the next decade, Kluwer Academic
Publishers, 2000.
12
Introduction
29. F. K. Ko, “Nanofiber technology: bridging the gap between nano and macro world,” in
Nanoengineered Nanofibrous Materials, S. Guceri, Y. G. Gogotsi, and V. Kuznetsov, Ed.
Dordrecht: Kluwer Academic Publishers, 2004 p. 544.
30. E. Gallo, A. Anwar, and B. Nabet, “Contact-induced properties of semiconducting nanowires
and their local gating,” Nanoengineered Nanofibrous Materials, pp. 313, 2004.
31. A. El-Aufy, B. Nabet, and F. Ko, Carbon nanotube reinforced (PEDT/PAN) nanocomposite
for wearable electronics,” Polymer Preprints, vol. 44(2), pp. 134–135, 2003.
32. W. H. Otto, “Relationship of tensile strength of glass fibers to diameter,” Journal of the
American Ceramic Society, vol. 38(3), pp. 122–125, 1955.
33. B. Yakobson and P. Avouris, “Mechanical properties of carbon nanotubes,” in Carbon
Nanotubes, M. Dresselhaus, G. Dresselhaus, and P. Avouris, Ed. Berlin Heidelberg:
Springer, 2001, pp. 287–327.
34. G. Berry, H. Nakayasu, and T. Fox, “Viscosity of poly (vinyl acetate) and its concentrated
solutions,” Journal of Polymer Science Polymer Physics Edition, vol. 17(11), pp. 1825–1844,
1979.
2
Fundamentals of polymers
In nanotechnology, polymers play a very important role as one of most often employed
materials, especially in the fields of nanofibers and nanocomposites. Hundreds of
polymers, including natural and synthetic polymers, have been fabricated into nanofibers and nanocomposites in the past 20 years. Thus a fundamental understanding of
polymers, especially fiber-making polymers, is essential for people in various fields
such as the biological, medical, electrical and material areas that are converging with
nanotechnology.
2.1
Polymeric materials
The first polymers to be exploited were natural products such as wood, leather, cotton
and grass for fiber, paper, construction, glues and other related materials. Then came
the modified natural polymers. Cellulose nitrate was the one that first attained
commercial importance for stiff collars and cuffs as celluloid in around 1885.
Notably, cellulose nitrate was later used in Thomas Edison’s motion picture film.
Another early natural polymer material was Chardonnet’s artificial silk, made by
regenerating and spinning of cellulose nitrate solution, which eventually led to the
viscose process that is still in use today. The first synthetic polymer was Bakelite,
manufactured from 1910 onward for applications ranging from electrical appliances
to phonograph records. Bakelite is a thermoset, that is, it does not flow after the
completion of its synthesis. The first generation of synthetic thermoplastics (materials
that could flow above their glass transition temperatures) are polyvinyl chloride
(PVC), poly(styrene–stat–butadiene), polystyrene (PS), and polyamide 66 (PA66).
Other breakthrough polymers include high modulus aromatic polyamides, known as
Kevlar™, and a host of high temperature polymers. Table 2.1 lists some of the
polymers currently often encountered.
Because of the low price, ease of fabrication, low density, unique and
tunable properties such as being insulating, chemical inert, etc., there has been huge
growth in the amount and applications of polymer materials during the past 50þ
years, and more and more metallic and ceramic objects are being replaced by
polymeric ones.
13
14
Fundamentals of polymers
Table 2.1 Commonly encountered polymers
Natural
polymers
Artificial
polymers
2.2
Proteins
Carbohydrates
Nuclei acids
Natural rubbers
Polyamids
Polyester
Polyolefin
Polyvinylchloride
Polyvinyldene
chloride
Polyacrylonitrile
Polyvinyl alcohol
Amino acids
Fibrous proteins
Globular protein
glycine, analine, serine etc
elastin, collagen, keratn, fibroin etc
casein, zein, insulin, egg albumin,
homoglobin etc
cellulose, starch. chitin, insulin, glycogen
DNA helix, bases
Nylon, 66, Nylon 6, Nomex
Dacron (polyethyleneterephalate), Kodel
Polyethylene, polypropylene
acrylic, modacrylic
Polymer flow, nonlinearity and heterogeneity
The most unique property of industrial and textile fibers, most plastics and polymers is
their combination of strength and toughness. Strength is characteristic of hard and brittle
materials, while toughness is characteristic of fluid that tends to flow. Strength and
toughness are mutually exclusive properties in most materials except for high polymers.
One condition uuder which materials can unify the strength of crystalline materials
(which are brittle) with the toughness of liquid-like materials (which are weak) is when
the material is heterogeneous, in another words, the constituent molecules can coexist in
crystalline and amorphous regions in the same materials. Only large polymer molecules
could be present in more-ordered and less-ordered regions at the same time, which
makes polymers unique from other types of materials. Furthermore, these large molecules would have to be anisotropic in their mechanical properties and structures; for
example, contrasting molecules in nylon with SiO2 in fused silica.
For many materials, the deformation response to a given external force may be
completely elastic and therefore temporary, or completely viscous and hence permanent. Polymers are unique in their responses due to the coexisting of crystalline and
amorphous regions in their long chain structures. Polymers exhibit elastic and viscous
responses simultaneously under ordinary stress conditions; this is said to be viscoelastic.
2.2.1
Linear kinetics
How a material responds to a load of stress can be described as being between two
limiting extremes: elastic or Hookean behavior, and viscous or Newtonian behavior. In
an ideally elastic body, Hooke’s law applies. Hooke’s law states that the deformation or
strain of a spring is linearly related to the force or stress applied by a constant specific.
Mathematically the stress as a function of the strain is expressed as:
2.2 Polymer flow, nonlinearity and heterogeneity
15
Fig. 2.1 Stress–strain curve of an elastic material.
σ ¼ Eε
ð2:1Þ
S ¼ Gγ,
ð2:2Þ
where σ is tensile stress, E is elastic tensile modulus and ε the strain. Accordingly S, G
and γ are respectively the stress, modulus and strain for shear response. Figure 2.1
shows the stress–strain curve of an elastic material. The elastic constants E and G are
related by the following expression:
E ¼ 2ð1 þ μÞG,
ð2:3Þ
where μ is the Poisson ratio. For elastic materials μ is about 0.5, i.e. the elastic tensile
modulus is three times greater than the shear modulus.
The other limiting extreme is the Newtonian behavior. Most small molecules and
ordinary solids are Newtonian or approach it within a fraction of one percent of
ordinary shear rate. They flow and obey Newton’s law. The material responds to the
applied stress by slowly deforming. As the rate of the shear increases, the rate of flow
of the material also increases. The Newton’s law is described by the following
equation:
_
S ¼ ηγ:
ð2:4Þ
Details can be found in Section 2.3.2.
2.2.2
Nonlinear behavior
Since they are viscoelastic, polymers behave in a nonlinear, non-Hookean way. The
most elementary manifestations of a polymer’s nonlinear viscoelastic behavior are
stress relaxation and a phenomenon known as creep, which describes how polymers
are strained under constant stress.
If a polymer is is subjected to a fixed strain, the material will exert a retractive
(elastic) stress response. As time passes, the stress required to hold the initial strain
decays from the initial value to zero. This deformation process is referred to “creep.”
16
Fundamentals of polymers
Fig. 2.2 Spring element.
This behavior can be explained on a molecular level: when strained, the polymer
molecular chain network is entangled, but then seeking to return to its unperturbed
state by giving rise to a retractive stress response. After continuous long exposure to the
stress, the molecules begin to slide from each other to reestablish an equilibrium status,
resulting in decaying stress response.
When the external stress is released, the polymer material will tend to return to its
original status. The extent to which the material will recover its original dimensions
depends on the magnitude of elastic stress remaining in it. The earlier the strain is
relieved, the higher the stress remains and the greater the extent of strain recovery will be.
2.2.3
2.2.3.1
Viscoelastic models
The basic elements: spring and dashpot
Spring (Fig. 2.2) and dashpot (Fig. 2.3) are two basic elements frequently used
mathematically to analyze the viscoelastic properties of materials. The inertia effects
are often neglected during analysis.
The spring element is analogous to ideal elastic or Hookean behavior. A spring stores
energy and responds instantaneously. For a spring,
σ ¼ Eε,
ð2:5Þ
as shown in Fig.2.4, where E can be interpreted as a linear spring constant or Young’s
modulus.
The dashpot is analogous to pure viscous or Newtonain behavior. Dashpots dissipate
energy and characterize the retarded nature of a response. A dashpot will deform
continuously at a constant rate when it is subjected to a step of constant stress, as
shown in Fig. 2.5a. On the other hand, when a step of constant strain is imposed on the
dashpot the stress will be of an infinite value at the instant and then rapidly diminish
with time to zero, as shown in Fig. 2.5b [1]:
2.2 Polymer flow, nonlinearity and heterogeneity
17
Fig. 2.3 Dashpot element.
Fig. 2.4 Behavior of a linear spring.
σ¼η
dε
_
¼ ηε,
dt
ð2:6Þ
where η is the coefficient of viscosity and ε_ is the to the stress.
2.2.3.2
Maxwell model
A Maxwell model consists of a spring and a dashpot in series, as illustrated in Fig. 2.6.
The Maxwell model assumes materials can undergo viscous flow and also respond
elastically: both the spring and the dashpot are assumed to be subjected to the same
stress but are permitted to respond with independent strains.
Since both elements are connected in series, the total strain will be
ε ¼ ε1 þ ε 2
ð2:7Þ
ε_ ¼ ε_ 1 þ ε_ 2
ð2:8Þ
or the strain rate will be
and the stress–strain rate relation is illustrated as
ε_ ¼
σ_ σ
þ :
E η
ð2:9Þ
18
Fundamentals of polymers
Fig. 2.5 Behavior of a linear viscous dashpot.
Fig. 2.6 Maxwell model.
2.2 Polymer flow, nonlinearity and heterogeneity
19
Fig. 2.7 Behavior of Maxwell model: (a) creep and recovery and (b) stress relaxation.
Given the condition that a constant stress σ ¼ σ0 is applied at t ¼ 0, Eq. (2.9) becomes a
first-order differential equation of ε. The following strain–time relation is obtained after
applying integration together with the initial condition [1]:
εðtÞ ¼
σ0 σ0
þ t:
E
η
ð2:10Þ
This equation describes the creep and recovery behavior of the Maxwell model [1]. If
the stress is removed from the model, the elastic strain returns to zero instantly, while
the viscous strain is withheld. The behavior of the Maxwell model under such condition
is as shown in Fig. 2.7a. In other words, the Maxwell model predicts that creep should
be constant with time, which is untrue.
If fixed strain, ε0, is applied to the model with an initial value of stress of σ0, the stress
response will be
σðtÞ ¼ σ 0 eEt=η ¼ Eε0 eEt=η :
ð2:11Þ
σ_ ¼ ðσ 0 E=ηÞeEt=η :
ð2:12Þ
The rate of stress change is
Then the initial rate of change in stress at t ¼ 0þ (where 0þ refers to the time right after the
application of the strain) is σ_ ¼ ðσ 0 E=ηÞ, which means the stress will decrease continuously at this initial rate. Therefore the following relaxation equation will be obtained:
20
Fundamentals of polymers
Fig. 2.8 Voigt (Kelvin) model.
σ_ ¼ ðσ 0 E=ηÞ þ σ 0
ð2:13Þ
The stress will reach zero at time tE ¼ η/E, which is the relaxation time of the Maxwell
model, as shown in Fig. 2.7b. The relaxation time characterizes one aspect of the
viscoelastic properties of the material. But the stress relaxing to zero is not always the
case for real polymers.
2.2.3.3
Voigt (Kelvin) model
A Voigt (Kelvin) model consists of a spring and a dashpot in parallel, as schemed in
Fig. 2.8. When a stress is applied, the stress is at first carried entirely by the viscous
element which elongates and gradually transfers the load to the elastic element. Finally
the elastic element carries the entire stress. This behavior is appropriately called delayed
elasticity [1]. It is intuitive to see that both elements are constrained to the same strain
but the stresses are additive:
σ ¼ σ1 þ σ2,
ð2:14Þ
E
σ
ε¼ :
η
η
ð2:15Þ
which can be expressed as
ε_ þ
If a constant stress σ0 is applied to the model at time t ¼ 0, the solution to Eq. (2.15)
will be
ε¼
σ0
ð1 eEt=η Þ:
E
ð2:16Þ
This equation implies that the strain increases with a decreasing rate and approaches
asymptotically the value of σ0/E when t tends to infinity, as shown in Fig. 2.9.
The strain rate in creep under a constant stress σ0 is
2.2 Polymer flow, nonlinearity and heterogeneity
21
Fig. 2.9 Behavior of Voigt (Kevin) model.
ε_ ¼
σ 0 Et=η
e
:
η
ð2:17Þ
Thus, the strain rate at t ¼ 0þ is ε_ ð0þ Þ ¼ σ 0 =η, which approaches asymptotically to zero
when t tends to infinity.
If the strain increases at an initial rate of σ0/η, it will converge the asymptotic value
σ0/E at time tc ¼ η/E, known as the retardation time. It is obvious that most of the total
strain σ0/E occurs within the retardation time period since eEt/η approachs the asymptotic value rapidly when t < tc. At t ¼ tc, ε ¼ (σ0/E)(1 1/e) ¼ 0.63σ0/R, which means
only 37% of the asymptotic strain remains to be accomplished after t ¼ tc [1].
If the stress is removed at time t1, the strain following stress removal can be
determined by the superposition principle [1].
The strain εa resulting from stress σ0 applied at t ¼ 0 is
εa ¼
σ0
ð1 eEt=η Þ:
E
ð2:18Þ
The strain εb resulting from a stress (σ0) exterted by the spring at time t ¼ t1 is
εa ¼
σ0
ð1 eEðtt1 Þ=η Þ:
E
ð2:19Þ
Then the stain for t > t1 during recovery will be
ε ¼ ε a þ εb ¼
σ 0 Et=η Et1 Þ=η
e
ðe
1Þ,
E
ð2:20Þ
which indicates the recovery trends toward zero when t approaches to infinity, as
illustrated in Fig. 2.9.
The Voigt (Kelvin) model does not show a time-dependent relaxation. Because of the
presence of the viscous element, an abrupt change in strain can be accomplished only by
an infinite stress. This makes the Voigt model as flawed as the Maxwell model.
2.2.3.4
Four-element model
Since neither the Maxwell nor the Voigt model can accurately describe the behaviors of
most viscoelastic materials, the two models are more often used in series, known as the
four-element model (schematically shown in Fig. 2.10), to describe the viscoelastic
deformation of polymers.
22
Fundamentals of polymers
Fig. 2.10 The four-element model.
Fig. 2.11 Creep behavior as predicted by the four-element model.
The four-element model is the simplest model that exhibits all the essential features of
viscolasticity, as shown in Fig. 2.11. When a stress, σ, is imposed, the model undergoes
an elastic deformation, followed by creep. The deformation due to a dashpot in series
(ηm) is nonrecoverable. Thus, on removal of the stress, the model undergoes a partial
recovery. The strain obtained from each of the components can be simply summed:
εðtÞ ¼
σ
σt
σ
þ þ ð1 et=τ Þ,
E m ηm E v
where τ ¼ ηv/Ev, which refers to the retardation time.
ð2:21Þ
2.3 Intrinsic structures of polymers
23
Table 2.2 General relationships between properties and structures of ploymer [2]
Properties
Crystallinity
Crosslinking
Molecular
weight
Abrasion resistance
Brittleness
Chemical resistance
Hardness
Tg
Solubility
Tensile strength
Toughness
Yield
þ
þ
þ
þ
þ
þ
þ
M
V
þ
þ
M
þ
þ
þ
þ
þ
þ
þ
þ
þ
Molecular
weight
distribution
Polar
backbone
units
Backbone
stiffening
groups
þ
þ
0
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ, increase as the factors increase; 0, little or no effect; , decrease in property; M, property passes through
a maximum; V, variable results dependent on particular sample and temperature.
The four-element model provides a crude qualitative representation of the phenomena generally observed with viscoelastic materials: instantaneous elastic strain, retarded
elastic strain, viscous flow, instantaneous elastic recovery, retarded elastic recovery and
permanent plastic deformation [1]. However, the quantities E and η of the models are
not real-life values. More complex arrangements of the elements are often employed,
especially when multiple relaxations are involved or accurate representations of engineering data are required.
2.3
Intrinsic structures of polymers
As seen in Section 2.2, the unique properties of polymers originate from their intrinsic
structures such as bondings, chain conformations in space, chain configuration, order
and disorder structures of polymer molecular chain, molecular weight and molecular
weight distribution. The bonds determine how strongly the atoms and molecules are
jointed. The configuration and conformation settle the arrangement of molecular chain.
The order and disorder structures give the polymer the combination of strength and
toughness. The molecular weight and weight distribution measure the viscoelasticity
and other physical properties of the polymer. Table 2.2 illustrates how the structures of
polymers may affect their properties.
2.3.1
Molecular bondings
The molecular interaction bondings include primary forces (valence bonds) and secondary forces. Atoms in individual polymer molecules are joined to each other by
relatively strong covalent bonds, while polymer molecules are attracted to each other by
intermolecular forces. The primary forces typically range from 50 to 200 kcal/mole; the
24
Fundamentals of polymers
secondary forces range typically from 0.5 to 10 kcal/mole by contrast [3]. Secondary
forces can also be found between segments of a long molecular chain.
Primary bonding forces are divided into ionic, metallic and covalent bonding,
including coordinate or dative bonding. Ionic bonds form between atoms with largely
differing electronegativites and are characterized by a lack of directional bonding. They
not typically present for polymer backbones. Similar to ionic bonds, metallic bonds lack
bonding direction and are typically absent from polymers. Metallic bonds are generated
between atoms having valence electrons that are too small to provide complete outer
shells. A covalent bond is a bond in which one or more pairs of electrons are shared by
two atoms. Atoms will covalently bond with other atoms in order to gain more stability
by sharing their outermost (valence) electrons to form a full electron shell. The bonding
lengths of primary bonds are usually about 0.90–2.0 Å. The carbon–carbon bond length
falls into the range of about 1.5–1.6 Å [2].
The potential of a polymer to exert strong intermolecular bonding and hence to
exhibit high mechanical strength resides fundamentally in the nature of the secondary
molecular bonding forces. The energies of secondary bondings are generally one
magnitude lower than those of primary bonding forces; however, when associated with
large molecules, secondary bonding forces are capable of providing a polymer with
strong mechanical properties. The secondary bonding forces most involved in polymers
are van der Waals forces and hydrogen forces.
2.3.1.1
Van der Waals forces
Van der Waals forces (or the van der Waals interaction), named after Dutch scientist
Johannes Diderik van der Waals, are the sum of the attractive or repulsive forces
between molecules. Van der Waals forces are the main type of secondary bonding
forces. Van der Waals forces are caused by electromagnetic interactions of nearby
particles [3]. A neutral atom or molecule is composed of a positively charged nucleus
enclosed by an outer shell of negatively charged electrons. The electrons are mobile,
and at any one instant they might find themselves towards one end of the molecule
causing deviation in the electron shell density. The deviation generates an infinitesimal
magnetic imbalance and turns the molecule as a whole into a small magnet or dipole,
and thus electromagnetic interaction is created between this molecule and the nearby
ones. In other words, van der Waals forces are the result of intermolecular polarities.
The degree of “polarity” that the temporary dipoles confer on a molecule depends on
its surface area [3]: the larger the molecule, the greater the number of temporary dipoles,
and thus the higher the intermolecular attractions generated. Molecules with straight
chains have a larger surface area, and therefore greater dispersion forces, than branched
molecules of the same molecular weight.
2.3.1.2
Hydrogen bonding
Hydrogen bonding occurs when a hydrogen atom is attached to an electronegative atom
such as oxygen, nitrogen or fluorine, and involves a hydrogen atom and another
electronegative atom. The sole electron of the hydrogen atom is drawn towards
the electronegative atom it is attached to, leaving the strongly charged hydrogen nucleus
2.3 Intrinsic structures of polymers
25
exposed, which imposes considerable attraction on electrons in another electronegative
atom from aother molecule or chemical group and form a protonic bridge, and thus
forms the hydrogen bond. A hydrogen bond is not a covalent chemical bond and ranges
from 5 to 10 kcal/mole which is substantially stronger than most other types of dipole
interaction [3].
2.3.2
Configuration and conformation
The geometric structure of a polymer is described in terms of configuration and
conformation. Configuration defines the arrangement of the atoms along a molecular
chain, which is permanent unless chemical bonds are broken and the molecule is
reformed. Conformation refers to the arrangements of atoms and substituents of a
polymer chain brought about by rotation about single bonds.
2.3.2.1
Configuration
The structure of a molecule joined by double bond cannot be changed by physical
means (e.g. rotation). The placement of the substituent groups differentiates the polymer
molecule into cis and trans configurations. When substituent groups are on the same
side of the double bond, the molecule is in the cis configuration, otherwise the trans
configuration.
In a polymer molecule chain, the structure of repeat units (monomers) is fixed by the
chemical bonds between adjacent atoms. The shape or shapes thus created is known as
the configuration. The monomers may be joined head-to-head, tail-to-tail or head-to-tail
during polymerization, resulting in different chain configurations. The head-to-tail
configuration is the thermodynamically and spatially preferred structure, although most
addition polymers contain a small percentage of head-to-head placements.
Stereoregularity is a term describing the substituent arrangements in a polymer chain.
Three distinct structures have been identified: isotactic, syndiotactic and atactic structures. Isotactic describes an arrangement where all substituents are on the same side of
the polymer chain, syndiotactic depicts a composion of alternating groups and atactic
refers to a random combination of the groups [4]. Figure 2.12 illustrates the three
stereoisomers of polymer chains.
2.3.2.2
Conformation
A single conformation is just a single shape that a chain can adopt. Polymers differing
only by rotations about single bonds are said to be two different conformations of that
polymer. A planar zigzag conformation (as shown in Fig. 2.13a) can be formed in
linear polymers such as polyehthylene, poly(vinyl alcohol) and polyamides since
alignment and packing of their crystallines are not disturbed by the presence of bulky
side groups. In many instances, the side groups are too bulky to be accommodated in a
zigzag conformation. To relieve the steric crowding, the molecules in the main chain
rotate and form either a right- or left-handed helix, as shown in Fig. 2.13b. The helical
conformation is ofen found in the isotactic and syndotactic α-olefin polymers.
A random conformation is a common form that most polymers adopt in amorphous
26
Fundamentals of polymers
Fig. 2.12 Diagram of three stereoisomers of polymer chain.
systems like solutions, melts or some solids, where rotation about primary valence
bonds is relatively free although restricted by bond length, bond angles and steric
crowding of side groups.
2.3.2.3
Other chain structures
The geometric arrangement of the bonds is not the only structure that a polymer can
alter [5]. A copolymer consists of at least two types of constituent units; therefore
copolymers can have different chain structures by changing the arrangement of the
units. As shown in Fig. 2.14, copolymers may assume three important types of chain
strucutures: random, block and graft structures. An example of a copolymer is Nylon.
Polymer molecules can also form a network structure by cross-links. Cross-links are
bonds that link polymer chains together. The bonds can be covalent bonds or ionic
bonds. It is notable that polymers with a high enough degree of cross-linking have
“memory” [5]. That is, when the polymer is stretched, the chains may straighten out; but
once the stress is released they will return to their original position and therefore resume
their original shape because the cross-links prevent the individual chains from sliding
2.3 Intrinsic structures of polymers
27
Fig. 2.13 (a) Zigzag conformation of HDPE and (b) helix conformation of isotactic vinyl polymers.
Fig. 2.14 Different structure types of copolymers: (a) block copolymer, (b) graft copolymer and (c)
random copolymer [5].
past each other. One example of cross-linking is vulcanization. In vulcanization, a series
of cross-links are introduced to an elastomer for improved strength. This technique is
commonly used to strengthen rubber.
2.3.3.
Order and disorder
2.3.3.1
Amorphous and crystal structure
Polymers in the solid state may be structurally amorphous or crystalline. During solidification, polymer molecules attract to each other and tend to cluster as closely as possible to
form a solid with the least possible potential energy. If the chains are folded and packed
regularly in a long-range, three-dimensional (3-D) and ordered arrangement, the polymer
has a crystalline structure. Otherwise the polymer is in an amorphous state with a
28
Fundamentals of polymers
Fig. 2.15 Fringed micelle model (modified from Ref. [7]).
disordered form. Since the polymer chains are too long to always have a perfect arrangement, most polymers have both amorphous and crystalline regions in their solid states.
The arrangement perfection, i.e. the fraction of the total polymer in crystalline
regions, is measured as the degree of crystallinity. The crystallinity for polymers may
vary from a few percent to about 90% depending on the crystallization conditions [2].
Rapid cooling often decreases the amount of crystallinity since there is not sufficient
time to allow the long chains to arrange themselves into a more-ordered structure.
Polymers with irregular bulky groups are seldom crystallizable. Ordered polymers are
seldom 100% crystalline because of the inability of the long chains to completely
disentangle and perfectly align themselves during the time the polymer chain is mobile.
Historically, various folded surfaces, generally imagined as a fringed micelle model,
as shown in Fig. 2.15, were important to explain many of the physical properties of
polymers, but such fringed micelle models are found not to be consistent with much of
the current experimental findings. For instance [6], this model accounts for the X-ray
diffraction behavior and the mechanical properties of semicrystalline polymers but fails
to explain the optical properties of polymer spherulites and their small-angle X-ray
scattering patterns. The actual structures of the amorphous and crystalline regions are
complex and still undergoing clarification.
2.3.3.2
Orientation
Orientation measures the alignment of the crystallites in the crystalline and amorphous
regions parallel to the material’s axis. If a bulk polymer crystallizes in the absence of
external forces, there is no preferred orientation of crystallites or molecules in the bulk.
If equal numbers of the axes of the chain segments or of the crystallites within this
polymer point in every direction in space, the polymer is isotropic. If an external force,
such as mechanical drawing, is imposed, the crystallites and molecules will orient in the
draw direction.
Orientation dominates the mechanical and physical properties of polymers. Desirable
changes in polymer properties such as tensile strength and impact strength may be
2.3 Intrinsic structures of polymers
29
Table 2.3 Methods for measuring order and disorder structures [9]
Structural element
Method
Dimensions
Fiber
Fibrils/lamellae
Voids
Crystalline regions
Non-crystalline regions
Optical microscopy
Electron microscopy
Small-angle X-ray scattering
Wide-range X-ray scattering
Small-angle X-ray scattering
Spectroscopy (IR, NMR, etc.)
X-ray scattering (wide- and small angle,
infrared, birefringence, sonic velocity)
>0.2 μm
>100 Å
1000–10 Å
10–1 Å
100–10 Å
Molecular groups
Lamellae, crystallites,
molecular segments or groups
Orientation
Table 2.4 Measurement method and their measurement range, perfection and time range
Method
Range
Perfection
Time(sec)
X-ray diffraction
0.7–2 Å
50–100 atoms
50 atoms
50 Å
2Å
200 Å
1%–2%
1 to 107
1%
?
100%
?
1 to 102
103 to 104
1 to 102
103 to 104
Electron diffraction
Light scattering
TEM
SEM
improved by orientation. However, orientation may also cause undesirable properties
including anisotropy properties and dimensional instabilities at elevated temperatures in
thermoplastic materials [8]. For instance, synthetic textile fibers shrink on heating above
their glass transition temperature due to randomization of the orientated amorphous
regions.
2.3.3.3
Measurement of order and disorder
Order and disorder structures in polymers are experimentally measurable. The methods
and the according measurable dimensions are listed in Table 2.3. It should be noted
that all measurements of order depend not only on the signal to noise ratio, but also
requires the polymers have some necessary minimum range of order, some minimum
perfection of order, and some minimum time over which the order structure can persist,
as shown in Table 2.4. Some of the main measurement methods are to be introduced in
Chapter 6.
2.3.4
Molecular weight and molecular weight distribution
The size of a single polymer molecule may be expressed in terms of its molecular
weight (MW) or its degree of polymerization (DP). The same bonding forces (intraand intermolecular) act in both low- and high-molecular-weight polymers. However,
the unique properties and disctinctive behaviors of polymers bear high relevance to
their MWs, as shown in Fig. 2.16. Knowledge of the MW and MW distribution along
30
Fundamentals of polymers
Fig. 2.16 The relationship between physical properties and molecular size.
with a good understanding of the chain conformation, will allow one to be able to
predict many of the properties of a polymer. Polymers with small molecules develop
hardly any strength. With increasing MW, a polymer will generate a steep rise in
performance until a certain level, thereafter the properties change very little with
increase of MW [6].
During synthesis, polymer molecules are subjected to a series of random events.
Therefore synthetic polymers are not composed of identical molecules, but of a mixture
of molecules with molecular weights in a certain range. Molecular weight distribution
(MWD) is often used to characterize this property of polymers. Average molecular
weight is computed to approximately measure the molecule size of a polymer.
Owing to the heterogeneity of polymer MW, average molecular weight is measured
in different ways depending on the way in which the heterogeneity is averaged. The
most commonly used ways are weight-average molecular weight, Mw, and numberaverage molecular weight, Mn. Others are the z-average molecular weight, Mz, and
viscosity-average molecular weight, Mv. Below are the formulas showing the relevances
between these average molecular weights:
X
NiMi
1
¼X
Mn ¼ X
ð2:22Þ
ðwi =M i Þ
Ni
X
X
X
W iMi
N i M 2i
¼
wi M i
ð2:23Þ
Mw ¼ X
¼ X
Ni
Wi
X
X
N i M 3i
W i M 2i
X
X
¼
ð2:24Þ
Mz ¼
N i M 2i
W iMi
2.3 Intrinsic structures of polymers
X
N i M i1þa
W i M 2i
¼X
,
Mv ¼ X
NiMi
W iMi
X
31
ð2:25Þ
where a is a constant between 0.6 and 0.8, Mi is the molecular weight of the ith species,
Ni is the number of moles of molecules with molecular weight Mi, Wi denotes the weight
of material with molecular weight Mi, and wi is the weight fraction of molecules of the
ith species, which is computed from the following formula
Wi ¼
N iMi W i
¼
,
Wt
Wt
ð2:26Þ
where Wt is the total weight of polymer.
The molecular weight of polymers can be determined by a number of physical and
chemical methods including end-group analysis, measurement of colligative properties,
light scattering, ultracentrifugation, dilute solution viscosity and gel permeation chromatography (GPC). The number-average weight, Mn, can be obtained from end-group
analysis, colligative property measurements and gel permeation chromotography. The
weight-average molecular weight, Mw, can be measured from light scattering, ultracentrifugation and gel permeation chromatography. The z-average molecular weight, Mz,
comes from GPC, while viscosity-average molecular weight, Mv, comes from the
measurement of polymer solution viscosity.
The following relationship holds for a polymer: M n < M v < M w < M z . For a random
molecular weight distribution, such as produced by many free radical or condensation
syntheses, M n : M w : M z ¼ 1 : 2 : 3. The ratio M w =M n , called the polydispersity index
(PDI), are often used to indicate the breadth of the distribution. For many linear
systems, M w =M n is about 2. But in highly nonlinear systems, M w =M n may range as
high as 20–50. Figure 2.17 illustrates the relationship between M w and M n .
Fig. 2.17 Number-average molecular weight Mn, and weight-average molecular weight, Mw.
32
Fundamentals of polymers
Fig. 2.18 Semi-log plot of polymer modulus (stiffness) versus temperature.
2.4
Thermal behavior
It is well known that matters composed of low-molecular-weight compounds have three
states: crystalline, liquid and gaseous. The three states transit upon certain changes of
temperature or pressure. By contrast, high-molecular-weight polymers have no gaseous
state. At high temperatures, they decompose rather than boil since their “boiling points”
are generally higher than their decomposition temperature. Their transition between the
solid and liquid forms is rather diffuse and occurs over a temperature range that depends
on their polydispersity. Furthermore, they are very viscous on melting and cannot freely
flow as in the case of low-molecular-weight materials [6].
Two important temperatures have been identified for polymers. The solid-to-liquid
transition temperature range is called the “glass transition temperature,” Tg. Glass
transition occurs only in an amorphous region. The temperature at which crystalline
regions start to melt is referred to as the “melting point,” Tm. Note that Tm usually
corresponds to the temperature at which the last trace of crystallinity disappears as
polymers melt over a range of several degrees. Amorphous polymers undergo glass
transition but have no melting point. At low temperatures, amorphous polymers are
glassy, hard and brittle. Above Tg, amorphous polymers soften and become rubbery
because of the onset of molecular motion. Semi-crystal polymers are elastic and flexible
at a temperature above Tg and below Tm. The amorphous regions contribute elasticity
and the crystalline regions contribute strength and rigidity. As the temperature increased
to Tm, all crystalline structures disappear; the chains are set free and become randomly
dispersed. Bulk crystallized polymers only melt. Figure 2.18 shows typical changes in
modulus for amorphous and semi-crystalline plastics as the temperature changes. Based
on their thermal behavior, polymers are usually classified as thermoplastics or thermosets. Thermoplastics soften and flow under heat and pressure, and harden upon cooling.
2.5 Polymer solutions
33
Thermosets are incapable of undergoing repeated cycles of softening and hardening.
Once the shape of thermosets is set by cross-link, it can’t be reformed by remelt.
Thermobehavior of polymers can be characterized by differential scanning calorimetry (DSC) or thermogravimetric analysis (TGA), which will be introduced in detail in
Chapter 6.
2.5
Polymer solutions
Dissolution of polymers mainly depends on the nature of the solute and solvent, while
the viscosity of the medium, the polymer stucture and polymer molecular weight also
have an effect to some extent. Dissolution of polymers is necessarily a slow, two-staged
process. First, the solvent molecules diffuse into the polymer producing a swollen gel
and, second, the gel slowly breaks down and forms a true solution. For polymers of high
molecular weight, this process may take several hours or longer depending on sample
size, temperature, and so on. Materials with strong polymer–polymer intermolecular
forces due to cross-linking (phenolics), crystallinity (Teflon), or strong hydrogen
bonding (native cellulose) will exhibit only a limited degree of swelling in any solvent
at ordinary temperatures and will not be truly dissolved without degradation.
2.5.1
Solubility parameter
“Like dissolves like” is one of the simplest and important notions in chemistry.
Generally, “like” is appreciated in terms of similar chemical groups or similar polarities
[10]. From thermodynamic considerations, the solubility of one component in another is
governed by the equation of the free energy of mixing:
ΔGm ¼ ΔH m TΔSm ,
ð2:27Þ
where ∆Gm, ∆Hm, and ∆Sm are respectively the change in Gibb’s free energy, the
enthalpy of mixing and the entropy of mixing. T is the absolute temperature.
Dissolution will occur if the free energy of mixing ∆Gm is negative. T∆Sm is always
positive because there is an increase in the entropy on mixing. Therefore, the sign of
∆Gm is determined by the sign and magnitude of ΔHm. In an ideal solution, the heat of
mixing ∆Hm is zero since the two types of molecules have the same force fields, i.e.
ΔGm ¼ TΔSm. However, in reality, the intermolecular forces working between similar
and dissimilar molecules give rise to a finite heat of mixing. Therefore, positive heats of
mixing are the more usual case for relatively nonpolar organic compounds [10].
Hildebrand and Scott proposed the enthalpy of mixing ∆Hm for regular solutions as:
"
#2
ΔE 1 1=2
ΔE 2 1=2
,
ð2:28Þ
ΔH m ¼ V m ϕ1 ϕ2
V1
V2
where V, V1, V2, are the volumes of the solution and components, and the subscripts 1
and 2 denote the solvent and polymer, respectively. ∆E is the molar energy of
34
Fundamentals of polymers
Table 2.5 Typical values of δ for various types of solvents [4, 10]
Solvent
δ
Poorly hydrogen bonded
Difluorodichloromethane
n-Decane
n-Pentane
n-Heptane
Apco thinner
Solvesso 150
Carbon tetrachloride
Xylene
Toluene
Benzene
Tetrahydronaphthalene
O-Dichlorobenzene
1-Bromonaphthalene
Nitroethane
Acetonitrile
Nitromethane
5.1
6.6
7
7.4
7.8
8.5
8.6
8.8
8.9
9.2
9.5
10
10.6
11.1
11.8
12.7
Solvent
δ
Solvent
Moderately hydrogen bonded
Strongly hydrogen bonded
Diethyl ether
Diisobutyl ketone
n-Butyl acetate
Methyl propionate
Dibutyl phthalate
Dioxane
Acetone
Dimethyl phthalate
2,3-Butylene carbonate
Propylene carbonate
Ethylene carbonate
Dibutyl amine
2-Ethylhexanol
Methyl isobutyl carbinol
2-Ethylbutanol
n-Pentanol
n-Butanol
n-Propanol
Ethanol
Methanol
Water
7.4
7.8
8.5
8.9
9.3
9.9
9.9
10.7
12.1
13.3
14.7
δ
8.1
9.5
10
10.5
10.9
11.4
11.9
12.7
14.5
23.4
vaporization to a gas at zero pressure, and ϕ1 and ϕ2 are volume fractions. The quantity
∆E/V represents the energy of vaporization per cubic centimeter, referred as the cohesive energy density. According to Eq. (2.28), it is easy to note that “like dissolves like”
means ∆E1/V1 and ∆E2/V2 have nearly the same numerical value.
The sequare root of the cohesive energy density is widely known as the solubility
parameter,
δ ¼ ðΔE=V Þ1=2 :
ð2:29Þ
Thus, the heat of mixing of two substances depends on the value of (δ1 δ2)2.
Generally, if δ1 δ2 is less than 1.7–2.0, the polymer will be expected to be soluble
in the solvent. Table 2.5 and Table 2.6 list the typical values of δ for various types of
solvents and some polymers respectively. The magnitude of the enthalpy of mixing can
be conveniently estimated from these tables.
The solubility parameter of a polymer can be determined by measuring the intrinsic
viscosity of the polymer in solvents, if the polymer is soluble in them. During the
measurement, several solvents of varying solubility parameter are selected and
the intrinsic viscosities of the solutions are plotted against the solubility parameter
of the solvents. Since the chain conformation is most expanded in the best solvent, the
intrinsic viscosity will be the highest for the best match in solubility parameter.
Values of the solubility parameter may also be calculated by using group molar
attraction constants, G, for each group,
X
ρ G
,
ð2:30Þ
δ¼
M
2.5 Polymer solutions
35
Table 2.6 Values of δ for various polymers [4, 10]
Polymers
Δ
Polyethylene
Polypropene
Polyisobutene
Polyvinylchloride
Polyvinylidene chloride
Polyvinyl bromide
Polyvinylfluoroethylene
Polychlorotrifluoroethylene
Polyvinyl alcohol
Polyvinyl acetate
Polyvinyl propionate
Polystyrene
Polymethyl acrylate
Polyethyl acrylate
Polypropyl acrylate
Polybutyl acrylate
Polyisobutyl acrylate
Poly-2,2,3,3,4,4,4, heptafluorobutyl acrylate
Polymethyl methacrylate
Polyethyl methacrylate
Polybutyl methacrylate
Polyisobutyl methacrylate
Poly-tert, butyl methacrylate
Polyethoxyethyl methacrylate
Polybenzyl methacrylate
Polyacrylonitrile
Polymethacrylonitrile
Poly-a-cyanomethyl
Polybutadiene
Polyisoprene
Polychloroprene
Polyepichlorohydrin
Polyethylene terephthalate
Polyhexamethylene adipamide
Poly(δ-aminocaprylic acid)
Polyformaldehyde
Polytetramethylene oxide
Polyethylene sulfide
Polypropylene oxide
Polystyrene sulfide
Polydimethyl siloxane
7.7
8.3
7.8
9.4
9.9
9.5
6.2
7.2
12.6
9.35
8.8
8.5
9.7
9.2
9.05
8.8
8.7
6.7
9.1
8.9
8.7
8.2
8.3
9.0
9.8
12.5
10.7
14.0
0.1
7.9
8.2
9.4
9.7
13.6
12.7
10.2
8.3
9.0
7.5
9.3
7.3
8.35
9.2
8.1
10.8
7.9
14.2
11.05
9.3
10.4
9.4
9.1
11.0
19.8
9.15
9.0
10.5
9.9
10.0
15.4
14.5
8.6
10.0
9.25
10.7
11.0
8.55
9.4
9.9
7.6
where ρ represents the density and M is the molecular weight or mer molecular weight
for a polymer.
For the value of a solvent mixture, the solubility parameter can be determined by
averaging the solubility parameters of the individual solvents by volume.
36
Fundamentals of polymers
2.5.2
Solution viscosity
The viscosity of a polymer solution strongly relates to the polymer's physical and
engineering properties, such as monomer molecular weight, molecular weight distribution and engineering processibility, etc. A good understanding of the viscosity of a
polymer is therefore vital for polymer processing.
2.5.2.1
Intrinsic viscosity
The viscosity of a polymer solution, η, relative to the viscosity of a pure solvent, η0, is
referred to as the relative viscosity, given by
ηr ¼
η
:
η0
ð2:31Þ
The viscosity of the polymer solution is always greater than that of the pure solvent.
Therefore the fractional increase in the viscosity resulting from the dissolution of the
polymer can be referred as the specific viscosity, ηsp,
ηsp ¼ ηr 1:
ð2:32Þ
The polymer coils and the solvent they enclose are assumed as behaving like an
Einsteinian sphere uder the action of a shear stress. Polymer chains in dilute solutions
are isolated and interact only with each other during brief times of encounter. It is easy
to speculate that there is no interaction of polymer coils in an infinite diluted solution.
Mathematically this can be achieved by defining intrinsic viscosity, [η]:
hη i
sp
½ η ¼
:
ð2:33Þ
c c¼0
For dilute solutions of which the specific viscosity is just over unity, the logarithm of the
relative viscosity can be written as:
ln ηr ¼ lnðηsp þ 1Þ ffi ηsp η2sp =2 þ :
Then the intrinsic viscosity is approximately:
ln ηr
½ η ¼
:
c c¼0
2.5.2.2
ð2:34Þ
ð2:35Þ
Intrinsic viscosity and molecular weight
Within a homologous series of linear polymers, the higher the molecular weight the
greater the increase in viscosity for a given polymer concentration. This capacity to
enhance viscosity or intrinsic viscosity is a reflection of the polymer molecular weight.
In the late 1930s and 1940s, Mark, Houwink and Sakurada Mark achieved an empirical
relationship between the molecular weight and the intrinsic viscosity:
½η ¼ KM α ,
ð2:36Þ
where K and α are constants for a particular polymer–solvent pair at a particular
temperature.
2.5 Polymer solutions
37
This equation has become one of the most important relationships in polymer science
and has been widely used. It is possible to calculate the molecular weight from intrinsic
viscosity measurements as long as K and α have been established for a particular
temperature.
Strictly speaking, this equation covers only a narrow molecular weight range.
However, it is relatively easier in practice to use intrinsic viscosity to determine the
molecular weights. In order to use it for molecularly heterogeneous polymers, the
equation was modified as
α
½η ¼ KM v ,
ð2:37Þ
where M v , is the viscosity average molecular weight.
The quantity K is often give in terms of the universal constant ϕ,
K¼ϕ
2 3=2
r0
,
M
ð2:38Þ
where r 0 2 represents the mean square end-to-end distance of the unperturbed coil. In
terms of the number-average molecular weight, ϕ equals 2.5 1021 dl/mol cm3.
For theoretical purpose, the Mark–Houwink equation is usually expressed in the
form:
2 3=2
r0
M 1=2 α3 ¼ KM 1=2 α3 :
½ η ¼ ϕ
M
ð2:39Þ
By choosing a theta-solvent or θ temperature, the influence of the molecular expansion
due to intramolecular interactions can be eliminated. Under these conditions, α equals 1,
and the intrinsic viscosity depends only on the molecular weight. Thus Eq. (2.39) is
reduced to
½η ¼ KM 1=2 :
2.5.2.3
ð2:40Þ
Measurement of intrinsic viscosity
Based on the Hagen–Poiseuille law, the viscosity of a liquid or solution can be
measured by using a viscometer. Essentially, this involves the measurement of the flow
rate of the liquid through a capillary tube which is part of the viscometer. In most
experiments, dilute solutions of about 1% polymer are prepared. By measuring the flow
time of the solution, t, and that of the pure solvent, t0, the relative viscosity is
determined by
ηr ¼
η
t
¼ :
η0 t 0
ð2:41Þ
The quantity ηr should be about 1.6 for the highest concentration used.
Several concentrations are run and ploted according to Fig. 2.19. The most frequently
used instrument is the Ubbelhode viscometer, which equalizes the pressure above and
below the capillary.
38
Fundamentals of polymers
Fig. 2.19 The relationship between concentration and viscosity.
Two practical points must be noted: (1) both lines must extrapolate to the same
intercept at zero concentration and (2) the sum of the slopes of the two curves is related
through the Huggins equation
ηsp
¼ ½ η þ k 0 ½ η 2 c
c
ð2:42Þ
ln ηr
¼ ½η þ k 00 ½η 2 c:
c
ð2:43Þ
k 0 þ k00 ¼ 0:
ð2:44Þ
and the Kraemer equation
Algebraically
If either of these requirements is not met, molecular aggregation, ionic effects, or other
problems may be indicated. For many polymer–solvent systems, k0 is about 0.35 and
k00 is about 0.15, although significant variation is possible.
2.6
Fiber, plastic and elastomer
Fibers are a class of materials that are continuous filaments or are in discrete elongated
pieces. In order to form fibers, the polymer must have a relative molecular weight
exceeding 12 000 and possess the ability to crystallize. Closely packed, linear and
symmetric polymers without side groups are favored for fiber formation. Consequently,
fibers are characterized by high modulus, high tensile strength and moderate extensibilities (usually less than 20%) resulting from the strong sencondary molecular attractive forces. Elastomers generally refer to polymers with irregular molecular structure,
2.7 Fiber formation
39
weak intermolecular attractive forces and very flexible polymer chains. Therefore
elastomers usually have a low initial modulus in tension, but exhibit very high extensibility (up to 1000%). Plastics fall between the two structural extremes represented by
fibers and elastomers. However, the boundary between fibers and plastics may sometimes be blurred. Polymers such as polypropylene and polyamides can be used as fibers
as well as plastics by adopting proper processing conditions.
2.7
Fiber formation
The process by which fibers are formed from bulk polymer material is termed spinning.
The three fundamental techniques for fiber manufacturing are melt spinning, wet
spinning and dry spinning.
2.7.1
Melt spinning
Melt spinning is a technique commonly used for polymers that can melt without heavy
degradation. This technique is used to fabricate materials that require extremely high
cooling rates in order to form a fiber. The cooling rates achievable by melt spinning are
on the order of 104–107 C/s. Nylon, polyester, olefin and saran are the common
polymers that produced via this process. Melt spinning is a straightforward process
with high spinning speeds and is the least expensive fundamental production technique.
Fibers produced from melt spinning do not need solvents and washing, and take the
shape of the spinneret hole. Figure 2.20 illustrates the scheme of melt spinning.
Fig. 2.20 Schematic of melt spinning.
40
Fundamentals of polymers
Fig. 2.21 Schematic of wet spinning.
2.7.2
Wet spinning
Wet spinning, as shown in Fig. 2.21, is a fiber-forming technique whereby the polymer
solution is spun into a bath where the fiber coagulates and solidifies. It is the oldest and
the most complex process for producing fibers. The fibers formed by wet spinning are
weak until they are dry. The fibers obtained from wet spinning process require washing
and bleaching. Rayon and acrylic are spun via wet spinning.
2.7.3
Dry spinning
Dry spinning is a direct process that spins fibers from a solution into warm air or inert
gas, as shown in Fig. 2.22. The fibers solidify as the solvent evaporates. Synthetic
polymers such as acetate, orlon acrylic and vinyon can be spun into fibers by using this
technique.
2.7.4
Fiber properties
The properties of a fiber rest not only on the polymer it made of, but also on the
manufacturing process. The processing determines the range within which a property of
the fiber may fall. The physical changes taking place in processing may proceed in
various ways. Figure 2.23 illustrates how the polymer structure and processing parameters affect fiber properties.
2.7.4.1
Polymer structure and fiber mechanical properties
As one of the most important properties, fiber modulus has been intensively studied.
Sakurada and Kaji indicated that the modulus of a polymer depends primarily on the
chain conformation. A method for calculating the modulus by building a relationship
between stress and the deformation of crystalline lattice detected by X-ray diffraction
has also been established [11]. Table 2.7 collects the values of calculated and experimental moduli of elasticity for polymer crystals of fiber-forming polymers.
Early in 1932, Mark presented the relationship between fiber tensile strength and
molecular mass as
2.7 Fiber formation
41
Fig. 2.22 Schematic of dry spinning.
Fig. 2.23 Relationship between fiber properties and polymer structures, and processing [9].
σ ¼ σ∞
B
,
Mn
ð2:45Þ
where σ is fiber tenacity originally expressed in the weight titer (denier), σ∞ is a constant
equal to the fiber tenacity at infinite molecular mass, B is a constant depending on the
polymer and Mn is number average molecular mass.
42
Fundamentals of polymers
Table 2.7 Modulus of elasticity of polymer crystals [11]
Modulus of elasticity (GPa)
Polymer
Chain deformation motion
Impulse propagation
X-ray diffraction
Nylon 66
Polyethylene terephthalate (PET)
157
182
146
340
160–200
—
98
180
77.5–121
57
58.9–88.3
147
210
—
216
194
124.5
168
93.5
138
—
—
82.4
132.5
137
74.5
137
235
—
245
40.7
—
—
—
—
—
Polyethylene (PE)
Syn-polyvinyl chloride (PVC)
Polyvinyl alcohol (PVA)
Polyvinylidene chloride (PVDF)
Cellulose
Cellulose triacetate
Polyacrynitrile (PAN)
Regarding brittle materials, the tensile strength is assumed to result from the simultaneous fracture of all the load-bearing bonds in the cross-section where the fracture
takes place
σ ¼ nf ,
ð2:46Þ
where σ is strength, n is number of loading-bearing bonds per unit of cross-section area
and f is the strength of an individual bond.
Bueche proposed a formula correlating the maximum strength to polymer density and
molecular entanglement:
ρN 2=3
σ ∞ ¼ nf ¼
f at M ¼ ∞,
ð2:47Þ
3M e
where ρ is the density of polymer, N is Avogadro’s number, and Me is the molecular
mass critical in respect to the entanglements.
Turner further applied Flory’s end correction factor to Bueche’s equation:
ρN 2=3
2M e
σ ∞ ¼ nf ¼
:
ð2:48Þ
f 1
3M e
M
The modulus of a polymer has also been theoretically calculated from molecule
conformation. The example can be found in Chapter 5.
Chain imperfections including chain branching may cause either a decrease of
crystallizability, or a change of crystallization rate, which ultimately lower the
moduli of polymers and fibers. Cross-linking normally causes difficulties in fiber
formation, and subsequently impairs the structure perfection and the mechanical
properties of the fiber. However cross-links generated after fiber formation usually
play a positive role.
References
43
Fig. 2.24 Schematic effect of process conditions on fiber properties [9].
2.7.4.2
Processing and fiber properties
In view of the importance of the process parameters in determining fiber properties, the
effects of temperature, tension and time on fiber properties are often evaluated, as
shown in Fig. 2.24.
The process temperature influences the shrinkage, birefringence, elongation and
tenacity at break, by affecting the crystal structures of polymers. High temperature
results in high crystallinity, large crystals and dense molecular packing in the crystal,
but coarse structure and disorientation in the amorphous phases [9].
Tension affects the orientation and crystal structures of polymer. Increasing tension
leads to increased orientation and length of crystal, and reduction in chain back-folding.
Cold drawing is the principal approach for building up the tensile properties of fibers.
The drawability of a fiber is limited to the density of entanglements, the number of tie
molecules and the crystalline structure in the undrawn fiber. Low crystalline material is
easier to draw. In addition to the importance of the degree of crystallinity, the crystal
size determines, to a large extent, how many link molecules will be formed. With
increased link molecules, the strength and possible modulus increase. Besides, the
smaller the crystallites are, the easier they can be relocated, and the better the distribution of tensile force that can be obtained.
It should be understood that an increase of elongational strength and modulus is
accompanied by a decrease of tranverse strength [11]. The more stretches applied on the
link molecules, the smaller is the cohesive force between the stretched molecules, and
hence a decreased tranverse strength.
References
1. W. Findley, J. Lai, and K. Onaran. Creep and Relaxation of Nonlinear Viscoelastic Materials:
With an Introduction to Linear Viscoelasticity. Dover Publications, 1989.
2. J. Charles, E. Carraher, Seymour/Carraher’s Polymer Chemistry (sixth edition). New York:
Marcel Dekker, Inc., 2003.
44
Fundamentals of polymers
3. J. Burke, “Solubility parameters: theory and application,” in AIC Book and Paper Group
Annual, ed. C. Jensen, vol. 3, 1984.
4. D. J. Williams, Polymer Science and Engineering. Englewood Cliffs, New Jersey: PrenticeHall, Inc., 1971.
5. Polymer structure. Polymers and Liquid Crystals [cited 2010 June 25]; Available from: http://
plc.cwru.edu/tutorial/enhanced/files/polymers/struct/struct.htm.
6. R. O. Ebewele, Polymer Science and Technology. CRC, 2000.
7. W. M. D. Bryant, “Polythene fine structure.” Journal of Polymer Science, vol. 2(6), pp. 547–
564, 1947.
8. D. I. Bower, An Introduction to Polymer Physics. Cambridge University Press, 2002.
9. P. M. Latzke, “Testing and influencing the properties of man-made fibers,” in Synthetic
Fibers: Machines and Equipment, Manufacture, Properties, F. Fourné, Ed. Munich: Hanser
Gardner Publications, 1999.
10. L. Sperling, Introduction to Physical Polymer Science. Wiley-Interscience, 2006.
11. Z. Walczak, Processes of Fiber Formation. Amsterdam: Elsevier, 2002.
3
Nanofiber technology
3.1
Nanofiber-forming technology
There are a number of techniques capable of fabricating nanofibers. These techniques
include conjugate spinning, chemical vapor deposition, drawing, template synthesis,
self-assembly, meltblowing and electrospinning.
3.1.1
Conjugate spinning (island in the sea)
Sea–island-type conjugate spinning involves extruding two polymer components from
one spinning die. The fiber islands are arranged in a sea component which is later
removed by extraction. Nakata et al. reported that continuous PET nanofibers with a
diameter of 39 nm could be obtained by sea–island-type conjugate spinning from the
flow-drawn fiber with further drawing and removal of the sea component. Figure 3.1
shows a TEM image of PET fiber island and Nylon-6 sea produced by conjugate
spinning and flow-drawing [1].
3.1.2
Chemical vapor deposition (CVD)
In a CVD process, a substrate is exposed to one or more volatile precursors, which react
and/or decompose on the substrate surface. The desired deposit is synthesized on the
substrate surface. The volatile by-products are produced during the process and are
removed by gas flow through the reaction chamber. The various forms of material that
can be produced via CVD include monocrystalline, polycrystalline, amorphous, and
epitaxial. Some examples of such CVD-fabricated materials are silicon, carbon fiber,
carbon nanofibers, filaments and carbon nanotubes [2]. Figure 3.2 shows a schematic
illustration of a plasma-enhanced CVD setup that can be used for fabricating singlewalled carbon nanotubes.
The two most important CVD technologies are low pressure CVD (LPCVD) and
plasma enhanced CVD (PECVD). The LPCVD process produces layers with excellent
uniformity of thickness and material characteristics at relatively high deposition
temperatures (higher than 600 C) and slow deposition rates. The PECVD process can
operate at lower temperatures (down to 300 C), attributed to the extra energy supplied
to the gas molecules by the plasma in the reactor. However, the quality of the films tends
to be inferior. A schematic diagram of a typical LPCVD reactor is shown in Fig. 3.2.
45
46
Nanofiber technology
A
B
100 nm
Fig. 3.1 Cross-sectional TEM image of conjugated-spun and flow-drawn fibers [1]: “A” indicates a
PET fiber island, and “B” indicates the nylon-6 sea.
3.1.3
Phase separation (sol–gel process)
The phase separation process is a wet-chemical technique. In this process, the sol (or
solution) evolves gradually towards the formation of a gel-like network containing both
a liquid phase and a solid phase. The process consists of dissolution, gelation, solvent
extraction, freezing, and freeze-drying. The basic solid phase, a nanoscale porous foam,
is formed as a result. The sol–gel approach is a cheap and low-temperature technique
3.1 Nanofiber-forming technology
47
Fig. 3.2 A schematic illustration of a LPCVD setup.
that allows for the fine control of the product’s chemical composition. But transforming
the solid polymer into the nanoporous form takes a relatively long period of time using
the phase-separation process [3].
3.1.4
Drawing
Drawing is a process similar to dry spinning in the synthetic fiber industry, that can
make long, continuous single nanofibers. However, only a viscoelastic material that can
undergo strong deformations while being cohesive enough to support the stresses
developed during pulling can be made into nanofibers through drawing [4].
3.1.5
Template synthesis
The template synthesis method uses a nanoporous membrane as a template to make
nanofibers of a solid fibril or a hollow tubule. The materials that can be used to grow
tubules and fibrils by template synthesis include electronically conductive polymers,
metals, semiconductors and carbon. However, continuous nanofibers cannot be fabricated by this method [5]. Fig. 3.3 shows a schematic diagram of the template preparation
for MCo2O4 nanotubes use in the application of gas sensors by Zhang et al. [6].
3.1.6
Self-assembly
Self-assembly is a process whereby individual, pre-existing components organize
themselves into desired patterns and functions. A broad range of peptides and proteins
have been shown to produce very stable nanofiber structures. These nanofibers are very
well ordered and possess remarkable regularity and, in some cases, helical periodicity.
The diameter and the surface structure of the nanofibers can be altered by controlling of
the molecular structures [7]. The potential applications of these composite nanofibers
include electronics, optics, sensing and biomedical engineering. Figure 3.4 illustrats the
self-assembly of various types of peptide materials.
3.1.7
Meltblown technology
Meltblown technology involves the production of fibers in a single step, by extruding a
polymer melt through an orifice die and drawing down the extrudate with a jet of hot air
48
Nanofiber technology
Fig. 3.3 A schematic diagram showing the template preparation for MCo2O4 nanotubes [6].
(a)
5 nm
500 nm
1 cm
(b)
2 nm
100 nm
(c)
10 cm
2 mm
150 mm
4 nm
Gold particles
(d)
5 nm
5 nm 2.4 nm
a-helix
b-sheet
Fig. 3.4 Self-assembly of various peptide materials [7]: (a) the ionic self-complementary
peptide, (b) a type of surfactant-like peptide, (c) surface nanocoating peptide and (d) molecular
switch peptide.
3.2 Electrospinning process
49
Fig. 3.5 Detailed schematic of meltblowing process.
(typically at the same temperature as the molten polymer). The air exerts the drag force
that attenuates the melt extrudate into fibers, which are then collected a few feet away
from the die in the form of a nonwoven mat. Figure 3.5 shows a schematic illustration of
the melt blowing process.
3.1.8
Electrospinning
Electrospinning is a well-established process for producing ultra-fine fibers, first
patented in 1934. The process currently produces micron-scale fibers at a commercially
viable level using electrostatic forces to pull fibers from a capillary of polymer solution.
According to Deitzel et al., the technique can be considered a variation of the electrospray process [8]. The polymer solution consists of a predetermined mixture of polymer
suspended in solvent. A drop of polymer solution forms at the tip of the capillary due to
gravity and is held in place by surface tension. Formation of the fiber begins when the
electrostatic force is greater than the surface tension of the droplet. The fiber is formed
as the ejected jet stream is narrowed by whipping itself as a result of an increase in
surface charge density due to the evaporation of the solvent. Nanofiber fabrication by
electrospinning will be described in detail in the following section.
3.2
Electrospinning process
The electrospinning process involves electrostatic forces in the fiber-formation process.
This is different from conventional fiber spinning techniques (melt, dry or wet spinning)
that rely on mechanical forces to produce fibers by extruding the polymer melt or
solution through a spinneret and subsequently drawing the resulting filaments as they
solidify and coagulate. Electrospinning involves the application of an electric field
between a capillary tip and a grounded collector via a high-voltage source. A pendant
droplet of polymer solution at the capillary tip is transformed to a hemispherical shape
and then into a conical shape (known as a Taylor cone) by the electric field. When the
intensity of the electric field causes a larger effect than the surface tension of the
50
Nanofiber technology
Fig. 3.6 An electrospinning setup.
polymer solution, the solution is ejected towards the grounded metallic collector. If the
concentration of the solution is too low, the jet breaks up into droplets. However, when
the viscosity is high enough, as indicated by entanglements of the polymer chains, a
continuous jet is formed. A series of electrically induced bending instabilities in the
air results in stretching and elongation of the jet in a cone-shaped volume. Rapid
evaporation of the solvent during the elongation process reduces the fiber diameter.
The nonwoven mat collected on the grounded surface contains continuous fibers from
the microscale to the nanoscale. Figure 3.6 shows the schematic diagram of an typical
electrospinning setup.
3.3
Processing parameters
There are numerous parameters that can affect the transformation of polymer solutions
into nanofibers through electrospinning. These parameters include (a) governing variables
such as applied voltage at the spinneret, the tip-to-ground distance, the hydrostatic
pressure in the capillary tube, (b) ambient conditions such as solution temperature, air
flow, and humidity in the electrospinning chamber, and (c) the solution(spinning dope)
properties such as viscosity, conductivity, surface tension and elasticity [9]. Spinnability
of various polymers has been widely investigated. Fong et al. found that electrospinning
of PEO solutions (using the cosolvents of water and ethanol) that have viscosities between
1–20 poises and a surface tension in the range of 35–55 dynes/cm was fiber-formable
[10]. However, for electrospinning of CA in 2:1 acetone/DMAc, viscosities between 1.2
and 10.2 poises were fiber-formable [11]. These two cases show that the spinnable set of
conditions for different polymer solutions is unique. In the following sections, the
processing parameters of the electrospinning process are discussed in greater detail.
3.3.1
Spinning dope concentration and viscosity
One of the more significant parameters influencing the fiber diameter is the spinning
dope viscosity. A higher viscosity results in a larger fiber diameter [9, 10, 12]. It is to be
noted that the spinning dope viscosity is directly proportional to the polymer
3.3 Processing parameters
51
Fig. 3.7 Average fiber diameter of polyurethane fibers is proportional to the cube of the polymer
concentration [13].
Fig. 3.8 SEM images of electrospun PEO nanofibers from different solutions with different
viscosity [10].
concentration. Thus, an increase in polymer concentration will also imply an increase in
fiber diameter. Deitzel et al. demonstrated that the fiber diameter increased with
increasing polymer concentration according to a power law relationship [8]. Demir
et al. then showed that the fiber diameter was proportional to the cube of the polymer
concentration [13]. Figure 3.7 shows that the average diameter of polyurethane fibers is
proportional to the cube of the polymer concentration.
Fong et al. found that the fiber morphology was influenced by the polymer concentration, thus by the viscosity as well. It was observed that many beads form at low
viscosity, resulting in a “beads-on-the-strings” morphology. At high viscosity, beadless
fibers were obtained. Figure 3.8 shows the SEM images of electrospun PEO fibers
obtained from solutions ranging from low to high viscosity [10].
52
Nanofiber technology
Fig. 3.9 Jet diameter as a function of applied voltage [13].
3.3.2
Applied voltage
Applied voltage is another parameter that affects the fiber diameter. Generally, a higher
applied voltage leads to a higher volume of spinning dope ejection, resulting in a larger
fiber diameter. Figure 3.9 shows that the jet diameter increases with increasing applied
voltage [13].
3.3.3
Spinning dope temperature
Uniformity of the fiber diameter is also a challenge posed by the electrospinning
process. Demir et al. reported that polyurethane fiber diameters were more uniform
when electrospinning was conducted at a high temperature (70 C) compared to room
temperature. The mechanism behind the increased fiber diameter uniformity with
increasing spinning dope temperature is not fully understood. It was also noted that
the spinnability of the fibers increased with increasing spinning dope temperature. The
highest polymer concentration that could be electrospun at room temperature was 12.8
wt% (weight percent), whereas a 21.2 wt% polyurethane polymer solution could be
electrospun at a high temperature of 70 C [13]. Clearly, the spinnability of the polymer
solution increases with decreasing viscosity of the solution.
3.3.4
Surface tension
It was observed by Doshi and Reneker that beadless fibers could be obtained by
reducing the surface tension of a polymer solution [9]. However, this generalization is
3.3 Processing parameters
53
not always true, as shown by Liu and Hsieh in their work producing cellulose fibers by
electrospinning of cellulose acetate using dimethylacetamide (DMAc) and acetone as
the solvents. Acetone has a surface tension of 23.7 dyne/cm, whereas DMAc has a
surface tension of 32.4 dyne/cm. When using either DMAc or acetone alone as the
solvent, beads or beaded fibers were obtained. When a cosolvent of DMAc and acetone
was used, the surface tension ranging between 23.7 and 32.4 dyne/cm, beadless fibers
were observed [11].
3.3.5
Electrical conductivity
The addition of salts into a polymer solution can result in fewer beads and finer fibers. It
was explained that a higher charge density on the jet surface was obtained with the
addition of the salts, bringing more electrical charge to the jet. An increase in the charge
carried by the jet leads to higher elongation forces on the jet under the electric field,
resulting in fewer beads and finer fibers. Zong et al. reported that, as the salt content in a
polymer solution increased, fewer beads and finer fibers were observed [14]. Lee et al.
also found that using a solvent with a higher electrical conductivity would result in PCL
fibers with smaller diameters [15]. Figure 3.10 shows that the PCL diameter decreases
as solvent electrical conductivity increases.
3.3.6
Molecular weight of polymer
As molecular weight (MW) is proportional to the polymer chain length, a high MW
implies a high degree of polymer chain entanglement. The Berry number, an indication
of the degree of polymer chain entanglement, is a product of the intrinsic viscosity and
polymer concentration. A high Berry number indicates a high degree of polymer chain
entanglement. The Berry number has been found to correlate positively with the fiber
morphology and diameter, implying that a higher MW would result in a larger fiber
diameter and fewer beads. Koski et al. reported that PVA fiber diameter increases with
MW and Berry number. At low MW and Berry number (<9), the beaded morphology
Fig. 3.10 The PCL diameter decreases with increasing electrical conductivity [15].
54
Nanofiber technology
(a) 25wt%, 9000-10000g/mol (b) 25wt%, 13000-23000g/mol
(c) 25wt%, 31000-50000g/mol
Fig. 3.11 PVA fiber morphology of 25 wt% polymer concentration with a molecular weight of
9000–50 000 g/mol [16].
Fig. 3.12 Plot of jet diameter as a function of distance from the apex of the Taylor cone for 4 wt%
aqueous PEO solution spun at 10 kV [9].
and circular cross-section are evident, as shown in Fig. 3.11a, b. At high MW and Berry
number (>9), flat fibers with large diameters were obtained, as shown in Fig. 3.11c [16].
3.3.7
Spinning distance
The further the distance between the spinneret and the grounded metal screen, the
finer the fiber diameter will be, as the fibers have more time and distance to elongate
themselves in the instable zone. Doshi and Reneker reported that the jet diameter would
decrease with increasing distance from the apex of the Taylor cone, suggesting that the
fiber diameter would decrease with increasing distance from the spinneret. Figure 3.12
shows the plot of jet diameter as a function of distance from the apex of the Taylor cone
for 4 wt% aqueous PEO solution spun at 10 kV [9].
3.3 Processing parameters
55
Fig. 3.13 Effect of orifice diameter on the diameter of PLAGA nanofibers with a 16-gauge
needle (ID ¼ 1.19 mm), an 18-gauge needle (ID ¼ 0. 84mm), and a 20-gauge needle
(ID ¼ 0.58 mm) [18].
3.3.8
Spinning angle
Many spinning angles have been previously studied, with 0 , 45 and 90 being the
most common. There is little experimental evidence that spinning angle affects fiber
diameter. However, uniformity of the electrospun fibers increased at 45 because the
flow rate was often lower and gravity did not allow for formation of as many beads [17].
3.3.9
Orifice diameter
The smaller the orifice diameter, the smaller the fibers tend to be. Dhirendra et al.
demonstrated that a smaller orifice diameter results in PLAGA nanofibers with smaller
diameter [18]. Figure 3.13 shows that the PLAGA fiber diameter decreases with
decreasing orifice diameter.
3.3.10
Solvent boiling point
A low boiling point is a desirable characteristic in electrospinning applications because
it promotes the evaporation of the solvent under conventional atmospheric conditions.
This property promotes the deposition of polymer fibers in an essentially dry state[19].
However, a solvent with an exceedingly low boiling point leads to frequent clogging of
the spinneret, due to quick evaporation of the solvent. Wannatong et al. reported that
polystyrene fiber diameter decreases with increasing solvent boiling point[20]. During
its flight to the grounded target, an ejected charged jet is stretched to a much lower
diameter. At the same time, the solvent gradually evaporates, causing the viscoelastic
properties of the jet to change. As soon as the viscoelastic force exceeds the electrostatic
56
Nanofiber technology
Fig. 3.14 The relationship between polystyrene fiber diameter and solvent boiling point [20].
Fig. 3.15 Average polystyrene fiber diameter versus relative humidity (data adapted
from Ref. [21]).
force, the jet cannot be stretched much further. Based on this explanation, solutions of
PS in both toluene and THF (which have low boiling points of 111 C and 65–66 C,
respectively) would give fibers with larger diameters than those in m-cresol and DMF
(Fig. 3.14).
3.3.11
Humidity
Kim et al. found that the average polystyrene fiber diameter increases with increasing
relative humidity in the air contained by the electrospinning chamber[21] (Fig. 3.15).
The higher relative humidity makes a thicker fiber because the higher electrostatic
charge density on the fiber surface is able to split the fibers more.
High humidity affects the evaporation rate of solvent in the jet. Thus when a fiber
reaches the receiver, some solvent remains inside. This subsequently evaporates and
leaves the fiber a porous structure. Jeun et al. reported that the number of pores on the
3.3 Processing parameters
(a)
(b)
57
(c)
Fig. 3.16 Effect of humidity of the 2.5 wt% PLDLA solution on the fiber morphology (flow
rate ¼ 0.005 ml/min, electric voltage ¼ 20 kV) [22]: (a) 30%, (b) 50% and (c) 70%.
Fig. 3.17 PCL fiber diameter decreases with increasing dielectric constant of the solvent [15].
fiber surface increases with increasing humidity. The influence of relative humidity on
the morphology of electrospun poly(L-lactide-co-D, llactide) (PLDLA) fiber is shown
in Fig. 3.16. It can be observed that increasing the amount of humidity causes an
increase in the number of pores on the surface, the pore diameter and the pore size
distribution [22].
3.3.12
Dielectric constant
The dielectric constant is a material property that describes a material’s ability to store
charge when used as a capacitor dielectric. An increase in the dielectric constant means
an increase in the charge storage capacity of the material. Methylene chloride (MC) has
a dielectric constant of 9.1, whereas the dielectric constant of dimethyl formamide
(DMF) is 36.7. By increasing the composition of DMF in the solvent, the dielectric
constant of the solvent increases as a result. The fiber diameter of polycaprolactone
(PCL) was found to decrease as the solvent’s dielectric constant increases [15], as seen
in Fig. 3.17.
3.3.13
Feeding rate
Zong et al. reported that a lower solution feeding rate yielded smaller fibers with
spindle-like beads. At a higher feeding rate, larger fibers and beads were observed.
Since the droplet suspended at the spinneret tip is larger with a higher feeding rate, the
58
Nanofiber technology
Fig. 3.18 SEM images showing the variation of beaded fibers at different feeding rates[14]:
(a) 20 ml/min and (b) 75 ml/min [14].
jet carries the fluid away at a higher velocity. As a result, the electrospun fibers are
harder to dry before they reach the grounded target. Consequently, a higher feeding rate
results in large beads and junctions in the final membrane morphology [14]. Figure 3.18
shows the SEM images of fibers at a feeding rate of 20 ml/min and 75 ml/min, where (a)
shows smaller beads and finer fibers and (b) shows bigger beads and larger fibers.
3.4
Melt electrospinning
The two major types of electrospinning include solution electrospinning and melt
electrospinning. Solution electrospinning involves the use of a polymer-and-solvent
system in the electrospinning process, whereas the spinning dope of melt electrospinning contains only a polymer melt with no solvent. Solution electrospinning has
attracted much more attention than melt electrospinning because of its viability in
producing fibers in the nanorange. The decrease in fiber diameter is believed to be
caused by the evaporation of the solvent in solution electrospinning, as evidenced by
many studies that concluded that fiber diameters increase with decreasing solvent
concentrations, thus explaining the difficulty in producing nanofibers by using melt
electrospinning [17].
In 2003, Huang et al. reported that nearly 100 polymer solutions and only six
polymer melts had been successfully electrospun [23]. Dissolution and electrospinning
of the polymer solutions were conducted at room temperature under atmospheric
conditions. However, electrospinning of polymer melts had to be carried out in a
vacuum condition and at a high temperature (200–290 C) [23]. Given such demanding
processing conditions, it is understandable that more nanofibers have been electrospun
from polymer solutions than from polymer melts.
Nevertheless, one advantage of melt electrospinning is that cytotoxic solvents are not
needed to prepare the spinning dope, and as such the fibers produced by melt electrospinning can be used in biomedical applications such as 3-D tissue scaffolds. In 2006,
Dalton et al. electrospun polymer melts directly onto fibroblast cells to form layered
scaffolds for tissue engineering [24]. The use of melt electrospinning eliminates the
possibility of introducing cytotoxic solvents into cell culture during fiber deposition.
3.5 Applications of nanofibers
3.5
59
Applications of nanofibers
Owing to their high surface area to volume ratio, nanofibers have the potential to
improve significantly current technology and to generate applications in new areas.
Potential applications for nanofibers include reinforcement fibers in composites, protective clothing, filtration, biomedical devices, electrical and optical applications, and
nanosensors.
3.5.1
Reinforcement fibers in composites
Traditional engineering fibers (such as Kevlar, carbon and glass) have been widely used
as reinforcements in composite designs. With reinforcement fibers, the resulting composite materials show superior structural properties including high modulus and
strength-to-weight ratios. It has been shown that nanosized materials (carbon nanotubes
[25, 26], cellulose nanofibrils [27]) have superior mechanical strength compared to their
larger counterparts. Undoubtedly, superior structural properties resulting from the
reinforcement with nanofibers can be expected. In 2004, Ko et al. demonstrated the
feasibility of producing recombinant spider silk fibers by co-electrospinning carbon
nanotubes (CNT) with the spider silk protein extracted from the milk produced by
Nexia’s transgenic goats [28]. The nanoeffects of the CNT were successfully transferred
to spider silk fibers, as it was reported that the Young’s modulus of the CNT-reinforced
spider silk fibers increased by up to 460%. However, this was achieved at the expense of
strength and strain to failure. Kim and Reneker investigated the reinforcing effect of
electrospun nanofibers of polybenzimidazole (PBI) in both epoxy and rubber matrices
[29]. It was found that, with increasing fiber content, the bending Young’s modulus and
the fracture toughness of the epoxy nanocomposite was only marginally increased,
whereas the fracture energy increased significantly. For the rubber nanocomposite,
however, the Young’s modulus was ten times higher and the tear strength was twice
as large as that of the unfilled rubber material. These two studies show that nanofibers
have huge potential in many structural applications based on their ability to improve
mechanical performances.
3.5.2
Protective clothing
Because of the superior mechanical properties of nanofibers, incorporating them into the
design of protective gear such as bullet proof vests and safety helmets holds promise for
improving performance and significantly reducing weight. Other protective clothing
applications of nanofibers rely on the small pore sizes of nonwoven nanofiber mats and
a high surface area. The small pore sizes of nonwoven fiber mats provide a good
resistance to penetration by chemical harm agents in aerosol form, and could thus
effectively protect users from nuclear, biological and chemical warfare [30, 31].
Because of their great surface area, nanofiber fabrics are capable of neutralizing
chemical agents without impeding their air or water vapor permeability [32].
60
Nanofiber technology
Fig. 3.19 The efficiency of a filter increases with decreasing fiber diameter [23].
Preliminary investigations have indicated that, compared to conventional textiles,
electrospun nanofibers present both minimal impedance to moisture vapor diffusion
and extreme efficiency in trapping aerosol particles [33].
3.5.3
Filtration
Filtration is widely used in many engineering fields. The future filtration market has
been estimated to be up to US $700 billion by 2020 [34]. Filter media are used to
produce clean compressed air in industry. These media are required to capture oil
droplets as small as 0.3 micron. The advantages of using fiber-based filter media include
high filtration efficiency and low air resistance [35]. Filtration efficiency, which is
closely related to fiber diameter, is one of the most critical factors for filter performance,
as shown in Fig. 3.19. Electrospinning is found to be able to fabricate fiber media for the
removal of submicron foreign particles. Since the channels and structural elements of a
filter must be comparable in size with the particles or droplets that are to be captured in
the filter, one direct way of developing highly efficient and effective filter media is by
using nanometer-sized fibers in the filter structure [36]. In general, due to the very high
surface-area-to-volume ratio and resulting high surface cohesion, tiny particles of the
order of <0.5 mm can be easily trapped in electrospun nanofibrous structured filters and
hence the filtration efficiency can be improved. Freudenberg Nonwovens, a major world
manufacturer of electrospun products, has been producing electrospun filter media from
a continuous web feed for ultra-high efficiency filtration markets for more than 20 years
[37, 38]. This is perhaps one of the earliest commercial businesses relevant to electrospinning. Recently, a US patent[39] has disclosed a method for making a dust filter bag,
constituting a plurality of layers, including a carrier material layer and a nanofiber
nonwoven tissue layer. Polymer nanofibers can also be electrostatically charged to
modify the electrostatic attraction of particles without an increase in pressure drop to
further improve filtration efficiency. In this regard, the electrospinning process has been
shown to integrate the spinning and charging of polymers into nanofibers in one step
[35, 40]. In addition to fulfilling the more traditional purpose of filtration, nanofiber
membranes fabricated from some specific polymers or coated with some selective
agents can also be used, for example, as molecular filters. Such filters can be applicable
for the detection and filtration of chemical and biological weapon agents [41].
3.5 Applications of nanofibers
3.5.4
61
Biomedical devices
From the perspective of nature, nearly all of the fundamental building blocks of human
tissues and organs, including dentin, collagen, bone, cartilage and skin, exist in the form
of nanofibers. These biological fibers are characterized by well-organized hierarchical
fibrous structures realigning at the nanoscale. As such, bioengineering has become the
major area of applications for electrospun nanofibers.
3.5.4.1
Wound dressing
In 2006, Dalton et al. electrospun polymer melts directly onto fibroblast cells to form
layered scaffolds for tissue engineering [24]. The use of melt electrospinning eliminates
any possibility of introducing cytotoxic solvents into the cell culture during fiber
deposition. With the aid of an electric field, biodegradable polymer fibers can be
electrospun onto the burned location to form a protective fibrous membrane, which
would aid wound healing by stimulating skin cell growth into the 3-D scaffolds and
eliminate the formation of scar tissue [32, 42]. Nonwoven fibrous membranes (produced
via aerosol particle capturing mechanisms) for wound dressing have pore sizes ranging
from 500 nm to 1 μm, which are sufficiently small for protecting a wound from bacterial
penetration. The high surface area of nanofiber mat is also very efficient for fluid
absorption and dermal delivery [43].
3.5.4.2
Medical prostheses
Soft tissue prosthesis applications such as blood vessels using nanofibers fabricated by
electrospinning have been proposed. Furthermore, biocompatible polymer nanofibers
can also be used as a coating film on an artificial biomedical device such as a hip
implant to serve as an interfacial layer between the prosthetic device and the host
tissues. The nanofiber coating film is expected to reduce the stiffness mismatch and
improve nutrient exchange between the device and host tissues. The use of nanofiber
coating films will reduce the chance of device failure after the implantation [44, 45].
3.5.4.3
Tissue scaffolds
Mimicking the structure and biological functions of the extracellular matrix has been one
of the challenges in tissue engineering with respect to the design of ideal scaffolds for the
treatment of damaged tissues and malfunctioning organs in the human body. Nanofiber
scaffolds can serve as a host for cells to seed, migrate and grow. Biocompatible 3-D
scaffolds for cell in-growth to achieve tissue repair and replacement are of particular
interest in tissue engineering. There is a growing interest in synthetic biopolymer
(proteins, starch) and biodegradable polymer (polycaprolactone, poly(lactic acid)) nanofibers, which are capable of forming networks that mimic native tissue structures [46, 47].
3.5.4.4
Controlled drug delivery
Drug delivery with polymer nanofibers is based on the principle that the dissolution rate
of a drug increases with increasing surface area of both the drug and the coating.
Kenawy and Abdel-Fattah investigated delivery of tetracycline hydrochloride based
62
Nanofiber technology
on the fibrous delivery matrices of poly(ethylene-co-vinylacetate) and poly(lactic acid),
and their blend [48]. As the drug and carrier materials can be mixed together for
electrospinning of nanofibers, the likely modes of the drug in the resulting nanostructured products are: (1) the drug as particles attached to the surface of the nanofiber-form
carrier, (2) both drug and carrier in nanofiber-form, with the end product being the two
kinds of nanofibers interlaced together, (3) a blend of drug and carrier materials
integrated into one kind of fiber containing both components, and (4) the carrier
material is electrospun into a tubular form in which the drug particles are encapsulated.
As drug delivery in the form of nanofibers is still in the early stages of exploration, a real
delivery mode, regarding production and efficiency, has yet to be determined.
3.5.5
Electrical and optical applications
The rate of electrochemical reaction is proportional to the surface area of the electrode.
Because of the high specific surface area of nanofibers, using nanofibers in electronic
devices such as sensors and actuators will potentially reduce the response time to a
stimulus. Conductive nanofibers have the potential for many applications, including
electrostatic dissipation, corrosion protection, electromagnetic interference shielding
and photovoltaic devices[49].
3.5.6
Nanosensors
The high surface-area-to-volume ratio of nanofibers, which improves sensitivity, makes
them ideal for sensor applications. Nanofibers from polymers with the piezoelectric effect,
such as polyvinylidene fluoride, will render the resulting nanofibrous devices piezoelectric
[50]. PLAGA nanofibers have been used as a new sensing interface for fabricating
chemical and biochemical sensors [51]. The sensitivities of the nanofiber sensors are
reported to be three orders of magnitude higher than those obtained from thin film sensors.
References
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15. K. Lee, et al., “Characterization of nano-structured poly (ε-caprolactone) nonwoven mats via
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pp. 232–238, 2002.
20. L. Wannatong, A. Sirivat, and P. Supaphol, “Effects of solvents on electrospun polymeric fibers:
preliminary study on polystyrene,” Polymer International, vol. 53(11), pp. 1851–1859, 2004.
21. G. Kim, et al., “Effect of humidity on the microstructures of electrospun polystyrene
nanofibers,” Microscopy and Microanalysis, vol. 10(S02), pp. 554–555, 2004.
22. J. Jeun, et al., “Electrospinning of Poly (L-lactide-co-D, L-lactide),” Journal of Industrial
and Engineering Chemistry, vol. 13(4), pp. 592–596, 2007.
23. Z. -M. Huang, et al., “A review on polymer nanofibers by electrospinning and their applications in nanocomposites,” Composites Science and Technology, vol. 63(15), pp. 2223–2253,
2003.
24. P. Dalton, et al., “Direct in vitro electrospinning with polymer melts,” Biomacromolecules,
vol. 7(3), pp. 686–690, 2006.
25. L. Forro, et al., Electronic and Mechanical Properties of Carbon Nanotubes, in Science and
Application of Nanotubes, D. Tománek and R. J. Enbody, Ed. Springer–Verlag, 2002,
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26. M.-F. Yu, et al., “Strength and breaking mechanism of multiwalled carbon nanotubes under
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4
Modeling and simulation
A key objective in electrospinning is generating fibers of nanoscale diameter consistently and reproducibly. Considerable effort has been devoted to understanding how the
parameters affect the spinnability and more specifically the diameter of the fibers
resulting from the electrospinning process. Many processing parameters that influence
the spinnability and the physical properties of nanofibers have been identified. These
parameters include process parameters such as electric field strength, flow rate and
spinning distance, spinning dope properties including concentration, viscosity and
surface tension, etc., the spinning environment factors like humidity and the spinning
setup factors such as the diameter of the orifice and the electrospinning angle. Through
observation of the electrospinning process and analyzing these parameters, some
governing models have been built and simulations of the motion of jet have been
carried out. In this chapter, several main existing models and simulation works will
be introduced to help readers build up a brief concept.
4.1
Electrospinning mechanism
For a long period, the mechanism of electrospinning for forming nanoscaled fibers was
believed to be a result of a “split” as seen by the naked eye (Fig. 4.1a). The “splitting” is
explained by Doshi and Reneker [1, 2] that, as the jet diameter decreases, the surface
charge density increases, resulting in high repulsive forces which split the jet into
smaller jets splay. When a high-speed camera was used in the investigation of electrospinning jet, unstable bending, also known as “whipping” of jet was observed, and the
“bending instability” started being widely accepted as the electrospinning mechanism,
as shown in Fig. 4.1b. As described [3, 4], the electrospun jet vigorously bent spirally
and stretched inside a conical envelop resulting in a huge stretch ratio and a nanoscale
diameter.
4.2
Fundamentals of process modeling
Many approaches have been achieved in analysis, modeling and simulation of electrospinning processes. According to the analyzing objects, theoretical and experimental
research mainly focused on three parts: the Taylor cone, the jet and the whole process.
65
66
Modeling and simulation
Fig. 4.1 Images of electrospinning jet [3]. (a) Splitting captured at 16.7 camera exposure time;
(b) bending instability captured at 1.0 ms camera exposure time.
No matter how simple or complicated the models are, the two major principles
employed in the all the analyzing works are very basic and simple: Newton’s law for
an object’s force analysis, and conservation laws for system analysis.
4.2.1
Newton’s law
Newton’s first law of motion predicts the behavior of objects for which all existing
forces are balanced. The first law states that, if the forces acting upon an object are
balanced, then the acceleration of that object will be 0 m/s/s. An object will accelerate
only if there is a net or unbalanced force acting upon it. The presence of an unbalanced
force will accelerate an object – changing its speed, its direction, or both its speed and
direction. Newton’s second law of motion pertains to the behavior of objects for which
all existing forces are not balanced. The second law states that the acceleration of an
object is dependent upon two variables – the net force acting upon the object and the
mass of the object. As the force acting upon an object is increased, the acceleration of
the object is increased. As the mass of an object is increased, the acceleration of the
object is decreased.
Newton’s second law of motion can be formally stated as follows.
The acceleration of an object as produced by a net force is directly proportional to the
magnitude of the net force, in the same direction as the net force, and inversely proportional
to the mass of the object.
This statement can be expressed in equation form as follows:
a ¼ F=m,
ð4:1Þ
where a is the acceleration of the object with a mass of m, and F is the net force applied
on the object. The above equation is often rearranged to a more familiar form as shown
below. The net force is equated to the product of the mass times the acceleration:
F ¼ ma:
ð4:2Þ
4.3 Taylor cone
67
As we will see in the following sections, in analyzing the equilibrium status of the
Taylor cone, Newton’s first law is a very powerful tool. When calculating the jet
profiles, Newton’s second law is mainly used. And for the whole electrospinning
process analysis, Newton’s second law also plays a very important role, known as the
conservation of linear momentum.
4.2.2
Conservation laws
According to physics, a conservation law is the principle that certain quantities within an
isolated system do not change over time. When a substance in an isolated system changes
phase, the total amount of mass does not change. When energy is changed from one form
to another in an isolated system, there is no change in the total amount of energy. When a
transfer of momentum occurs in an isolated system, the total amount of momentum is
conserved. The same is true for electric charge in a system: charge lost by one particle is
gained by another. Conservation laws make it possible to predict the macroscopic
behavior of a system without having to consider the microscopic details of a physical
process or chemical reaction.
In modeling the electrospinning processes, conservation of mass, charge, momentum
and energy are the four elements to be analyzed.
4.3
Taylor cone
The Taylor cone is one of the main characters of electrospinning process, from which
the jets are ejected and formed in fibers. It is believed that through controlling the
Taylor cone the electrospun jet profile is controllable and, furthermore, the diameter
of electrospun nanofibers is adjustable. Therefore a great deal of theoretical and
experimental work has been done on the Taylor cone. Most of the obtained equations
were derived from analyzing the equilibrium status of the Taylor cone or, in another
words, the force balance analysis of the Taylor cone using Newton’s first law.
Electric forces and surface tension are the two forces that are considered to be the
essential factors.
In the 1960s, Taylor pioneered the theoretical and experimental research into electrically charged jets [5, 6]. He derived the condition for the critical electric potential
needed to transform the droplet of liquid into a cone, commonly referred to as the
Taylor cone, and for it to exist in equilibrium under the presence of both electric and
surface tension forces as
H2
2L 3
V 2c ¼ 4 2 ln ð0:117πγRÞ,
ð4:3Þ
R 2
L
where Vc is the critical voltage, H is the distance between the capillary exit and the
ground, L is length of the capillary with radius R, and γ is the surface tension of the
liquid. In this calculation only inviscid fluids were considered.
68
Modeling and simulation
A similar equation was found by Hendricks et al. [7]:
pffiffiffiffiffiffiffiffiffiffiffiffi
V ¼ 300 20πγr,
ð4:4Þ
where r is the radius of the pendant drop. Above this potential, the drop becomes fluid
dynamically unstable.
In 1995, Doshi and Reneker [1] investigated the electrospinning process, and calculated the electric field strength at the apex of the cone, as shown below:
rffiffiffiffiffiffiffi
4γ
E¼
:
ð4:5Þ
ε0 R
Hayati investigated the velocity profile inside the liquid cone at the base of an electrically driven jet both numerically and experimentally [8, 9] and calculated the electric
stress on the surface of the cone as
τ t ¼ ε0 E n E t ,
ð4:6Þ
where ε0 is the permittivity of free space, and En and Et are respectively the normal and
tangential electric field subjected to the jet surface.
4.4
Jet profile
Since the most important product parameter in electrospinning is the diameter of
nanofibers, there has been considerable interest in understanding the jet profile of
electrically charged jets. Investigation of the behavior of thin liquid jets in electric
fields can be traced back to the work of Rayleigh [10, 11] in 1882, later studies of
Zeleny [12], and today’s studies of electrospun jet. Studies of electrically charged
jets are mainly based on Newton’s second law by considering how the forces such as
the solution viscosity, flow rate and electronic force accelerate a jet as it flows
downward.
Baumgarten was the first to systematically investigate the electrospinning process
[13]. Using microflash photographs, he observed that “fiber loops shot out radially from
the jet” for the first time. After a series of studies of the effects of solution viscosity,
flow rate, voltage and gap, Baumgarten calculated the radius of the jet as
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
r / 4εVm=ρ,
ð4:7Þ
where r is the radius of the jet, m/ρ is the flow rate and V is the voltage.
By assuming all electric energy is transformed into kinetic energy and neglecting the
effect of viscosity, the acceleration of the jet was obtained [13] as
a¼
F 4εV 2
,
m ρy2 r
ð4:8Þ
where y is the axial distance from the center of the hemisphere. The boundary conditions for this equation are v ¼ 0 and y ¼ r at t ¼ 0.
4.5 Models
69
Fridrikh et al. [14] obtained the terminal jet radius rt as follows:
rt ¼
γε
Q2
2
I 2 π ð2ln χ 3Þ
1=3
,
ð4:9Þ
where γ is the surface tension, ε is the dielectric permittivity, Q is the flow rate, I is the
electric current, and χ R/h is the dimensionless wavelength of the instability responsible for the normal displacements.
e ¼ R=R0 and axial coordinate ez ¼ z=z0 ,
By introducing the dimensionless jet radius R
3
2 4
where z0 ¼ ρQ =2π R0 E 0 I, and the characteristic jet radius, R0, is taken as equal to the
radius of the capillary orifice, Spivak and Dzenis [15] came up with a dimensionless
equation for the jet radius:
d e 4
1 1 d e 2 m
1
2
e
e
R þ W eR
R
¼ 1:
ð4:10Þ
γR
d ez
Re 2 d ez
The dimensionless parameters in this equation are defined as follows. The Weber
number, W e ¼ 2π 2 R30 σ s = ρQ2 , describes the ratio of the surface tension forces to
the inertia forces. The parameter γ ¼ π 2 I 2 R60 =4ε0 ρQ4 describes the ratio of the
electric forces to the inertia forces. The effective Reynolds number for the fluid is
characterized by the power-law relationship, known as the Oswald–deWaele law,
h 2 im 1=2
^γ , where µ is a constant, ^γ is state of strain tensor, m is flow
^τ c ¼ μ tr ^γ
m
describes the ratio of the inertia
index. Re ¼ Q2 ρ = 2π 2 R40 ϖ 4πE 0 IR2 = Q2 ρ
forces to the viscous forces.
Rutledge et al. [16] found that the jet behavior near the nozzle is sensitive to
operating conditions, jet geometry and fluid properties. However, far from the nozzle,
the scaling for the radius of the jet r is determined mainly by operating conditions:
r¼
4.5
dρQ3 e 1=4
z
b2π 2 IE∞ c
1=4
:
ð4:11Þ
Models
The foundational axioms of fluid dynamics are the conservation laws. In electrospinning, the pendant droplet of the polymer solution at the capillary tip is deformed into a
Taylor cone under the influence of an electrostatic field. When the voltage overcomes
the surface tension, a fine charged jet is ejected and moves towards the receiver. Thus,
apart from the three common conservation relationships: the mass conservation, the
energy conservation and the momentum conservation, there is another very important
conservation relation in electrospinning: the charge conservation due to the applied
electric field. The influence factors involved in electrospinning are very complicated,
and include hydrostatic pressure, flow rate, electric field strength, viscosity and conductivity of polymer solutions, and even temperature gradient effects. For a thorough
understanding of the electrospinning process, enormous efforts have been involved in
70
Modeling and simulation
Fig. 4.2 Conservations in electrospinning process.
Fig. 4.3 Mass conservation.
modeling this process and building models. Figure 4.2 shows the conservations occuring in the electrospinning process.
4.5.1
One-dimensional model
For a simple and quick analysis, the one-dimensional (1-D) steady governing model for
electrospinning jet can be written as follows [4, 17–23].
Assuming the jet is steady, as shown in Fig. 4.3, according to the law of conservation
of mass, the mass (flow rate Q) flows in the jet channel will equals the mass flows out.
Denote the instantaneous flow velocity by v and the radius of jet by R. Then, we write
the conservation equation of mass as
πR2 ρv ¼ Q:
ð4:12Þ
For the conservation of charges, the equation is usually in the form of current I. The
current flowing through the jet channel at a given time is considered composed of two
part: the current generated by the motion of surface charges, and the current generated
by the motion of body charges. (Fig. 4.4)
4.5 Models
71
Fig. 4.4 Charges distributed on the surface of the jet (a) and in the body of jet (b).
Fig. 4.5 Hydrostatic pressure on fluid segment.
πR2 KE þ 2πRvσ ¼ 1,
ð4:13Þ
where E is the intensity of applied electric field and σ is the density of surface charge.
In linear momentum analysis, the internal pressure p, the electric force, the viscous
force τ and the body force g are considered, as shown in Fig. 4.5.
The conversion of linear momentum is written as
d v2
dp 2σE dτ
¼
ρ
þ
þ þ ρg:
ð4:14Þ
dz 2
dz
r
dz
72
Modeling and simulation
4.5.2
Three-dimensional models
According to the same rules, 3-D models for the electrospinning process have also been
established respectively by Spivak and Dzenis, Rutledge’s group and Wan et al.
4.5.2.1
Spivak–Dzenis model
In this model, the conservation of mass and charge are simply written as below [15, 24]:
(1)
(2)
conservation of mass
r v ¼ 0,
ð4:15Þ
r J ¼ 0:
ð4:16Þ
ρðv rÞv ¼ rT m þ rT e :
ð4:17Þ
conservation of electric charge
Regarding linear momentum analysis, only viscous and electric forces are
considered:
The right-hand side of the linear momentum balance equation is the sum of viscous and
electric forces. Rheologic behavior of polymer fluids is described by the power-law
constitutive equation, known as the Oswald–deWaele model:
T m ¼ μ II γ_
m 1
2
γ_
pI,
ð4:18Þ
_ m is
where µ is viscosity, II γ_ is the second invariant of the rate of strain tensor γ,
the flow index, p is hydrostatic pressure, and I is the unit tensor. Viscous Newtonian
fluids are described by m ¼ 1, pseudoplastic (shear thinning) fluids are described by
0 < m < 1, and dilatant (shear thickening) fluids are described by m > 1.
The electric stress tensor T e is the Maxwell tensor:
! !!
!
!
E E
e
:
ð4:19Þ
T ¼ ε E⊗ E
2
4.5.2.2
Rutledge’s model
Assuming that the jet is a long, slender object and substituting leading order terms of a
perturbative expansion in the aspect ratio, Rutledge’s group [22, 25] derived another set
of equations for electrospinning process:
(1)
(2)
(3)
conservation of mass:
qt πh2 þ qz πh2 v ¼ 0,
ð4:20Þ
qt ð2πhσ Þ þ qz 2πhσv þ πh2 KE ¼ 0,
ð4:21Þ
conservation of charge:
momentum balance, or the Navier–Strokes equation :
2
v
1
2σE 3v
¼
qz p þ g þ
þ 2 qz h2 qz v :
qt v þ qz
2
ρ
ρh
h
ð4:22Þ
4.5 Models
73
By introducing non-dimensional parameters, they obtained non-dimensionalized equations for the above equations:
2
0
qt h þ h2 v ¼ 0,
ð4:23Þ
qt v þ vv0 ¼
0
@1
h
0
k*
qt ðσhÞ þ σhv þ h2 E ¼ 0,
2
10
E2
2σE
qv*2
00
0
2πσ 2 A þ pffiffiffi þ g* þ 2 h2 v0 ,
h
8π
βh
h
ð4:24Þ
ð4:25Þ
where h is the radius of the jet at axial coordinate z, ν is the axial velocity of the jet and
is constant across the jet cross-section to leading order, σ is the surface charge density
and E is the electric field in the axial direction. The prime (0 ) denotes differentiation with
respect to z.
The non-dimensional equations are made bypchoosing
ffiffiffiffiffiffiffiffiffiffiffi a length scale r0, related to
the diameter of the capillary; a time scale t 0 ¼ ρr30 =γ, where γ p
is ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
the surfaceffi tension
and ρ is the density of the fluid; an electric field strength E 0 ¼ γ=ðε pεÞr
0 , ffiwhere
ffiffiffiffiffiffiffiffiffiffi
ε=ε is the permittivity of the fluid(air); and a surface charge density γε=r 0 . The
dimensionless asymptotic field is Ω0 ¼ E∞/E0. The material properties of the fluid are
characterizedpby
four dimensionless parameters: β ¼ ε=ε 1; the dimensionless visffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
cosity v* ¼ v2 =ργrp
the dimensionless gravity g* ¼ gρr 20 =γ; and the dimensionless
0 ; ffiffiffiffiffiffiffiffiffiffiffiffiffi
*
conductivity K ¼ K ρr30 =βγ.
4.5.2.3
Wan–Guo–Pan model
Upon the application of a very high electric field, molecular polarization induced by the
electric field is likely. And, accordingly, the current and momentum of jet will be
different (Fig. 4.6). Thus a more integral model was established [21]:
qqe
þ r J ¼ 0,
qt
ð4:26Þ
Fig. 4.6 Electric field induced molecular polarization: (a) unoriented molecular chains and
(b) polarized oriented molecular chains.
74
Modeling and simulation
ρ
Dv
¼ r t þ ρf þ qe E þ ðrE Þ P,
Dt
ð4:27Þ
DP
DT
¼ Qh þ r q þ JE þ E
,
Dt
Dt
ð4:28Þ
ρcp
where qqe/qt, (rE)P and E DP/Dt are respectively the current, force and energy
generated by the polarized molecular chains.
This set of conservation laws can constitute a closed system when it is supplemented
by appropriate constitutive equations for the field variables such as polarization. The
most general theory of constitutive equations determining the polarization, electric
conduction current, heat flux and Cauchy stress tensor has been developed by Eringen
and Maugin:
P ¼ εp E,
ð4:29Þ
J ¼ KE þ σv þ σ T rT,
ð4:30Þ
q ¼ κrT þ κE ,
ð4:31Þ
t¼
e
p ¼I þ η rv þ ðrvÞt ,
ð4:32Þ
where εp, K, μm, σ, σT, κ, κE and η are coefficients that relate to the material properties
and depend only on temperature in the case of an incompressible fluid.
4.6
Application of models in parametric analysis
By simplifying and analyzing models, the relationships between process parameters are
predictable, as well as the relationship between jet diameters and process parameters.
The most systematic work on parameter relationship analysis and parameter prediction
has been done by He and Wan based on the one-dimensional model [23, 26–28].
Fig. 4.7 Three stages of electrospun jet.
4.7 Computer simulation
75
As shown in Fig. 4.7, the jet was divided into three stages: the initial, unstable and
final stages. Each stage was investigated by analyzing the 1-D model.
At the initial stage of the electrospinning, the jet is stable and straight, and the
electrical force is dominant over all the other forces on the jet. So Eq. (4.14) may be
rewritten as:
d v2
2σE
:
ð4:33Þ
¼
dz 2
ρr
From Eqs. (4.1), (4.11) and (4.36), the scaling law for the jet diameter was obtained as
r/z
1=2
:
ð4:34Þ
During the unstable stage, the electric force and viscous force reach a balance, thus
2σE dτ
þ 0:
ρr
dz
Assume the gradient of pressure is constant, then
d v2
¼ constant:
dz 2
ð4:35Þ
ð4:36Þ
we can rewrite it as
d
r
dz
4
/ r0 ,
ð4:37Þ
which shows the scaling law for jet diameter in the unstable stage is
r/z
1=4
:
ð4:38Þ
If the distance approaches infinity, the acceleration in the axial direction vanishes, thus
d v2
! 0,
ð4:39Þ
dz 2
which leads to the scaling law for the jet diameter in the final stage:
r / z0 :
ð4:40Þ
The scaling laws satisfy the data obtained by Shin et al. [22] very well (Fig. 4.8).
4.7
Computer simulation
Modeling the jet by a system of connected viscoelastic dumbbells and analyzing the
momentum balance of these dumbbells, Reneker and Yarin studied the jet motion
through computer simulation [29].
In this model [29], they took electrical forces and surface tension effects into
consideration, but neglected the Earth’s gravity and aerodynamic forces. The segment
76
Modeling and simulation
Fig. 4.8 Relationship between the jet radius and the axial distance (data adapted from Ref. [22]).
Fig. 4.9 Viscoelatic dumbbell representing a segment of the rectilinear part of the jet [29].
of the jet was modeled as a viscoelastic dumbbell, as shown in Fig. 4.9. The dumbbell
comprises two beads, A and B, each possessing a charge e and mass m. The position
of bead A is fixed and bead B is acting by Coulomb repulsive forces from bead
A, e2/l2 and the force due to the external field eV0/h. The dumbbell, AB, models a
viscoelastic Maxwellian liquid jet. Therefore the stress, σ, pulling B back to A is
given by
4.7 Computer simulation
77
Fig. 4.10 Elctrospun jet modeled by a system of beads connected by viscoelastic elements [29].
dσ
dl
¼G
dt
ldt
G
σ,
μ
ð4:41Þ
where t is time, G and µ are the elastic modulus and viscosity, respectively, and l is the
filament length.
The momentum balance for bead B is
m
dv
¼
dt
e2
l2
eV 0
þ πr2 ,
h
ð4:42Þ
where r is the cross-sectional radius of the filament, and v is the velocity of bead B that
satisfies the kinematics equation
dl
dt
v:
ð4:43Þ
The electrospun jets was then modeled by a system of beads possessing charge e and
mass m connected by viscoelastic elements [29], as shown in Fig. 4.10, which generalizes the models of Fig. 4.9 and Fig. 4.11.
The parameters corresponding to the element connecting bead i with bead (iþ1) are
denoted by subscript u (up), those for the element connecting bead i with (i 1) by
subscript d (down). The rates of strain of the elements are given by (dlui/dt)/lui and (dldi/
dt)/ldi. The viscoelastic forces acting along the elements are similar to Eq. (4.41):
Modeling and simulation
120
100
80
60
40
4
0
0
–4
–20
2
1
0
–1
Y (dimensionless)
–2
ensio
nless
)
20
m
X (di
Z (dimensionless)
78
–3
Fig. 4.11 Temporal growth of the bending instability along the straight segment of a charged jet
subject to a small perturbation that is initially periodic in space [3].
dσ ui
1 dlui
¼G
dt
lui dt
G
σ ui ,
μ
ð4:44aÞ
dσ di
1G
σ di :
¼G
ldi μ
dt
ð4:44bÞ
According to Newton’s second law, the equation governing the radius vector of the
position of the ith bead ri ¼ ixi þ jyi þ kzi in the following form:
m
X e2
d2 ri
¼
R3ij r i
dt 2
j¼1, N
rj
J6¼1
¶ r 2di σ di
ðr i
ldi
ri 1 Þ
e
V0
πr 2 σ ui
k þ ui ðr iþ1
lui
h
aπ ðr2 Þrv ki
ðx2i þ y2i Þ
1=2
:
ri Þ
ð4:45Þ
For the first bead, i¼1, and N, the total number of beads, is also 1. As more beads are
added, N becomes larger and the first bead i ¼ 1 remains at the bottom end of the
growing jet. For this bead, all the parameters with subscript d should be set equal to zero
since there are no beads below i ¼ 1.
References
79
To consider the temporal instability of an established jet, the calculation began from a
long rectilinear filament 0 z h containing a fixed number of beads [29]. The filament was
perturbed by moving it laterally, at t ¼ 0, everywhere along its axis by the functions [29]
2π h z
3
x ¼ 10 L cos
z
λ
h
ð4:46Þ
2π h z
z
,
y ¼ 10 3 L sin
λ
h
where λ is the wavelength of the perturbation. Then the temporal evolution of the path
was calculated.
In all cases, the system of Eqs. (4.44) and (4.45) was solved numerically, assuming
the stresses σui and σdi and the radial velocity dri/dt were zero at t ¼ 0. The calculation
shows that small perturbations increased dramatically as the Earnshaw-like instability
grew. As the calculation progresses, the beads move further and further apart. The graph
becomes quite irregular when the separation between the beads is larger than the radius
of the spiral path because the beads are connected by straight lines in this model [29].
The simulation results in reasonable agreement with the experimental evidence and
shows that viscoelastic forces along the jet and the surface tension tend to stabilize the
charged jet (Fig. 4.11).
References
1. J. Doshi, and D. Reneker, “Electrospinning process and applications of electrospun fibers,”
Journal of Electrostatics, vol. 35(2), pp. 151–160, 1995.
2. X. Fang, and D. Reneker, “DNA fibers by electrospinning,” Journal of macromolecular
science. Physics, vol. 36(2), pp. 169–173, 1997.
3. D. H. Reneker, et al., “Bending instability of electrically charged liquid jets of polymer
solutions in electrospinning,” Journal of Applied Physics, vol. 87, p. 4531, 2000.
4. J. J. Feng, “The stretching of an electrified non-Newtonian jet: a model for electrospinning,”
Physics of Fluids, vol. 14, p. 3912, 2002.
5. G. Taylor, “Disintegration of water drops in an electric field,” Proceedings of the
Royal Society of London. Series A. Mathematical and Physical Sciences, vol. 280(1382),
pp. 383–397, 1964.
6. G. Taylor, “Electrically driven jets,” Proceedings of the Royal Society of London. Series
A. Mathematical and Physical Sciences, vol. 313(1515), pp. 453–475, 1969.
7. C. Hendricks, et al., “Photomicrography of electrically sprayed heavy particles,” AIAA J,
vol. 2(4), pp. 733–737, 1964.
8. I. Hayati, A. I. Bailey, and T. F. Tadros, “Investigation into the mechanisms of electrohydrodynamic spraying of liquids,” Journal of Colloid and Interface Science, vol. 117(1),
p. 205, 1987.
9. I. Hayati, A. I. Bailey, and T. F. Tadros, “Mechanism of stable jet formation in electrohydrodynamic atomization,” Nature, vol. 319(6048), pp. 41–43, 1986.
10. Lord Rayleigh, Philosophical Magazne and Journal, vol. 44, p. 184, 1882.
11. Lord Rayleigh, Further Observations Upon Liquid Jets, 1882.
80
Modeling and simulation
12. J. Zeleny, “Instability of electrified liquid surfaces,” Physical Review, vol. 10 (Copyright (C)
2010 The American Physical Society), p. 1, 1917.
13. P. Baumgarten, “Electrostatic spinning of acrylic microfibers,” Journal of Colloid and
Interface Science, vol. 36(1), 1971.
14. S. V. Fridrikh, et al., “Controlling the fiber diameter during electrospinning,” Physical
Review Letters, vol. 90 (Copyright (C) 2010 The American Physical Society), p. 144 502,
2003.
15. A. F. Spivak, and Y. A. Dzenis, “Asymptotic decay of radius of a weakly conductive viscous
jet in an external electric field,” Applied Physics Letters, vol. 73(21), pp. 3067–3069, 1998.
16. G. Rutledge, et al., “Electrostatic spinning and properties of ultrafine fibers,” National Textile
Center Research Briefs-Materials Competency, 2003.
17. A. M. Gañán-Calvo, “On the theory of electrohydrodynamically driven capillary jets,”
Journal of Fluid Mechanics, vol. 335, pp. 165–188, 1997.
18. A. M. Gañán-Calvo, “Generation of steady liquid microthreads and micron-sized monodisperse sprays in gas streams,” Physical Review Letters, vol. 80 (Copyright (C) 2010 The
American Physical Society), p. 285, 1998.
19. A. M. Gañán-Calvo, Cone-jet analytical extension of Taylor’s electrostatic solution and the
asymptotic universal scaling laws in electrospraying, Physical Review Letters, vol. 79
(Copyright (C) 2010 The American Physical Society), p. 217, 1997.
20. J. J. Feng, “Stretching of a straight electrically charged viscoelastic jet,” Journal of NonNewtonian Fluid Mechanics, vol. 116(1), pp. 55–70, 2003.
21. Y. Q. Wan, Q. Guo, and N. Pan, “Thermo-electro-hydrodynamic model for electrospinning
process,” International Journal of Nonlinear Sciences and Numerical Simulation, vol. 5(1),
pp. 5–8, 2004.
22. Y. M. Shin, et al., “Experimental characterization of electrospinning: the electrically forced
jet and instabilities,” Polymer, vol. 42(25), pp. 09955–09967, 2001.
23. J. H. He, Y. Q. Wan, and J. Y. Yu, “Allometric scaling and instability in electrospinning,”
International Journal of Nonlinear Sciences and Numerical Simulation, vol. 5, pp. 243–252,
2004.
24. A. F. Spivak, Y. A. Dzenis, and D. H. Reneker, “A model of steady state jet in the
electrospinning process,” Mechanics Research Communications, vol. 27(1), pp. 37–42, 2000.
25. M. M. Hohman, et al., “Electrospinning and electrically forced jets. I. Stability theory,”
Physics of Fluids, vol. 13(8), pp. 2201–2220, 2001.
26. J. H. He, and Y. Q. Wan, “Allometric scaling for voltage and current in electrospinning,”
Polymer, vol. 45(19), pp. 6731–6734, 2004.
27. J. H. He, Y. Q. Wan, and J. Y. Yu, “Application of vibration technology to polymer
electrospinning,” International Journal of Nonlinear Sciences and Numerical Simulation,
vol. 5, pp. 253–262, 2004.
28. J. H. He, Y. Q. Wan, and J. Y. Yu, “Scaling law in electrospinning: relationship between
electric current and solution flow rate,” Polymer, vol. 46(8), pp. 2799–2801, 2005.
29. D. H. Reneker, et al., “Bending instability of electrically charged liquid jets of polymer
solutions in electrospinning,” Journal of Applied Physics, vol. 87(9), pp. 4531–4547, 2000.
5
Mechanical properties of fibers
and fiber assemblies
Polymer, metallic and ceramic materials in fibrous form are of fundamental importance
in materials engineering. Fibrous materials are the basic building blocks for the backbone of most natural and man-made engineering structures, ranging from the skeletal
structure of animals to advanced fiber-reinforced composites.
To fulfill themselves as textile materials, fibers and fiber assemblies possess some
unique properties. They are combinations of strength and toughness, they are flexible,
soft, porous (permeable to air, vapor and light, etc.), lightweight and mostly textured.
As an essential requirement to fiber and fiber assemblies, mechanical properties are
undoubtedly the most important properties that need to be characterized and investigated. In this chapter, we will look at the mechanical properties of fiber assemblies from
single fiber to fiber fabrics.
5.1
Structure of hierarchy of textile materials
Traditionally fibers are defined as soft materials with a length-to-diameter ratio above
103 and a diameter ranging from several to 100 microns. The emergence of nanofibers
broadens the span of fibers to the nanoscale world.
For engineering applications, fibers are usually employed in different forms such as
yarns/ropes, woven textiles and nowoven textiles. The structure hierarchy of textile
materials is shown in Fig. 5.1.
5.2
Size effect on mechanical properties
Similar to the Hall–Petch relationship, fiber diameter plays a very important role in
nanofiber strength and surface energy [2], as shown in Fig. 5.2:
τ ¼ τ0 þ
κτ
, when d < 100 nm,
dα
κE
E ¼ E0 þ β ,
d
ð5:1Þ
ð5:2Þ
where κτ and κE are the fitting parameters (material constants), τ0 and E0 are the strength
and surface energy of the bulk material, respectively; d is the fiber diameter and
0 < d < 100 nm; and α > 0 and β > 0 are scaling exponents.
81
82
Mechanical properties of fibers and fiber assemblies
Fig. 5.1 Structure hierarchy of textile materials [1].
Fig. 5.2 The relation between the radius and modulus of nanofiber [2].
5.3
Theoretical modulus of a fiber
Assuming that the total deformation is due to intramolecular dilation, the calculation of
the tensile modulus along the c-axis of the crystal will reduce to the calculation of the
effective force constant of a single molecule along its axis. The required information for
5.3 Theoretical modulus of a fiber
83
Fig. 5.3 Model for the calculation of the tensile modulus of a polymer chain (modified from
Ref. [3]).
this calculation comprises of the unit cell dimensions, bond lengths and angles and the
force constants for bond-length and bond-angle deformation.
Take polyethylene, for example, according to the procedure of Trealore [3]. Let the
bond lengths be l and the angle between successive bonds α, as shown in Fig. 5.3.
Forces F are applied at the two ends of n segments along the axis of the molecules. Let
the bonds make an angle θ with the direction of F. If the total length of the molecule is
L ¼ nl cos θ, where n is the number of carbon atoms, then the total deformation ΔL
under a force F is
ΔL ¼ nð cos θΔL
l sin ΔθÞ:
ð5:3Þ
The deformation of each bond, ΔL, is produced by the force F cos θ and is
given by
Δl ¼
F
cos θ,
kl
ð5:4Þ
where kl is the force constant governing the separation of two carbon atoms. The
1
deformation Δθ ¼ Δα/2, is produced by a torque of Fl sin θ acting around each
2
of the bond angles and is given by
Δθ ¼
1 Fl
sin θ,
4 kα
ð5:5Þ
where kα is the valence-angle deformation constant. The tensile modulus is defined
E¼
F=A
:
ΔL=L
ð5:6Þ
From Eqs. (5.3),(5.4) and (5.5),
ΔL
cos 2 θ l2 sin 2 θ
:
þ
¼n
F
kl
4kα
ð5:7Þ
The equation is simplified if the valence-angle force constant kα is replaced by another
force constant kp. When the valence angle opens by Δα, one of the carbon atoms moves
a distance lΔα with respect to the other. The energy involved in the distortion, Δω, may
be written in terms of lΔα,
1
1
Δw ¼ kα ðΔαÞ2 ¼ k p ðlΔαÞ2 :
2
2
ð5:8Þ
84
Mechanical properties of fibers and fiber assemblies
Substituting kp in Eq. (5.7),
ΔL
cos 2 θ sin 2 θ
¼n
:
þ
F
kl
4kp
ð5:9Þ
Therefore, the modulus is given by
E¼
1
l cos θ cos 2 θ sin 2 θ
þ
:
A
kl
4k p
ð5:10Þ
The cross-sectional area of each chain A ¼ 18.25 Å2. The C–C bond length l ¼ 1.53Å
and valence angle α ¼ 112 . For values of kl ¼ 4.36 102 N/m and kp ¼ 0.35 102
N/m, we have E ¼ 182 GPa.
Using force constants determined from low-frequency Raman shifts of normal
hydrocarbons, Shimanouchi, Asahina and Enomoto obtain the theoretical value,
E ¼ 340 GPa.
5.4
Mechanical properties of nonwovens
Nonwoven is a word that described a fibrous structure fabricated directly from fiber to
fabric. The fibers are held together mechanically or chemically in a nonwoven, resulting
in a mechanically stable, self-supporting, flexible, web-like structure. According to the
bonding method, nonwovens can be classified as chemically bonded, mechanically
bonded, spun bonded and stitch bonded. Meltblown nonwovens are a type of spunbonded nonwoven. They are produced by extruding melted polymer materials through a
spin net or die to form long thin fibers which are stretched and cooled by passing hot air
as they fall from the die.The fibers fall onto a conveyor belt and bond into a web that is
collected into rolls and subsequently converted to finished products. From a process
point of view, electrospinning is a similar technique to meltblown, and from the
standpoint of structure, electrospun nanofiber webs are the same as spun-bonded
nonwovens. Therefore understanding of the analytical methods of mechanical properties of nonwovens will be a great help for analysis of the mechanical properties of
electrospun nanofiber products.
5.4.1
Geometry of nonwovens
The geometry of nonwovens can be analyzed by investigating a unit circle (a circle of
unit diameter).
Let N(θ) be the number of continuous fibers through the entire thickness of the fabric
that lie within (Δθ/2) of any prescribed angle θ per unit width perpendicular to the
direction of the fibers. The sum total of N(θ) over all angles is:
X
Nf ¼
N ðθÞ,
ð5:11Þ
where Nf is the total number of fibers in a unit circle drawn on the fabric, as shown in
Fig. 5.4a. With a dimension of length1, Nf is obtained by redistributing all the fibers in
5.4 Mechanical properties of nonwovens
85
Fig. 5.4 (a) Nf fibers through the thickness of the fabric in a unit circle. (b) The Nf fibers being
redistributed in the circle to be parallel to each other in a two-dimensional (2-D) plane.
the unit circle such that they lie parallel to their real direction in the fabric plane, shown
in Fig. 5.4b. Every fiber maintains its original length in the circle. The number of these
fibers per unit width perpendicular to the fibers equals exactly Nf in a statistical sense.
These fibers can be taken spaced a distance 1/Nf apart. If the ith fiber of the set of Nf
fibers has a length li, then area Ai allocated to the ith fiber is approximately
1
A i ffi li
:
ð5:12Þ
Nf
The summation of Ai over all the N fibers gives approximately the total area A u of the
unit circle, i.e.
Au ¼
XN f
A ffi
i¼1 i
1 XN f
l:
i¼1 i
Nf
ð5:13Þ
From this, Nf is found to be
Nf ffi
XN f
l
i¼1 i
Af
:
ð5:14Þ
XN f
l represents the total length of the fibers in A u, Nf, in fact, represents the
Since
i¼1 i
total fiber length per unit area.
The fiber-orientation distribution function ϕ(θ), i.e. the fraction of total fibers that lie
in the direction of θ, is defined as [4]:
N ðθ Þ
:
ΔθNf
ð5:15Þ
1 dN
:
Nf dθ
ð5:16Þ
ϕðθ Þ ¼
As Δθ approaches zero, ϕ(θ) becomes:
ϕðθ Þ ¼
It is, therefore, concluded that
dN
¼ Nf ϕðθÞ:
dθ
ð5:17Þ
86
Mechanical properties of fibers and fiber assemblies
Nf can also be expressed as
π
W b2
Nf ¼ 4 ,
λLf
ð5:18Þ
where W is fabric weight, b is cell width, λ is fiber diameter and
π
λ ¼ b:
4
ð5:19Þ
XN
N
ðθ Þ ¼
A cos i ðθÞ:
i¼1 i
Nf
ð5:20Þ
The fiber distribution function will be
5.4.2
Deformation of nonwovens
In the case that the nonwoven is under uniaxial loading, the stress and strain can be
analyzed based on the assumption that there is no lateral contraction during the
deformation.
Assuming the fibers have discrete orientation, as shown in Fig. 5.5, the axial strain,
fiber starin and elementral tranverse strain will, respectively, be:
axial strain
εF ¼
δyi
,
yi
ð5:21Þ
εf ¼
δij
,
li
ð5:22Þ
fiber strain
Fig. 5.5 Strain of fiber.
5.5 Mechanical properties of yarns
87
elemental transverse strain
εT ¼
δxi
¼ 0,
xi
ð5:23Þ
and the strain along the fiber axis [5] is
εf ¼ εF cos 2 θ:
ð5:24Þ
Then the corresponding stress on the nonwovern along the fiber axis will be [5]
ð π=2
ð π=2
σF ¼
E f εf cos 2 ðθÞ dθ ¼
Ef εF cos 4 ðθÞϕðθÞdθ:
ð5:25Þ
π=2
π=2
The subscripts, f and F, respectively, denote fiber and fabric.
For uniform fiber orientation distribution, we have
1
ϕðθ Þ ¼ :
π
ð5:26Þ
Substitute Eq. (5.26) into Eq. (5.25) to obtain
3
σ F ¼ E f εF ,
8
ð5:27Þ
EF 3
¼ :
8
Ef
ð5:28Þ
which implies
According to Eq. (5.20), the total fabric load will be
3
F ¼ Nf Af E f εf :
8
5.5
ð5:29Þ
Mechanical properties of yarns
A yarn is a linear fibrous assembly consisting of textile fiber of discrete length (staple
yarn or spun yarn) or continuous filaments (continuous filment yarn). Yarns are the
building blocks for the formation of knitted, woven or braided textile structures.
Accordingly, the conversion of nanofibers into yarn will create a pathway to the
formation of higher-order two-dimensional (2-D) and three-dimensional (3-D) textile
structures. Yarns can be produced by various yarn formation systems including spun
yarn, twisted filament yarn and textured yarns [6]. The fiber material properties and the
resulting yarn geometry (twist, denier, etc.) determine the performance properties of
a yarn and, consequently, dictate the processing methods and applications for which a
yarn is suited. As shown in Fig. 5.6, the mechanical properties of the various types
of yarns differ, which can be attributed to the variation in yarn structural features.
88
Mechanical properties of fibers and fiber assemblies
Fig. 5.6 Stress–strain curves of some synthetic filament yarns [7].
5.5.1
Yarn geometry
The geometric parameters that describe a linear fiber assembly include the shape of the
bundle cross-section, the number of fibers in the cross-section, the bundle twist level,
the degree of fiber migration in the radial direction and the fraction of interfiber packing.
Usually, the fiber bundles are assumed to be circular in cross-section but, in reality, the
fibers or filaments can be packed in various shapes. Most engineering fibers, such as
glass and carbon, have a circular or near-circular cross-section with a constant diameter.
In the case of yarns produced from electrospun fibers or from carbon nanofiber and
nanotubes, the fiber packing geometry in the yarn is more complex due to the ultrafine
nature of the fibers and the high level of cohesion between fibers due to friction and
secondary force effect. In a nanofiber yarn, the constituent fiber itself is often an
assembly of fibrils. Keeping this in mind, as a first approximation, we assume classical
yarn mechnics applied in the analysis of yarns consisting of nanofibers.
The geometry of interfiber packing in fiber bundles has been studied by a number of
researchers. Three basic idealized forms of circular fiber packing were identified: open
packing, in which the fibers are arranged in concentric layers (Fig. 5.7a); square packing,
in which the fibers are enclosed by a square (Fig. 5.7b); and close packing, in which
the fibers are arranged in a hexagonal pattern, as in the case of carbon nanotube fibers
(Fig. 5.7c).
In open-packed bundles the fiber volume fraction, defined as fiber-to-bundle area
ratio, has been computed as a function of the number of fibers. If the outer ring is
completely filled and the fibers are circular, the fiber volume fraction will be [9]
Vf
open
¼1
3N r ðN r
2N f
1Þ þ 1
2 ,
1
ð5:30Þ
where Nr is the number of rings and its relationship to the number of fibers Nf is
given by
5.5 Mechanical properties of yarns
89
Fig. 5.7 Idealized fiber packings: (a) open packing; (b) square packing; (c) close packing [8].
Nr ¼
1
1 1
þ þ 2N f
2
4 3
1
1=2
:
ð5:31Þ
For large numbers of fibers the fiber volume fraction approaches 0.75.
In squared-packed bundles, the fibers are arranged in a square. For any number of
circular fibers, if the outer layer is completely filled, the fiber volume fraction can be
shown to equal the area ratio of a circle to an enclosing square [8]:
V fsquare ¼
π
¼ 0:785:
4
ð5:32Þ
Similar to square packing, the fiber volume fraction of a close-packed bundle is equal to
the area ratio of a circle to an enclosing hexagon [8]:
π
V fsquare ¼ pffiffiffi ¼ 0:907:
2 3
ð5:33Þ
The level of bundle fiber volume fraction predicted by the above models applies equally
to other shapes if the number of fibers is sufficiently large.
Twists are inserted into fiber bundles (yarns) to maintain the integrity of the yarn
structure, improve the tensile strength of spun yarn and resist lateral stresses in continuous filament yarns. Staple yarns usually require more twist than filament yarns. For
twisted fiber bundles, the fibers are no longer aligned along the bundle axis. Instead, the
fibers are oriented in a helical configuration within the bundle, as shown in Fig. 5.8. The
assumptions underlying this geometric model are characterized by the following postulates: (1) the yarn is circular in cross-section and is uniform in the yarn structure; (2) the
axis of the circular cylinders coincides with the yarn axis; and (3) the fibers fall into a
rotationally symmetrical array in the cross-sectional view (all yarns the fibers in each
layer assume the same distance from central axis of the yarn).
Define twist (T, tpi or tpm) the number of twist within a unit length, then we have:
h¼
1
,
T
ð5:34Þ
l2 ¼ h2 þ ð2πr Þ2 ,
ð5:35Þ
L2 ¼ h2 þ ð2πRÞ2 ,
ð5:36Þ
tan ϕ ¼
2πr
¼ 2πrT,
h
ð5:37Þ
90
Mechanical properties of fibers and fiber assemblies
Fig. 5.8 (a) Idealized yarn geometry; (b) “opened out” diagram of cylinder at radius r; and
(c) “opened out” at yarn surface [10].
tan α ¼
2πR
¼ 2πRT:
h
ð5:38Þ
According to Hearle et al. [10], yarn diameter is related to the number of filaments (n) in
the yarn and the packing fraction of the fibers (κ) in the following relationship:
1=2
κ
D¼
,
ð5:39Þ
nd 2
where D is the diameter of the yarn, and d is the diameter of the fiber.
The fiber volume fraction of a yarn is actually equal to its fiber packing fraction, i.e.
Vf ¼ κ. Combining Eqs. (5.38) and (5.39), we have [8]
Vf ¼ n d þ
tan α
πdT
2
:
ð5:40Þ
Clearly, for a given twist level inserted into the fiber bundle, as fiber orientation angle
increases, yarn diameter increases whereas the fiber volume fraction decreases, as can
be seen in Fig. 5.9, which is useful in determining the twist level of fiber bundles. For
example, to obtain a fiber volume fraction (or fiber packing fraction) 0.8 and a fiber
orientation angle l0 , a twist level of 3 tpi should be used for the 12k, 7 μm fiber
diameter carbon yarns [8, 11].
5.5.2
5.5.2.1
Mechanical properties of linear fiber assemblies
Stress analysis
The application of an external tensile load, P, along the axis of a yarn results in
forces being applied to the various fibers. In general, the only stresses that
can possibly act on the cross-section of a fiber are illustrated in Fig. 5.10. They
are: (1) a tensile force, pr, in the direction along the fiber axis and normal to the fiber
5.5 Mechanical properties of yarns
91
Fig. 5.9 Relationship of fiber volume fraction to fiber orientation at various twist levels [8].
Fig. 5.10 General stresses in fibers.
cross-section; (2) a shear force, τ, acting tangential to the fiber cross section; (3) a
bending moment, M; and (4) a torsional moment, Γ [12].
Assuming the fibers are perfectly flexible and incapable of resisting any axial
compressive forces, in addition to the extremely large ratio of fiber length to fiber
diameter, the stresses M, τ, and Γ vanish and the only force acting is a direct tension, pr.
Then the contribution of pr to the total load P acting on the yarn in tension is pr cos ϕr
per fiber, therefore [12]
XN f
P¼
p cos ϕr :
ð5:41Þ
n¼1 r
In reality each filament in the yarn would assume a different orientation. To simplify the
analysis, the continuum approach is used by assuming the yarn consists of rotationally
symmetric rings of fibers and the fibers in the yarn are consolidated into an equivalent
solid continuous medium rather than a group of discretely individual fibers. Thus the
yarn is treated with the use of differential elements of area as opposed to individual
fibers. Then the force dpr is written as
92
Mechanical properties of fibers and fiber assemblies
dpr ¼ σ r dN f Af ,
ð5:42Þ
where σr is stress intensity at the radial position, dNf is the number of fibers in an
element of area A, and Af is the cross-sectional area of fiber.
So, dA is written as
dA ¼ 2πrdr
ð5:43Þ
Nf
2πrdr cos ϕr :
dN f ¼
πR2
ð5:44Þ
and dNf can be written as
Substitute Eq. (5.44) into Eq. (5.42),
N f Af
2πr cos ϕr dr:
dpr ¼ σ r
πR2
ð5:45Þ
Since the packing factor of the yarn is Vf ¼ Nf Af /πR2,
dpy ¼ σ r V f 2πr cos ϕr dr:
ð5:46Þ
Tension in the yarn direction dpy ¼ dpr cos ϕr will be
dpy ¼ 2πV f σ r r cos 2 ϕr dr:
And the total load on the yarn is therefore
ðR
py ¼ 2πV f σ r r cos 2 ϕr dr:
0
ð5:47Þ
ð5:48Þ
From the identity cos 2 ϕ ¼ 1=ð1 þ tan 2 ϕÞ and Eq. (5.37), it is apparent that
cos 2 ϕr ¼
Thus
py ¼ 2πV f
5.5.2.2
1
:
1 þ 4π 2 r 2 T 2
ð5:49Þ
σr r
dr:
1 þ 4π 2 r 2 T 2
ð5:50Þ
ðR
0
Strain analysis
Considering the change in fiber angle is very small, i.e. ϕ ϕ0 , as shown in Fig. 5.11,
then [5]
l¼
L
cos ϕ
ð5:51Þ
and
Δl ¼ ΔL cos ϕ:
Since εy ¼ ΔL/l,
ð5:52Þ
5.5 Mechanical properties of yarns
93
Fig. 5.11 Deformation in twisted yarns.
εf ¼
Δl ΔL cos ϕ
¼
¼ εy cos 2 ϕ,
l
L=cos ϕ
ð5:53Þ
εy
:
1 þ 4π 2 R2 T 2
ð5:54Þ
substitute into Eq. (5.49) to give
εf ¼
Assume fibers and yarns are Hookean material, then
σ f ¼ E f εf ,
ð5:55Þ
σ f ¼ E f εy cos 2 ϕ:
ð5:56Þ
which can be written as
Substitute Eq. (5.56) into Eq. (5.48),
ðR
It can be also shown that
5.5.3
py ¼ 2πV f Ef εf cos 4 ϕdr
0
¼ πV f R2 Ef εf cos 2 ϕ:
ð5:57Þ
Ey ¼ E f cos 2 ϕ:
ð5:58Þ
Mechanical properties of staple yarns
In many nanofiber structures such as carbon nanotubes and carbon nanofibers, the fibers
are not continuous. The discrete fibers in the assembly are held together by secondary
forces and or frictional cohesions. Using classical staple yarn mechanics one may think
that cohesive forces between the fibers/fibrils would include secondary forces between
fibers.
Taking a single fiber in the yarn as our object of analysis, and assuming
the compressive stress applied to the this fiber is uniform due to twist effect, and the
frictions between fibers obey Amontons’ laws, the distribution of tensile stress on
the fiber is as shown in Fig. 5.12.
94
Mechanical properties of fibers and fiber assemblies
Fig. 5.12 Distribution of tensile stress on the fiber.
We define the slippage factor, S, as the fractional reduction in fiber contribution to the
longitudinal tension T, then express S in terms of energy, and we then have
S¼
ABCD
AED
ABCD
BFC
¼1
AGED
:
ABCD
ð5:59Þ
Here
ABCD ¼ LT
AGED ¼ lc T,
ð5:60Þ
where L is the fiber length, and lc is the slippage length, which is defined as the critical
length of the fiber on which the total surface friction equals the breaking strength of
this fiber.
Therefore
S¼1
lc
:
L
ð5:61Þ
At E and F, the force resisting slippage should be equal to the constant fiber tension
resulting from the strain in the structure. That is
F ¼ T ¼ 2πrlc G,
ð5:62Þ
where F is force-resisting slippage, r is fiber radius, and G is the force per unit surface
area resisting slippage.
Since
T ¼ πr2 σ f
ð5:63Þ
rσ f
:
2G
ð5:64Þ
we have
lc ¼
The slippage depends on the frictional resistance, thus
5.5 Mechanical properties of yarns
95
Fig. 5.13 Numerical plot of equations derived in Ref. [10].
G ¼ μN,
ð5:65Þ
where N is the normal stress acting on the fiber surface.
The slippage factor can be rewritten as
S¼1
rσ f
:
2LμN
ð5:66Þ
Considering the slippage of fibers in yarn, Hearle et al. [10] obtained the following
relationships:
σy
¼ A cos 2 ϕð1
σf
2
¼ A cos αð1
k cosec ϕÞ
k cosec αÞ
ð5:67Þ
and
Ey
¼ cos 4 αð1
Ef
k cosec αÞ,
ð5:68Þ
pffiffiffi
where k ¼
2=3Lf ðrQ=μÞ1=2 , Q is the migration factor, and μ is the fiber coefficient
of friction which increases as slippage increases and decreases when slippage decreases.
Q is an experimentally determined factor.
Figure 5.13 shows a numerical plot with k ¼ 0.01 and 0.1. The separate effects of
obliquity (cos2 α) and slip (1 k cosec α) are clearly indicated.
Using a different approach, Platt et al. obtained:
r
Py ¼ N f Af σ f 1
,
ð5:69Þ
2μGL
where r is the fiber radius, μ is the fiber friction, G is shear resistance and L is the fiber
length.
96
Mechanical properties of fibers and fiber assemblies
Fig. 5.14 Geometry of the twist effect.
This relation involves the concept of critical length. The fiber must be long enough
to be locked into the structure, otherwise deformation could occur only due to
slippage.
As shown in Fig. 5.14, for a staple fiber, the twist angle is related to its twist and
radius:
12
0
1
C
B
C
B
1
2
T
B
ffiC
cos ϕ ¼ Bsffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð5:70Þ
C ¼ 1 þ 4π 2 r 2 T 2 :
2
A
@
1
2
þ ð2πr Þ
T
According to Eq. (5.58), the translational efficiency for spun yarns can be estimated
by the following relationship:
σy
TM
2
,
ð5:71Þ
¼ cos θ
5
σf
where TM is the twist multiplier.
Figure 5.15 shows the translation efficiency of a continuous filament and staple yarn.
5.6
Mechanical properties of woven fabrics
The performance characteristics of a fabric are functions of the fiber materials’ properties, the yarn geometry and the geometry of the fabric. The finishing (chemical and
mechanical modification) that the fabric receives is equally important for the appearance
and the functional properties of a fabric. Before one can make effective use of the
combination of materials, geometry and finishing technique to design a fabric that has
the desirable properties (engineering of a fabric), we need to be acquainted with the
materials, the geometry and the finishing techniques. In this section, the geometry and
mechanical properties of fabrics is introduced.
5.6 Mechanical properties of woven fabrics
97
Fig. 5.15 Effect of twist on the breaking tenacity of filament yarn and staple yarn.
5.6.1
Woven fabric geometry
Figure 5.16 gives the unit cell geometry for plain biaxial weave, as proposed by Dow
and Ramnath [13]. In their analysis, Dow and Ramnath assumed circular yarn crosssection, the same yarn diameter and pitch length for both fill and warp yarns. The
expression of the fiber volume fraction was derived as
l
2 þ 4θ
π
d
Vf ¼
κ 2 ,
4
L
T
d
d
ð5:72Þ
where κ is the fiber packing fraction, d is the yarn diameter, L is the pitch length, T is the
fabric thickness and l is the dimension, as shown in Fig. 5.16. The yarn inclination angle
to the fabric plane, θ, is given by
θ ¼ tan
1
2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðL=dÞ2 3
tan
1
ðd=LÞ:
ð5:73Þ
The fabric thickness is very close to two yarn diameter, i.e.
T 2d
ð5:74Þ
l
1
¼
:
d
tan θ
ð5:75Þ
and approximately
Equation (5.72) is then simplified to
98
Mechanical properties of fibers and fiber assemblies
Fig. 5.16 Unit cell geometry of plain weave proposed by Dow and Ramnath [13].
Fig. 5.17 Relationship of fiber volume fraction to fiber orientation for plain weave [8].
1
π tan θ þ 2θ
Vf ¼
κ 2 :
4
L
d
ð5:76Þ
Figure 5.17 plots the fiber volume fraction against the yarn inclination angle. It can be
seen that as the inclination angle increases, the pitch length becomes longer, which
results in a lower fiber volume fraction. The woven fabric has the tightest structure at the
inclination angle of 60 (when L/d ¼ 3 in Eq. (5.73). In this calculation, the fiber packing
fraction κ is assumed to be 0.8. The above analysis is given only for the simplest of
woven structures. Different weave patterns, non-circular yarn cross-sectional shape,
different yarn dimensions and pitch lengths for fill and warp yarns can be analyzed [8].
5.6 Mechanical properties of woven fabrics
99
Fig. 5.18 Unit cell geometry of plain weave proposed by Peirce [14, 15].
While the Dow–Ramnath model provides a modern analsyis of woven fabrics from
the composite structure point of view, one should recognize the original contribution of
Peirce [14, 15] and Haas and Dietzius [16] who pioneered the quantitative analysis of
textile fabric structures. Figure 5.18 shows Peirce’s model of unit cell of plain weave.
According to Peirce, the main parameters relates to unit cell geometry are: crimp, C,
which determines the amount of elongation; diameter, D, which determines the relationship between the paths of two sets of yarns; texture, P; and thickness, H, the
distance between center of fabric and the center of yarn. These parameters can be
expressed as follows.
(1)
Crimp (C):
C1 ¼
L1
P2
P2
C2 ¼
(2)
(3)
(4)
L2
P1
¼
L1
P2
1,
ð5:77Þ
1:
ð5:78Þ
Diameter (D):
Texture (P)
Thickness (H):
D ¼ H þ H 2 ¼ d1 þ d2 :
P1 ¼ ðL2
P2 ¼ ðL1
H 1 ¼ ðL1
H 2 ¼ ðL2
ð5:79Þ
Dθ2 Þ cos θ2 þ D sin θ2 ,
Dθ1 Þ cos θ1 þ D sin θ1 :
Dθ1 Þ sin θ2 þ Dð1
Dθ2 Þ sin θ2 þ Dð1
cos θ1 Þ,
cos θ2 Þ:
ð5:80Þ
ð5:81Þ
Here, θ is the yarn inclination angle, L is the modular length, and the subscripts 1 and 2
are, respectively, warp and filling.
There are seven equations and 11 unknowns included in these relationships. In order
to solve the seven simultaneous nonlinear algebraic equations, at least four parameters
need to be determined. The solutions can be obtained by graphical methods (graphical
means [14, 17, 18] or nomograph [19]) or by a computer method. The computational
flow is shown in Fig. 5.19.
100
Mechanical properties of fibers and fiber assemblies
Fig. 5.19 (a) Computational flow for Peirce’s equations. (b) Subroutine computational flow.
References
1. F. Ko, and M. Gandhi, “Producing nanofiber structures by electrospinning for tissue engineering,” in Nanofibers and Nanotechnology in Textiles, P. J. Brown and Kathryn Stevens, Ed.
Woodhead Publishing, in association with The Textile Institute, 2007: p. 22.
2. J. H. He, Y. Q. Wan, and L. Xu, “Nano-effects, quantum-like properties in electrospun
nanofibers,” Chaos, Solitons and Fractals, vol. 33(1), pp. 26–37, 2007.
3. L. Treloar, “Calculations of elastic moduli of polymer crystals: I. Polyethylene and nylon 66,”
Polymer, vol. 1, pp. 95–103, 1960.
4. W. D. Freeston, and M. M. Platt, “Mechanics of elastic performance of textile materials:
Part XVI: bending rigidity of nonwoven fabrics,” Textile Research Journal, vol. 35(1), pp. 48–
57, 1965.
5. D. W. R. Petterson, “Mechanics of nonwoven fabrics,” Industrial & Engineering Chemistry,
vol. 51(8), pp. 902–903, 1959.
References
101
6. B. C. Goswami, J. G. Martindale, and F. Scardino, Textile Yarns; Technology, Structure, and
Applications. John Wiley and Sons, 1977.
7. Piller, B., Bulked Yarns: Production, Processing and Applications. Czechoslovakia: SNTLPublishers of Technical Literature, in coedition with the Textile Trade Press, 1973.
8. Ko, F. and G. Du, Textile Preforming Handbook of Composites, 1998, p. 397.
9. G. -W. Du, T. -W. Chou, and P. Popper, “Analysis of three-dimensional textile preforms for
multidirectional reinforcement of composites,” Journal of Materials Science, vol. 26(13),
pp. 3438–3448, 1991.
10. J. Hearle, P. Grosberg, and S. Backer, Structural Mechanics of Fibers, Yarns, and Fabrics.
New York: Wiley-Interscience, 1969.
11. F. Ko, and G. Du, Processing of textile preforms, in Advanced Composites Manufacturing,
T. G. Gutowski, Ed. New York. John Wiley & Sons, Inc., 1997.
12. M. M. Platt, “Mechanics of elastic performance of textile materials: III. Some aspects of
stress analysis of textile structures – continuous-filament yarns,” Textile Research Journal,
vol. 20(1), pp. 1–15, 1950.
13. N. Dow, and V. Ramnath, Analysis of woven fabrics for reinforced composite materials
(NASA CR-178275). National Aeronautics and Space Administration, Hampton (VA 23681–
0001), Washington, DC, 1987.
14. F. Peirce, “Geometrical principles applicable to the design of functional fabrics,” Textile
Research Journal, vol. 17(3), p. 123, 1947.
15. F. Peirce, “The geometry of cloth structure,” Journal of the Textile Institute Transactions,
vol. 28(3), pp. 45–96, 1937.
16. R. Haas, and A. Dietzius, The stretching of the fabric and the deformation of the envelope in
nonrigid balloons. National Advisory Committee for Aeronautics, 1918.
17. L. Love, “Graphical relationships in cloth geometry for plain, twill, and sateen weaves,”
Textile Research Journal, vol. 24(12), p. 1073, 1954.
18. E. V. Paintert, “Mechanics of elastic performance of textile materials: Part VIII: Graphical
analysis of fabric geometry,” Textile Research Journal, vol. 22(3), pp. 153–169, 1952.
19. D. P. Adams, E. R. Schwarz, and S. Backer, “The relationship between the structural
geometry of a textile fabric and its physical properties: Part VI: Nomographic solution of
the geometric relationships in cloth geometry,” Textile Research Journal, vol. 26(9),
pp. 653–665, 1956.
6
Characterization of nanofibers
Knowing the basic properties of nanofibers (such as morphology, molecular structure
and mechanical properties) is crucial for the scientific understanding of nanofibers and
for the effective design and use of nanofibrous materials. In order to evaluate and
develop the manufacturing process, the composition, structure and physical properties
must be characterized to decide whether the produced fibers are suitable for their
particular application. Evaluation of the various production parameters in processes
such as electrospinning is a critical step towards production of nanofibers commercially.
Many common techniques used to characterize conventional engineering materials, as
well as some not so common techniques, have been employed in the characterization of
nanofibers. Table 6.1 shows the scales of fibers and the corresponding characterizations
techniques. To provide an overall understanding, some of the general characterization
techniques for structural, chemical, mechanical, thermal and other properties will be
introduced in this chapter.
6.1
Structural characterization of nanofibers
The morphological characterization techniques briefly discussed herein are: optical
microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and scanning tunneling microscopy
(STM). These methods characterize the morphology and determine fiber diameter, pore
size and porosity, all of which are necessary to evaluate the various production parameters. The techniques for the characterization of order/disorder of molecular structures
using X-ray diffraction (XRD) are also covered in this section. Furthermore, mercury
porosimetry, a special technique for porosity measurement, is introduced.
6.1.1
Optical microscopy (OM)
An optical microscope, otherwise known as a “light microscope,” is a type of microscope that is operated by using visible light and a system of objective lenses to achieve
image magnification of small samples. Optical microscopes are the oldest and simplest
of microscopes. The resolution of an optical microscope is given by:
102
6.1 Structural characterization of nanofibers
103
Table 6.1 Scale of fibers and according characterization techniques
Fiber
Process
Scale
SWCNT
CVD
MWCNT NF Whisker
Electrospinning
Fiber
Wire
Spinning Drawing Extrusion
Testbeds Composition EELS AES XPS EDX Raman FTIR XRD
Elemental composition Chemical bonds Crystal structure
Structure
TEM STM AFM SEM Light microscopy
Phisical
AFM Nanoindentation MEMS Test devices Conventional
properties
Mechanical, electrical, magnetic, optical
R ¼ 0:61
λ
λ
¼ 0:61
,
n sin α
NA
ð6:1Þ
where R is the resolution of the image, λ is the wavelength of white light, n is the
refractive index, α is the refracted angle and NA is the numerical aperture. The best
possible resolution that can be resolved by an optical microscope is 200 nm. [1].
Figure 6.1 shows an optical microscope.
Generally speaking, geometric properties of nanofibers such as fiber diameter, diameter distribution, fiber orientation and fiber morphology can be characterized with a
fairly high degree of accuracy by using an optical microscope. Figure 6.2 shows an
optical micrograph of cellulose acetate nanofibers at 100 magnification. The red scale
bar has a length of 1 micron.
6.1.2
Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) is capable of producing high-resolution images of
a sample surface. Compared to OM, SEM provides a better understanding of the
microstructure of a sample due to higher resolution, greater depth of field, and its
sensitivity to surface morphology. In a scanning electron microscope, electrons from a
tungsten or lanthanum hexaboride (LaB6) cathode are thermionically emitted and
accelerated towards an anode. Alternatively, the electrons can be emitted via field
emission (FE), hence FE-SEM. The main two reasons for using tungsten as the cathode
in SEM are that it confers the highest melting point and lowest vapor pressure of all
metals. The electron beam typically has an energy ranging from several hundred eV to
100 keV. The beam is focused by one or two condenser lenses into a beam with a very
fine focal point, with diameter ranging from 0.4 nm to 5 nm. When the primary electron
beam interacts with the sample, electrons lose energy by repeated scattering and
absorption within a pear-shaped volume of the sample. This pear-shaped volume is
known as the interaction volume, which extends from less than 100 nm to around 5 μm
104
Characterization of nanofibers
Fig. 6.1 An optical microscope.
Fig. 6.2 An optical micrograph of cellulose acetate nanofibers at 100 magnification.
into the surface. The size of the interaction volume depends on the electron energy
(accelerating voltage), the atomic number of the sample and the sample’s density.
The energy exchange between the electron beam and the sample results in electron
emission and electromagnetic radiation. The electrons are detected and an image is
produced. A larger interaction volume causes backscattered electrons to emit from a
larger area of the specimen. The backscattered electrons interact with the specimen on
6.1 Structural characterization of nanofibers
105
Fig. 6.3 Interaction volume of the sample with increasing accelerating voltage [3].
Fig. 6.4 Nanofiber sample preparation for SEM.
their way out, producing secondary electrons which are further away from the original
spot size. Consequently, the resolution of the image obtained using a higher voltage is
reduced. Figure 6.3 shows the interaction volume of the sample at different accelerating voltages [2].
An SEM sample must be conductive to prevent charging and it must be vacuum
compatible. Thus a nanofiber sample must be dried and coated with gold before it can
be observed with SEM. Figure 6.4 illustrates the general procedure.
In recent years, SEM has become one of the most widely used techniques and has been
reported in the literature for measuring fiber diameter and the study of general fiber
morphological characteristics [4–6]. Owing to the capability of measuring nanometer
dimensions, SEM has also been combined with AFM cantilevers as a nanomanipulator,
which can be worked as the load sensor to measure the mechanical properties of
nanofibers or just manipulate nanofibers [7]. Yu et al. reported that the tensile properties
of individual multi-walled carbon nanotubes were successfully measured using AFM
cantilever tips under SEM [8]. They designed a nanomanipulator so that the carbon
nanotubes could be manipulated in three dimensions inside the microscope, by attaching
the tips of the AFM [9]. Samuel et al. reported mechanical testing of pyrolyzed polymer
nanofibers with the aid of SEM [10]. They utilized a micro-device with a leaf-spring load
cell, which was actuated externally with a piezo-motor. The average length of a sample
was 10 μm, and the maximum engineering strain on the sample was 15%. Because of the
106
Characterization of nanofibers
Fig. 6.5 Fabricating carbon nanotube transistors on pre-patterned electrodes on a Si substrate [7].
(a) Adhesion of a bundle at the end of tip. (b) Positioning the MWCNT on electrodes and
pressing the MWCNT by the counter CNT bundle.
small sample length, SEM was used for high magnification imaging to measure displacements. Lim et al. [7] installed two nanomanipulators inside a field-emission scanning
electron microscope (FE-SEM) for manipulation of nanostructured materials. The nanomanipulators can travel about 20 mm with a minimum increment of 1 nm, providing
various manipulation freedoms such as moving, bending, cutting and biasing. With these
nanomanipulators, they conducted in situ characterization of the electrical breakdown of
multi-walled carbon nanotubes (MWCNTs). Figure 6.5 illustrates the SEM images of an
MWCNT mounted between two AFM cantilevers for mechanical testing.
6.1.3
Transmission electron microscopy (TEM)
Transmission electron microscopy (TEM) is an imaging technique whereby a beam of
electrons is transmitted through a specimen and forms an image that is magnified and
directed to appear on either a fluorescent screen or a layer of photographic film, or to
be detected by a sensor such as a CCD camera. The theoretical resolution of TEM is
0.02 nm. However, due to spherical aberration, the practical resolution is approximately
0.2 nm [11].
In TEM, a crystalline material interacts with the electron beam mostly by diffraction
rather than absorption. The intensity of the transmitted beam is affected by the volume
and density of the material through which it passes. The intensity of the diffraction
depends on the orientation of the planes of atoms in a crystal relative to the electron beam.
At certain angles the electron beam is diffracted strongly from the axis of the incoming
beam, while at other angles the beam is largely transmitted. Modern microscopes are
often equipped with specimen holders that allow the user to tilt the specimen to a range of
angles in order to obtain specific diffraction conditions, and apertures placed below the
specimen allow the user to select for electrons that are diffracted in a particular direction.
A high-contrast image can therefore be formed by blocking electrons deflected away
from the optical axis of the microscope by adjusting the aperture to allow only
unscattered electrons through. This produces a variation in electron intensity that
reveals information about the crystal structure, and can be viewed on a fluorescent
screen, recorded on photographic film or captured electronically.
6.1 Structural characterization of nanofibers
107
Fig. 6.6 Components of a TEM instrument.
This technique (known as bright field or light field) is particularly sensitive to
extended crystal lattice defects (such as dislocations) in an otherwise ordered crystal.
As the local distortion of the crystal around the defect changes, the angle of the crystal
plane and intensity of scattering will vary around the defect. As the image is formed by
the distortion of the crystal planes around the defect, the contrast in these images does
not normally coincide exactly with the defect, but is slightly to one side.
It is also possible to produce an image from electrons deflected by a particular crystal
plane. By either moving the aperture to the position of the deflected electrons, or by
tilting the electron beam so that the deflected electrons pass through the centred
aperture, an image can be formed consisting of only deflected electrons, known as a
dark field image. Figure 6.6 shows the basic components of a TEM instrument.
In the most powerful diffraction contrast TEM instruments, crystal structure can be
investigated by high-resolution transmission electron microscopy (HRTEM), also
known as phase contrast imaging, as the images are formed due to phase differences
of electron waves scattered through a thin specimen [12].
Like SEM, TEM can be used to characterize geometric properties of nanofibers such
as fiber diameter, diameter distribution, fiber orientation and fiber morphology (e.g.
cross-section shape and surface roughness). The use of TEM does not require the
sample to be in a dry state, as in the SEM. Hence, nanofibers electrospun from a
polymer solution can be directly observed under TEM [13].
TEM can also be used to study the fiber alignment in composite nanofibers. McCullen et al. [14] utilized TEM to examine the presence and alignment of MWCNT in
PEO nanofibers. Figure 6.7a shows that MWCNT were embedded and aligned in PEO
108
Characterization of nanofibers
SWNT
5 nm
Graphitized
PAN fiber
(a)
(b)
5 nm
Fig. 6.7 (a) TEM image of a core–shell structured PAN nanofibers with MWCNTs embedded
in both core and shell and (b) TEM image of SWCNT stick out of PAN nanofibers [15].
nanofibers. As shown in Fig. 6.7b, aligned SWCNT embedded in PAN nanofibers were
also investigated with TEM [15].
When preparing a nanofiber sample for AFM, a carbon-coated metal mesh for AFM
is usually positioned between the tip of the syringe and the receiver when electrospinning. As the nanofibers move towards the receiver, some fibers will stick to the mesh,
and therefore a mesh with nanofibers attached is obtained for AFM observation.
Figure 6.8 shows a metal mesh for AFM.
Nanomanipulators can be installed in an atomic force microscope as well as in a
scanning electron microscope. Kuzumaki et al. [16] selectively performed deformation,
cutting off and bonding of individual CNT using a dual-nanomanipulation system
installed in a high-resolution transmission electron microscope. These processes were
directly observed in situ at a lattice resolution of 0.1 nm.
6.1.4
Atomic force microscopy (AFM)
The atomic force microscope (also known as a scanning force microscope) is a very
high-resolution scanning probe microscope. The atomic force microscope was invented
by Binnig, Quate and Gerber in 1986, and is one of the foremost tools for imaging,
measuring and manipulating matter at the nanoscale [17]. The theoretical resolution of a
TEM is at a fraction of a nanometer, which is more than 1000 times better than the
optical diffraction limit.
An AFM instrument is composed of a microscale cantilever with a pointed tip at its
end that is used to scan the specimen surface. Typically, the cantilever is made of silicon
or silicon nitride, with a tip radius of curvature on the nanoscale. When the tip is
brought sufficiently close to a sample surface, the cantilever deflects due to forces
between the tip and the sample, as governed by Hooke’s law. Depending on the
situation, forces that are measured in AFM include the mechanical contact force, van
der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic
forces, Casimir forces and solvation forces [18]. In addition to force, other quantities
may be simultaneously measured through the use of specialized probes. Typically, the
deflection is measured by using a laser spot reflected from the top of the cantilever into
6.1 Structural characterization of nanofibers
109
Fig. 6.8 A metal mesh for TEM.
Fig. 6.9 Schematic diagram of AFM.
an array of photodiodes. Other methods used include optical interferometry, capacitive
sensing or piezoresistive AFM cantilevers. These cantilevers are fabricated with piezoresistive elements that act as a strain gage. Using a Wheatstone bridge, strain in the
AFM cantilever due to deflection can be measured, but this method is not as sensitive as
laser deflection or interferometry.
If the AFM instrument tip were scanned at a constant height, there would be a risk that
the tip would collide with the surface, causing damage. Hence, in most cases a feedback
mechanism is employed to adjust the tip-to-sample distance, in order to maintain a
constant force between the tip and the sample. Traditionally, the sample is mounted on
a piezoelectric tube, which can move the sample in the z direction for maintaining a
constant force, and the x and y directions for scanning the sample. Alternatively a
“tripod” configuration of three piezo crystals may be employed, with each responsible
for scanning in the x, y and z directions. This eliminates some of the distortion effects seen
with a tube scanner. The resulting map of the area s ¼ f(x, y) represents the topography of
the sample. Figure 6.9 illustrates the schematic diagram of AFM.
Similar to SEM and TEM, AFM can be used to characterize geometric properties of
nanofibers such as fiber diameter, diameter distribution, fiber orientation and fiber
110
Characterization of nanofibers
Fig. 6.10 AFM image of MWCNT-reinforced electrospun cellulose fibers.
morphology (e.g. cross-section shape and surface roughness) [13]. However, an accurate measurement of the nanofiber diameter with AFM requires a rather precise procedure. The fibers appear larger than their actual diameters because of the AFM instrument
tip geometry [19]. For a precise measurement, fibers crossing to each other on the
surface are generally chosen. The upper horizontal tangent of the lower fiber is taken as
a reference, and the vertical distance above this reference is considered to be the exact
diameter of the upper nanofibers [20]. Figure 6.10 shows an AFM image of an
MWCNT reinforced electrospun cellulose fibers. Single MWCNTs coated on the
surface of the fibers are distinguishable. The diamension of MWCNTs can be measured
with AFM as well.
6.1.5
Scanning tunneling microscopy (STM)
Scanning tunneling microscopy (STM) is a powerful technique for viewing surfaces at
the atomic level. Its development in 1981 won its inventors, Gerd Binnig and Heinrich
Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986 [21]. The technique of
STM probes the density of states of a material by using tunneling current. For STM,
good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth
resolution [22]. The STM can be used not only in ultra-high vacuum but also in air
and various other liquid or gas ambients, and at temperatures ranging from near zero
to several hundred kelvin [23].
The STM instrument is based on the concept of quantum tunneling. When a conducting tip is brought very near to a metallic or semiconducting surface, a bias between
the two can allow electrons to tunnel through the vacuum between them. For low
voltages, this tunneling current is a function of the local density of states (LDOS) at the
Fermi level, Ef, of the sample [23]. Variations in current as the probe passes over the
surface are translated into an image. However, STM can be a challenging technique, as
it requires extremely clean surfaces and sharp tips.
6.1 Structural characterization of nanofibers
111
The components of an STM instrument include a scanning tip, piezoelectric controlled height, an x,y scanner, coarse sample-to-tip control, a vibration isolation system
and a computer [24]. The resolution of an image is limited by the radius of curvature of
the scanning tip of the microscope. Additionally, image artifacts can occur if the tip has
two tips at the end, rather than a single atom; this leads to “double-tip imaging,” a
situation in which both tips contribute to the tunneling [22]. Therefore it has been
essential to develop processes for consistently obtaining sharp, usable tips. The tip is
often made of tungsten or platinum–iridium, although gold is also used. Tungsten tips
are usually made by electrochemical etching, and platinum–iridium tips are made by
mechanical shearing [22]. Owing to the extreme sensitivity of tunnel current to height,
proper vibration isolation is imperative for obtaining usable results. In the first scanning
tunneling microscope, by Binnig and Rohrer, magnetic levitation was used to keep the
instrument free of vibrations; now spring systems are often used, and additional
mechanisms for reducing eddy currents are implemented [23]. The maintenance of tip
position with respect to the sample, scanning the sample in raster fashion and acquring
the data are all computer controlled [25]. The computer is also used for enhancing the
image with the help of image processing as well as performing quantitative morphological measurements.
To use an STM instrument, the tip is first brought into close proximity of the
sample by some coarse sample-to-tip control. The values for common sample-to-tip
distance, W, range from about 4 Å to 7 Å, which is the equilibrium position between
attractive (3 Å < W < 10 Å) and repulsive (W < 3Å) interactions [23]. Once
tunneling is established, piezoelectric transducers are implemented to move the tip
in three directions. As the tip is rastered across the sample in the x–y plane, the density
of states, and therefore the tunnel current, changes. This change in current with respect
to position can itself be measured, or the height, z, of the tip corresponding to a
constant current can be measured. These two modes are called the constant height
mode and the constant current mode, respectively [23]. In the constant current mode,
feedback electronics adjust the height by applying a voltage to the piezoelectric height
control mechanism [25]. This leads to a height variation, and thus the image comes
from the tip topography across the sample and gives a constant charge density surface;
this means contrast on the image is due to variations in charge density [24]. In the
constant height mode, the voltage and height are both held constant while the current
changes to keep the voltage from changing; this leads to an image made of current
changes over the surface, which can be related to charge density. The benefit to using
a constant height mode is that it is faster, as piezoelectric movements require more
time to register the change in constant current mode than the voltage response in
constant height mode [24]. In addition to scanning across the sample, information on
the electronic structure of the sample can be obtained by sweeping voltage and
measuring current at a specific location [22]. This type of measurement is called
scanning tunneling spectroscopy (STS).
The technique of STM has primarily been used to provide direct 3-D imaging
of the surface topography of nanomaterials such as organically modified SWCNT
[26, 27]. Specifically, the first STM study of fluorinated SWCNT revealed a banded
112
Characterization of nanofibers
(a)
1 nm
(b)
(c)
1 nm
2 nm
Fig. 6.11 Constant current STM images (Vbias, 200–1000 mV, It, 0.2–1 nA, T, 298 K, under
UHV), of purified p-SWCNT showing the carbon lattice on the sidewall (a), the terminals (b), and
some irregular structures (c) which are attributed to amorphous carbon or catalyst particles
adhered to the carbon surface [29].
structure which indicates the regions of fluorination.[28] Figure 6.11 shows the STM
images of p-SWCNT.
Although current literature surrounding the characterization of nanofibers by using
STM is sparse, STM is a powerful technique for studying the surface modification and
topography of functionalized nanofibers, which can eventually assist in the understanding of interfacial reaction of the nanofibers. Moreover it has been demonstrated that
STM is capable of moving and arranging atoms as designed, which shows the potential
of utilizing STM to move nanofibers and design nanostructures in the very near future.
Figure 6.12 shows UBC written with STM provided by Yan’s group in UBC.
6.1.6
X-ray diffraction
X-ray diffraction (XRD) is a very important technique that has long been used to
address all issues related to the crystal structure of solids, including lattice constants
and geometry, identification of unknown materials, orientation of single crystals,
preferred orientation of polycrystals, defects and stresses, etc.
In XRD, a collimated beam of X-rays, with a wavelength typically ranging from 0.7
Å to 2 Å, is incident on a specimen and is diffracted by the crystalline phases in the
specimen according to Bragg’s law:
λ ¼ 2d sin θ,
ð6:2Þ
where d is the spacing between atomic planes in the crystalline phase and λ, is the
X-ray wavelength. The intensity of the diffracted X-rays is measured as a function of
the diffraction angle 2θ and the speciment’s orientation. Strong diffraction from a set
of planes results when the angles of incidence and diffraction, θ, are equal, and the
path difference AOB between the two beams is equal to an integral number of
wavelengths, nλ, Figure 6.13 illustrates the principle of XRD. The X-ray diffraction
pattern is used to identify the specimen’s crystalline phases and to measure its
structural properties.
The crystallite size, D, can be estimated from the peak width with Scherrer’s
formula [30]:
6.1 Structural characterization of nanofibers
113
Fig. 6.12 UBC written with STM (copyright authorized by Dr. Yan Pennec).
Fig. 6.13 The principle of XRD.
D¼
Kλ
,
B cos θB
ð6:3Þ
where λ is the X-ray wavelength, B is the full width of height maximum (FWHM) of
a diffraction peak, θB is the diffraction angle, and K is Scherrer’s constant, of the
order of unity for usual crystal. However, one should be alerted to the fact that
nanoparticles often form twinned structures; therefore, Scherrer’s formula may produce results different from the true particle sizes. In addition, X-ray diffraction
provides only the collective information of the particle sizes and usually requires a
sizable amount of powder. It should be noted that since the estimation would work
only for very small particles, this technique is very useful in characterizing nanoparticles. Similarly, the film thickness of epitaxial and highly textured thin films can also
be estimated with XRD [31].
The technique of XRD is nondestructive and does not require elaborate sample
preparation, which partly explains the wide usage of the method in characterization of
materials, including nanofibers [32, 33].
114
Characterization of nanofibers
Fig. 6.14 WAXD of pristine and electrospun silk fibers [34].
6.1.6.1
Wide-angle X-ray diffraction
Wide-angle X-ray scattering (WAXS) or wide-angle X-ray diffraction (WAXD) is an
X-ray diffraction technique that is often used to determine the crystalline structure of
polymers. This technique specifically refers to the analysis of Bragg peaks scattered to
wide angles, which implies that they are caused by sub-nanometer sized structures. The
diffraction pattern generated allows the determination of the chemical composition or
phase composition of the film, the texture of the fiber (preferred alignment of crystallites), the crystallite size and presence of fiber stress.
Figure 6.14 shows the WAXD patterns of pristine and electrospun fibers. The
similarities between the patterns of the two systems strongly suggest structural similarity between the pristine and electrospun fibers. The crystallinity of the fibers was
determined through this pattern [34].
6.1.6.2
Small-angle X-ray scattering
Small-angle X-ray scattering (SAXS) is another powerful technique for characterizing
nanostructured materials. Strong diffraction peaks result from constructive interference
of X-rays scattered from ordered arrays of atoms and molecules. A lot of information can
be obtained from the angular distribution of scattered intensity at low angles. The elastic
scattering of X-rays (wavelength 0.1. . . 0.2 nm) by a sample that has inhomogeneities at
the nanometer level, is recorded at very low angles, typically, 0.1 –10 . The technique of
SAXS is capable of delivering structural information of macromolecules between 5 nm
and 25 nm, of repeat distances in partially ordered systems of up to 150 nm [35]; USAXS
(ultra-small-angle X-ray scattering) can resolve even larger dimensions.
6.1 Structural characterization of nanofibers
115
We can use SAXS for the determination of the microscale or nanoscale structure of
particle systems in terms of such parameters as averaged particle sizes, shapes, distribution and surface-to-volume ratio. The materials can be solid or liquid and they can
contain solid, liquid or gaseous domains (so-called particles) of the same or another
material in any combination. Not only particles, but also the structure of ordered
systems likes lamellae, and fractal-like materials can be studied. The method is accurate,
nondestructive and usually requires only a minimum of sample preparation.
Small-angle X-ray scattering has also been widely used in characterization of
nanofibers for superstructure investigations [36, 37]. Figure 6.15 shows a SAXS
pattern recorded from a bundle of electrospun PA66 nanofibers. The image does not
reveal any lamellar superstructure (a structure on a scale larger than the unit cell with a
repeating long period). The elliptical shape of the diffuse small-angle scattering
indicates elongated nanostructures, possibly nanofibrils or voids between them. Meltdrawn PA66 fibers exhibit a long period of 100 Å. Therefore, it is believed that the
extremely rapid structural formation and the high draw ratio have no significant effect
on crystallite structure formation within the nanofibers during the electrospinning
process [37].
6.1.7
Mercury porosimetry
Nanofiber membranes have potential applications in many areas including filtration, cell
culture matrix and as battery separators, etc. For such applications, pore structures are
very important characteristics.
A variety of techniques can be used to investigate the continuous porosity of porous
materials. The most direct way is image analysis of TEM or SEM micrographs. But this
technique has a drawback relating to the fact that it provides only a 2-D projection of a
3-D structure. Mercury porosimetry is a well-established technique for the characterization of porous materials. It is widely accepted as a standard measure of total pore
volume and pore size distribution in the macropore and mesopore ranges.
Mercury intrusion measurements are extremely simple in principle, although a
number of experimental complications need to be considered. In the usual procedure,
a small specimen is first dried to empty the pores of any existing fluid. It is then
weighed, transferred to a chamber, which is then evacuated, and mercury is introduced
to surround the specimen. Since mercury does not wet the sample spontaneously, it does
not intrude into empty pores unless pressure is applied. Pressure in progressive increments is then applied to the mercury, and the intrusion of mercury at each step is
monitored. The set of pressure steps and corresponding volumes intruded provides the
basic data for pore size distribution calculations, as shown in Fig. 6.16 [38]. The
analysis of the distribution of the specific surface area and of the specific pore volume
in relation with the pore size from mercury porosimetry data is classically based on
Washburn’s equation [39]:
d¼
4γ cos θ
,
P
ð6:4Þ
116
Characterization of nanofibers
Fig. 6.15 SAXS pattern recorded from a bundle of electrospun PA66 nanofibers [37].
where d is the diameter of the cylinder being intruded, γ is the surface tension of
mercury, θ is the contact angle of mercury on the solid, and P is the applied pressure.
Figure 6.17 shows a representative plot of pore diameter distribution of electrospun
PLGA fibers. The data were obtained from mercury porosimetry measurements. From
this figure, pore diameter distribution, total pore volume, total pore area, and porosity of
the structure were calculated.
6.2
Chemical characterization of nanofibers
6.2.1
Fourier transform infra-red spectroscopy (FTIR)
Fourier transform infra-red spectroscopy is a measurement technique whereby spectra
are collected based on measurements of the temporal coherence of a radiative source,
using time-domain measurements of the infra-red radiation. A Fourier transform infrared (FTIR) spectrometer is a Michelson interferometer with a movable mirror. By
scanning the movable mirror over some distance, an interference pattern is produced
that encodes the spectrum of the source, which turns out to be its Fourier transform. In
its most basic form, a Fourier transform spectrometer consists of two mirrors located at
a right angle to each other and oriented perpendicularly, with a beam-splitter placed at
the vertex of the right angle and oriented at a 45 angle relative to the two mirrors.
Radiation incident on the beam-splitter from one of the two “ports” is then divided into
two parts, each of which propagates down one of the two arms, and is reflected off one
of the mirrors. The two beams are then recombined and transmitted out of the other port.
When the position of one mirror is continuously varied along the axis of the
6.2 Chemical characterization of nanofibers
117
Fig. 6.16 Principle of mercury porosimetry.
Fig. 6.17 Representative plot of pore diameter distribution. Each log differential intrusion value
indicates the relative quantity of pores of a specific diameter [40].
corresponding arm, an interference pattern is swept out as the two phase-shifted beams
interfere with each other. Figure 6.18 illustrates a Michelson (or Fourier transform)
interferometer that forms the basic setup of an FTIR spectrometer.
The energy released by an infra-red photon is small but it is sufficient to excite
vibrations and rotations and other collective motions of the molecules in the sample.
A net change in molecular dipole due to vibration and rotation is required for
absorption of the infra-red radiation. The electric field interacts with the molecular
dipole and absorption of infra-red radiation occurs at discrete frequencies. When there
118
Characterization of nanofibers
Fig. 6.18 A Michelson interferometer.
is no net change in the dipole moment, there is no infra-red absorption. Quantum
h qffiffik
mechanical solution of a harmonic oscillator, which is governed byE ¼ ν þ 12 2π
μ,
yields discrete energy levels, where ν is the vibrational quantum number (integer 0).
The absorption of infra-red radiation involves transition between adjacent energy
levels (Δν ¼ 1). Wavenumbers (cm 1, which are proporational to energy or frequency), are usually used in FTIR. The governing equation for the change in the
qffiffi
h
k
energy due to infra-red absorption is ΔE ¼ 2π
μ ¼ hν. Like any other absorption
spectroscopy, the infra-red absoption spectroscopy relies upon Beer’s law, where
AðλÞ ¼
logTðλÞ ¼
log PPðλÞ
¼ εðλÞbc.
0 ðλÞ
To measure the absorbance, P (intensity of light after passing through sample) and P0
(intensity of incident light) have to be determined. The absorption frequency is related
to the type of bond, whereas the absorption intensity is related to the concentration of
species in the sample. The FTIR spectrum has a group frequency region and a
fingerprint region. The fingerprint region is sensitive to differences in molecular structure, which is useful for identification of molecular structural changes. The group
frequency region is weakly dependent on the environment, which is useful as the
absorbance band for a particular functional group.
There are two operation modes for an FTIR spectrometer: (1) transmission and (2)
reflection. Typically FTIR is performed in the transmission mode using the KBr pellet
method. Alternatively, FTIR can also be done in the reflection mode, where the specular
reflection can be obtained from reflective samples with thin organic coating.
In the reflection mode, P0 is more difficult to quantify. As such, the R0 of an
uncoated surface is measured, followed by the measurement of R of the surface of
the sample of interest. Absorbance of the infra-red radiation is governed by
R0 R
R
A ΔR
10 A [41].
R ¼ R0 ¼ 1
R0 ¼ 1
The limitations of FTIR include: (1) strong IR radiation absorption by H2O and CO2;
(2)a good blank sample is needed because P0 determination is important; and (3)thick
samples cause too much scattering and thus very little light is transmistted.
The technique of FTIR has been widely used to study the changes of the conformational structures that occur to a polymer during the electrospinning process. Ayutsede
6.2 Chemical characterization of nanofibers
119
Fig. 6.19 The FTIR spectra of (1) dialyzed silk fibroin in water, (2) 6% silk fibroin in calcium
chloride solution, (3) degummed silk fiber and (4) 12% silk fibroin in formic acid [34].
et al. used FTIR to compare the structural and concentration changes between pre-spun
fibroin solutions and post-spun fibers of Bombyx mori silk [34]. Figure 6.19 illustrates
the FTIR spectra of the silk fibroin in four different forms.
6.2.2
Raman spectroscopy (RS)
Raman spectroscopy is a spectroscopic technique used in condensed matter physics
and chemistry to study vibrational, rotational, and other low-frequency modes in a
system [42]. It depends on inelastic scattering, or Raman scattering, of monochromatic
light, usually from a laser in the visible, near infra-red, or near ultra-violet range. The
laser light interacts with phonons or other excitations in the system, resulting in the
energy of the laser photons being shifted up or down. The shift in energy gives
information about the phonon modes in the system. Infra-red spectroscopy yields
similar, but complementary information.
Typically, a sample is illuminated with a laser beam (Arþ ion, 488 nm) from
irradiation of CCl4. The emitted radiation includes Rayleigh scattering, Stokes scattering and anti-Stokes scattering. Light from the illuminated spot is collected with a lens
and sent through a monochromator. Wavelengths close to the laser line, due to elastic
Rayleigh scattering, are filtered out while the rest of the collected light is dispersed onto
a detector.
Spontaneous Raman scattering is typically very weak, and as a result the main
difficulty of Raman spectroscopy is separating the weak inelastically scattered light
120
Characterization of nanofibers
Fig. 6.20 Energy level diagram showing the states involved in the Raman signal. The line thickness
is roughly proportional to the signal strength from the different transitions.
from the intense Rayleigh scattered laser light. Raman spectrometers typically use
holographic diffraction gratings and multiple dispersion stages to achieve a high degree
of laser rejection. In the past, photomultiplier tubes were the detectors of choice for
dispersive Raman setups, resulting in long acquisition times. However, the recent use of
CCD detectors has made dispersive Raman spectral acquisition much more rapid.
The Raman effect occurs when light impinges upon a molecule and interacts with
the electron cloud of the bonds of that molecule. The incident photon excites one
of the electrons into a virtual state. For the spontaneous Raman effect, the molecule
will be excited from the ground state to a virtual energy state, and will relax into a
vibrational excited state, which generates Stokes Raman scattering. If the molecule
was already in an elevated vibrational energy state, the Raman scattering is then called
anti-Stokes Raman scattering.
A molecular polarizability change or a certain amount of deformation of the electron
cloud with respect to the vibrational coordinate is required for the molecule to exhibit
the Raman effect. The amount of the polarizability change will determine the intensity,
whereas the Raman shift is equal to the vibrational level that is involved. Figure 6.20
illustrates an energy level diagram showing the states involved in the Raman signal.
Raman spectroscopy is commonly used for chemical structure investigation since
chemical bonds in molecules have specific vibrational information that therefore provides fingerprints by which the molecule can be identified. The fingerprint regions of
organic molecules is in the range 500–2000 cm 1 [43]. Like FTIR, Raman spectroscopy
is capable of following the changes in conformational structure and chemical bondings
of a sample.
The Raman effect is based on polarizability of the bond, which is a measure of
deformability of bond. To have Raman scattering, a momentary distortion of electrons
6.2 Chemical characterization of nanofibers
121
Fig. 6.21 Raman spectra of composite nanofibrils [15]. (a) Raman spectra of original purified
SWCNTs and PLA with 5 wt% SWCNT, obtained using 514.5 nm excitation wavelength, and
(b) Raman spectra of PAN with 4 wt% SWCNT before and after graphitization, using 780 nm
excitation wavelength. The spectra of composite nanofibrils with SWCNT show all the typical
peaks of SWCNT.
in a bond (polarization) is required to create a termporarily induced dipole moment. The
Raman effect, therefore, needs to be a function of the distance between the two nuclei.
Because water is not a strong Raman scatterer, samples with water in them can be
analyzed using Raman spectroscopy. However, samples with a source of fluorescence
can swamp much smaller Raman signals, which is a major problem for Raman
spectroscopy.
Raman spectroscopy has been commonly used for comparing the structural and
concentration changes between pre-spun fibroin solutions and the post-spun fibers [44].
The two modes that are used in Raman spectroscopy are the radial breathing mode
(RBM) and the tangential (stretching) mode. Ko et al. [15] used Raman spectroscopy to
confirm the inclusion of SWCNTs in PAN and PLA nanofibers, see Fig. 6.21.
RBM peaks can be used to estimate the diameter of SWCNT by following this
equation [45, 46]:
ωR ¼ 224 cm 1 =d:
6.2.3
ð6:5Þ
Nuclear magnetic resonance (NMR)
Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the
quantum mechanical magnetic properties of an atomic nucleus. NMR spectroscopy
is one of the principal techniques used to obtain physical, chemical, electronic and
structural information about molecules due to the chemical shift and Zeeman effect on
the resonant frequencies of the nuclei. It is a powerful technique that can provide
detailed information on the topology, dynamics and 3-D structure of molecules in
solution and the solid state. In NMR, a spin interacts with a magnetic or an electric
field. Spatial proximity and/or a chemical bond between two atoms can give rise to
interactions between nuclei. In general, these interactions are orientation dependent. In
media with no or little mobility (e.g. crystals powders, large membrane vesicles,
122
Characterization of nanofibers
molecular aggregates), anisotropic interactions have a substantial influence on the
behavior of a system of nuclear spins. In contrast, in a classical solution-state NMR
experiment, Brownian motion leads to an averaging of anisotropic interactions. In
such cases, these interactions can be neglected on the timescale of the NMR experiment. Consider nuclei which have a spin of one-half, like 1H, 13C or 19F. The nucleus
has two possible spin states: m ¼ ½ or m ¼ ½ (also referred to as up and down or α
and β, respectively). The energies of these states are degenerate – that is to say, they
are the same. Hence the populations of the two states (i.e. number of atoms in the two
states) will be exactly equal at thermal equilibrium. If a nucleus is placed in a
magnetic field, however, the interaction between the nuclear magnetic moment and
the external magnetic field mean the two states no longer have the same energy. The
energy of a magnetic moment μ when in a magnetic field B0 (the zero subscript is used
to distinguish this magnetic field from any other applied field) is given by the negative
scalar product of the vectors:
E¼
B0 μ ¼
μz B0 ¼
mhγB0 :
ð6:6Þ
As a result, the different nuclear spin states have different energies in a non-zero
magnetic field. In hand-waving terms, we can talk about the two spin states of a spin
½ as being aligned either with or against the magnetic field. If γ is positive (true for most
isotopes) then m ¼ ½ is the lower energy state. The energy difference between the two
states is
ΔE ¼ hγB0 :
ð6:7Þ
Resonant absorption will occur when electromagnetic radiation of the correct frequency
to match this energy difference is applied. The energy of a photon is E ¼ hν, where ν is
its frequency. Hence absorption will occur when
ν¼
ΔE γB0
:
¼
2π
h
ð6:8Þ
These frequencies typically correspond to the radiofrequency range of the electromagnetic spectrum. It is this resonant absorption that is detected in NMR [47].
For characterization of electrospun nanofibers, NMR allows the determination of the
various coordinate sites and local environment of specific nuclei. Ohgo et al. used
13
C solid state NMR to identify the structures of the as-spun and chemically treated
Bombyx mori silk fibers [48] Figure 6.22 illustrates the 13C CP/MAS NMR spectra of
nonwoven B. mori silk fibers prepared (a) as-spun and (b) after methanol treatment.
6.3
Mechanical characterization of nanofibers
Mechanical properties of materials describe their characteristic responses to applied
loads and displacements. Mechanical tests serve all aspects of the science and
technology of materials and their utilization. Depending on how the loads are applied,
materials have different deformation modes: tensile, compression, bending, shear and
6.3 Mechanical characterization of nanofibers
123
13
C CP/MAS NMR spectra of nonwoven Bombyx mori silk fibers prepared (a)as-spun
and (b) after methanol treatment [48].
Fig. 6.22
torsion. Since fibers and fiber assemblies are 1-D structures and therefore the main
form of applied loads is stretching, we will discuss only tensile testing of nanofibers
herein.
6.3.1
Microtensile testing of nanofiber nonwoven fabric
Considering an axial tensile test for a fiber of cross-section area A, in which a force F
is applied and the fiber ungoes an elongation Δl comparing the orginal testing length
of l. For comparing the mechanical behavior, the nominal stress σ (¼ F/A, SI unit is
Pa ¼ N/m2) is usually used for object force measurement. In the case of fibers, specific
stress σsp is often introduced by dividing the force by the fiber linear density (ρl), the
accepted practical unit is N/tex or N/denier. The conversion for the specific stress unit to
engineering stress unit can be obtained by simply multiplying specific stress by fiber
density:
σðGPaÞ ¼ σ sp ðN=texÞ densityðg=cm3 Þ:
ð6:9Þ
Two types of curves are used: the load–elongation(F–Δl) curve and the stress–strain
(σ–ε, where ε¼Δl/l) curve. Useful parameters obtained from stress–strain curves
are initial modulus E, strength (the stress at failure σf), strain at failure and toughness εf.
124
Characterization of nanofibers
Fig. 6.23 Yield point of the stress–strain curve.
The yield point (Y) is defined as the point on the stress–strain curve at which the
material begins to deform plastically. The strength at the yield point is called the yield
strength. Before the yield point the material deforms elastically and the initial modulus
can be calculated from Hooke’s law:
E¼
dσ
:
dε
ð6:10Þ
Elastic deformation is recoverable. The deformation above the yield point, known as
plastic deformation, is not recoverable, and the material will not return to its initial
shape. Fibers and fiber assemblies usually do not have an obvious yied point. In order
to locate a precise position, Meredith [49] [2] suggested defining the yield point as
the point at which the tangent to the curve is parallel to the line joining the origin to the
breaking point, as in Fig. 6.23a. Coplan [50] defined the yield point as occurring
at the stress given by intersection point of the tangent at the origin with the tangent
having the least slope, as shown in Fig. 6.23b. Alternatively, when there are considerable linear regions both above and below the yield region, the point of intersection of
the tangents may be taken as the yield point.
Toughness can be determined by measuring the area underneath the stress–strain
curve. It is defined as the energy of mechanical deformation per unit volume prior to the
fracture of material. Toughness describes the ability to absorb mechanical or kinetic
energy up to failure. Its mathematical description is:
ð εf
Energy
K¼
σdε:
ð6:11Þ
¼
Volume
0
In the case of nonwoven nanofibrous structures the specific stress can be determined by
first obtaining the membrane stress (force/specimen width) and then obtain the membrane stress by areal density as shown below. Please note the units to be used in order to
obtain the linear density unit. For example, in order to express specific stress in N/tex:
σ sp ðN=texÞ ¼
forceðNÞ
1
:
widthðmmÞ areal densityðg=m2 Þ
ð6:12Þ
For microtensile testing of nanofiber nonwoven fabric, the KES-G1 microtensile tester
is one of the more commonly used microtensile testing devices, as shown in Fig. 6.24.
6.3 Mechanical characterization of nanofibers
125
Fig. 6.24 A KES-G1 microtensile tester.
The outputs of KES-G1 are voltage and time, which correspond to force (stress) and
displacement (strain), respectively. The ultimate tensile strength and its corresponding
strain can be obtained through the microtensile tester. Figure 6.25 shows the stress–
strain curves of nonwoven silk fibers reported by Ohgo et al. [48].
6.3.2
Mechanical testing of a single nanofiber
The AFM cantilever can also be used to characterize mechanical properties such as the
strength and stiffness of nanofibers. By clamping the nanofiber between an AFM tip and
a tungsten wire, a tensile test can be conducted on the fiber [51]. Figure 6.26 shows an
SEM image of a carbonized nanofiber clamped between an AFM tip and a tungsten
wire. Figure 6.27 shows a stress–strain curve of a PAN single nanofiber.
Pushing an AFM tip into nanofibers to apply a transverse compression load on the
fiber can be a method for evaluation of the elastic modulus of a single nanofiber based
on the approach of Kracke and Damaschke [15]. This method utilizes the following
relationship:
dF=dðΔzÞ ¼ 2=π 1=2 E * A1=2 ,
ð6:13Þ
where F is the normal force, d is the tube diameter, Δz is the indentation depth, A is the
contact area and E* is the effective Young’s modulus of the contact as defined by
126
Characterization of nanofibers
Fig. 6.25 Stress–strain curves of nonwoven silk fibers after methanol treatment: (a) B. mori silk and
(b) S. c. ricini silk. The stress–strain experiments were performed twice for each fiber [48].
Fig. 6.26 An SEM image of a carbonized nanofiber clamped between an AFM tip and a tungsten
wire. A smaller AFM cantilever on the same AFM chip is present in the background [51].
1=E * ¼ 1
ν21 =E1 þ 1
ν22 =E2 :
ð6:14Þ
Here E1, E2, ν1 and ν2 are the elastic moduli and the Poisson’s ratios of the sample and
the tip respectively.
Figure 6.28 shows the load indentation curves from a set of stabilized experiments
measured with CNT/PAN and neat PAN fibers placed on a mica substrate.
The AFM technique can also be used to characterize the roughness of fibers. The
roughness value is the arithmetic average of the deviations of height from the central
horizontal plane given in terms of millivolts of measured current. [20].
6.3 Mechanical characterization of nanofibers
127
Fig. 6.27 Engineering strain–stress curve of an electrospun PAN nanofiber [52].
Fig. 6.28 Load indentation curves from a set of experiments [15].
As is well known, AFM is not specially designed for the mechanical testing of
nanofibers. Therefore there is strong demand for a nanotensile tester specialized for
the testing of nanoscaled fibrous materials. Several companies have realized that this
strong demand implies a promising market in both academic and industrial areas, and
have invented some nanotensile instruments such as the Agilent T150 universal testing
machine (UTM) (MTS Nano Bionix 858) and the Hysitron nanoTensile 5000. These
instruments offer superior means of nanomechanical characterization and enable
researchers to understand dynamic properties of compliant fibers via the largest
dynamic range in the industry.
Here we take the T150 UTM as an example. The T150 system from Agilent
Technologies, as pictured in Fig. 6.29a, is a universal testing machine. It employs a
nanomechanical actuating transducer (NMAT) head to produce tensile force (load on
sample) using electromagnetic actuation combined with a precise capacitive gauge,
delivering outstanding sensitivity over a large range of strain. Figure 6.29b shows a
schematic of the NMAT. During tensile elongation, the T150 UTM holds the NMAT
head stationary and moves the crosshead, providing a very stable system that ensures
the lowest noise floor. The force applied is determined by the current applied to the
NMAT’s voice coil, which pulls the test specimen downward. The displacement is then
128
Characterization of nanofibers
Fig. 6.29 (a) The Agilent T150 universal testing machine (UTM); (b) nanomechanical actuating
transducer (NMAT) head of the Agilent T150 UTM.
measured by the NMAT’s capacitive gauge. The NMAT provides a maximum linear
displacement of 1 mm with resolution of <0.1 nm and a maximum load of 500 mN
with resolution of 50 nN. The T150 is equipped with a micropositioner stage, which is a
helpful sample guide that ensures the sample is orthogonal and aids in the positioning of
the upper grip. It is capable of testing in three-point bend, four-point bend, compression
and tension. In addition, an indentation kit including an inversion footer is available,
allowing the system to be used as an indenter. The whole system is usually placed on an
antivibration table for vibration isolation and enclosed to minimize the air flow influence during testing.
Tan et al. [53] performed tensile tests on single electrospun PCL microfibers using the
MTS Nano Bionix 858, the predecessor of the Agilent T150 UTM. Aligned electrospun
fibers were collected and prepared for tensile tests. The PCL microfibers exhibited the
characteristic low strength and low modulus but high extensibility nature of PCL at
room temperature. The mechanical properties were found dependent on fiber diameter,
as shown in Fig. 6.30. The fibers with smaller diameter had higher strength but lower
ductility due to the higher “draw ratio” applied to the fiber during electrospinning. Ko’s
group drew the same conclusion in their investigation of tensile stress-strain response of
small-diameter electrospun PCL fibers using the Agilent T150 UTM [54].
6.4
Thermal analysis
Thermal properties are important for material engineering. For thermal analysis, several
methods are commonly used: differential thermal analysis (DTA, temperature difference), differential scanning calorimetry (DSC, heat difference), thermogravimetric
analysis (TGA, mass), thermomechanical analysis (TMA, dimension), dilatometry
6.4 Thermal analysis
129
Fig. 6.30 Plot of stress against strain for electrospun PCL microfibers at various fiber
diameters [53].
(DIL, volume), dynamic mechanical analysis (DMA, mechanical stiffness and
damping), dielectric thermal analysis (DEA, dielectric permittivity and loss factor),
evolved gas analysis (EGA, gaseous decomposition products) and thermo-optical
analysis(TOA, optical properties). Among those methods, TGA and DAS are the two
most-used, and they will be introduced in this section.
6.4.1
Thermogravimetric analysis (TGA)
The technique of TGA measures changes in sample mass with temperature by using a
thermobalance, which is sometimes referred to as a thermogravimetric analyzer. One
should note that mass is a measure of the amount of matter in a sample, whereas weight
refers to the effect of the gravitational force on a mass, and thus varies from one
geographical location to another. A thermobalance is a combination of a suitable
electronic microbalance with a furnace, a temperature programmer and computer for
control, that allows the sample to be simultaneously weighed and heated or cooled in a
controlled manner, and the mass, time, and temperature data to be captured. The balance
should be in a suitably enclosed system so that the nature and pressure of the atmosphere surrounding the sample can be controlled. Figure 6.31 shows a schematic
thermobalance.
Results from TGA experiments can be presented in a variety of graphical ways. Mass
or mass percent is usually plotted as the ordinate (y-axis) and temperature or time as the
abscissa (x-axis). Mass percent has the advantage that results from different experiments
can be compared on normalized sets of axes. When time is used as the abscissa, a
second curve of temperature versus time needs to be plotted to indicate the temperature
programme used [55].
Actual TGA curves obtained may be classified into various types, as shown in
Fig. 6.32. Possible interpretations of the curves can be found in Ref [55].
Knowledge of the thermal stability range of materials provides information on
problems such as the hazards of storing explosives, the shelf-life of materials and the
130
Characterization of nanofibers
Fig. 6.31 A schematic thermobalance [55].
conditions for drying them. By using an atmosphere of air or oxygen, the conditions
under which oxidation and degradation of polymers become catastrophic can be
determined. The TG curves for more complex materials, composite polymers, are not
always immediately interpretable in terms of the exact reactions occurring. Such curves
can, however, be used for “fingerprint” purposes.
As observed in Fig. 6.33, pure PVA fibers exhibited two weighted-loss steps. The
weight loss around 350 C and 450 C was considered to reflect the decomposition of
side chain (T-ds) and main chain (T-dm) of PVA, respectively. However, for PVA/silica
fibers, three degradation steps could be observed. The first step was between 50 C and
300 C, the second step was between 300 C and 400 C, and the third step was above
400 C. The first weight loss process, which was associated with the loss of absorbed
moisture and/or with the evaporation of trapped solvent (H2O or C2H5OH from TEOS),
was independent of the composition for all samples. The second weight loss process
corresponded to the degradation of PVA by dehydration on the polymer side chain,
whose decomposition temperature was around 350 C. In the third weight loss process,
the polymer residues were further degraded at approximately 450 C, corresponding to
the decomposition of main chain of PVA (T-dm). As seen in Fig. 6.33, the degradation
temperature was ambiguous with the increase of the silica. These suggested that some
bonding between the polymer and silica formed [56].
6.4 Thermal analysis
131
Fig. 6.32 Main types of TG curves [55].
6.4.2
Differential scanning calorimetry (DSC)
Differential scanning calorimetry is a thermoanalytical technique in which the
difference in the amount of heat required to increase the temperature of a sample
and reference is measured as a function of temperature. Both the sample and
reference are maintained at nearly the same temperature throughout the controlledtemperature programme. Any difference in the independent supplies of power to the
sample and the reference is then recorded against the programmed (reference)
temperature. Generally, the temperature program for a DSC analysis is designed
such that the sample holder temperature increases linearly as a function of time. The
reference sample should have a well-defined heat capacity over the range of temperatures to be scanned. The apparatus is shown schematically in Fig. 6.34, and a
schematic DSC curve demonstrating the appearance of several common features
shows in Fig. 6.35.
Thermal events in the sample thus appear as deviations from the DSC baseline,
in either an endothermic or exothermic direction, depending upon whether more or
132
Characterization of nanofibers
Fig. 6.33 TG curves of various PVA/silica fibers with different silica content: (a) 0 wt% (pure
PVA), (b) 22 wt%, (c) 34 wt%, (d) 40 wt%, (e) 49 wt%, (f) 59 wt% [56].
Fig. 6.34 Schematic DSC.
less energy has to be supplied to the sample relative to the reference material. In
DSC, endothermic responses are usually represented as being positive, i.e. above the
baseline, corresponding to an increased transfer of heat to the sample compared to
the reference [55]. By observing the difference in heat flow between the sample and
reference, differential scanning calorimeters are able to measure the amount of heat
absorbed or released during a physical transformation such as phase transitions. DSC
may also be used to observe more subtle phase changes, such as glass transitions.
6.4 Thermal analysis
133
Fig. 6.35 Schematic DSC curve.
The result of a DSC experiment is a curve of heat flux versus temperature or versus
time. This curve can be used to calculate enthalpies of transitions. This is done by
integrating the peak corresponding to a given transition as expressed by [57]
ΔH ¼ KA,
ð6:15Þ
where ΔH is the enthalpy of transition, K is the calorimetric constant and A is the area under
the curve. The calorimetric constant will vary from instrument to instrument, and can be
determined by analyzing a well-characterized sample with known enthalpies of transition
The technique of DSC is used widely for examining polymers to check their
composition. Melting points and glass transition temperatures for most polymers are
available from standard compilations, and the method can show up possible polymer
degradation by the lowering of the expected melting point, Tm, for example. The
percentage crystallinity of a polymer can also be found by using DSC since the heat
of fusion can be calculated from the area under an absorption peak from the DSC graph.
Impurities in polymers can be determined by examining thermograms for anomalous
peaks, and plasticizers can be detected at their characteristic boiling points.
Figure 6.36 shows the DSC thermograms of semi-crystalline PLLA resin and
electrospun PLLA membranes dried under vacuum at room temperature, and of a
quenched sample, which was prepared by first melting PLLA at 220 C and then rapidly
quenching with ice water. It was found that the as-received PLLA exhibits a crystallinity of 35.5%, whereas the electrospun PLLA membrane exhibits barely any crystallinity. The electrospun membrane shows a large crystallization peak at 103 C, but the
quenched samples does not show any apparent crystallization peak under the temperature scan with the same heating rate (20 C/min), which suggested that cold crystallization of the electrospun membranes during heating is enhanced, probably due to the
chain orientation. The decrease in glass transition temperature can be attributed to
the very large surface-to-volume ratio of the electrospun membranes having air as the
134
Characterization of nanofibers
Fig. 6.36 DSC thermograms of as-received PLLA (line A); electrospun PLLA dried at room
temperature (line B) and quenched PLLA in ice water (line C) [36].
plasticizer. The extent of crystallinity in electrospun PLLA membrane is very low,
indicating that the majority of the chains are in the non-crystalline state. This was
explained by the rapid solidification process of stretched chains under a high elongation
rate during the later stages of electrospinning hindering the development of crystallinity,
as the chains do not have time to form crystalline registration [36].
6.5
Characterization of other properties
6.5.1
Wettability and contact angle
Wettability is a very important property of nanofiber products when the applications
relate to fluids in such fields as filtration and bioengineering with regards to the
requirements set by converting and end use.
Consider a drop of water resting on a horizontal membrane, the drop of water will
adopt a position between completely spreading out on the surface or a round drop
resting lightly on the surface. Between these two extremes, the general shape of the drop
will exhibit a measurable contact angle. The force exerted by the air–water interfacial
tension acts at a tangent to the surface of the drop with its horizontal component, pulling
the circumference of the drop toward its center. The water–solid interfacial tension pulls
the circumference of the water drop area of contact with the solid toward the center. At
the equilibrium these two forces are balanced by the air–solid interfacial tension acting
to pull the circumference of the drop’s area of contact away from its center [58].
Figure 6.37 shows the schematic of the wetting of solid surfaces. The surface is
water-wet (hydrophilic) when the contact angle is less than 90 and the pressure
difference across the interface is positive; it is neutral-wet at contact angles close to
90 ; and the surface is hydrophobic for angles greater than 90 . The contact angle and
interfacial tensions of a sessile drop are related according to Young’s equation:
6.5 Characterization of other properties
135
Fig. 6.37 Schematic of wettability.
Fig. 6.38 Schematic of a pc-based wettability system.
Fig. 6.39 Wettability of (a) an electrospun PLA nanofiber, (b) a 1 wt% chitosan coated electrospun
PLA nanofiber and (c) an 8 wt% chitosan coated electrospun PLA nanofiber
γs ¼ γsl þ γl cos θ:
ð6:16Þ
The wettability of a nanofiber product can be tested by measuring the contact angle.
The TRI Wettability System is a pc-based wettability characterization system.
Figure 6.38 shows the schematic of a pc-based system. It provides a visualization
system suitable for the characterization of wettability and absorbency of porous fibers/
fiber bundles/yarns; has the ability to obtain dynamics of complete droplet shape for
slow and fast absorption processes; and from the source data and absorption process
modeling, structural and transport parameters of nanofibrous system can be extracted.
Distilled water and/or different fluid droplets can be utilized for the measurement.
Figure 6.39 shows the images captured with a pc-based wettability characterization
system. The contact angle of an electrospun PLA nanofiber decreased from 119.05 to
60.84 as the weight ratio of coated chitosan increased from 0% to 8%.
136
Characterization of nanofibers
Fig. 6.40 Schematic of 4-point probe configuration.
6.5.2
Electrical conductivity
For general purpose resistance measurements and current–voltage (I–V) curve generation, 2-point electrical measurements are normally used. However, when the resistance
being measured is relatively low, or the resistance of the probes or the contacts is
relatively high, a 4-point probe will yield more accurate results.
The purpose of the 4-point probe is to measure the resistivity of any semiconductor
material. It can measure either bulk or thin film specimen. A typical 4-point probe setup
consists of four equally spaced tungsten metal tips with finite radius. Each tip is
supported by springs on the other end to minimize sample damage during probing.
The four metal tips are part of an auto-mechanical stage that travels up and down during
measurements. A high-impedance current source is used to supply current through the
outer two probes; a voltmeter measures the voltage across the inner two probes to
determine the sample resistivity, as shown in Fig. 6.40. Typical probe spacing s is
around 1 mm.
For a very thin film (thickness t << s), assume that the metal tip is infinitesimal,
samples are semi-infinite in lateral dimension and a ring protrusion of current emanaties
from the outer probe tips. The differential resistance is:
dx
ΔR ¼ ρ
,
ð6:17Þ
A
where ρ is the resistivity and A is the area of the ring, which is expressed as A ¼ 2πxt.
The integration between the inner probe tips will be:
R¼
xð2
x1
dx
ρ
¼
2πxt
2ðs
s
ρ dx
ρ
¼
ln 2:
2πt x
2πt
Consequently, for R¼V/2I, the resistivity for a thin sheet is:
ð6:18Þ
6.5 Characterization of other properties
πt V
:
ρ¼
ln 2 I
137
ð6:19Þ
This expression is independent of the probe spacing s. In general, the sheet resistivity
can be expressed as:
Rs ¼ ρ=t ¼ k ðV=I Þ,
ð6:20Þ
where k is a geometric factor that will be different for different samples.
As a simple and useful tool, the 4-point probe system has been widely used to
measure the electrical conductivity (resistivity) of nanofiber membranes [59, 60].
6.5.3
Electrochemical properties
6.5.3.1
Linear-sweep voltammetry and cyclic voltammetry
The terms voltammetry covers a range of techniques involving the application of a
linearly varying potential between a working electrode and a reference electrode in an
electrochemical cell with electrolyte in between. The current through the cell is monitored continuously. A graph is traced on a recorder of current against potential, this is
known as a voltammogram [61].
The most straightforward technique is linear-sweep voltammetry (LSV). In linearsweep voltammetry (LSV) the current at a working electrode is measured while the
potential between the working electrode and the reference electrode is swept linearly in
time. The voltage is scanned from a lower limit to an upper limit, and the current response
is plotted as a function of voltage. The characteristics of the linear-sweep voltammogram
recorded depend on a number of factors including the rate of the electron transfer reaction
(s), the chemical reactivity of the electroactive species and the voltage scan rate.
Choi et al. [62] used linear-sweep voltammetry to determine the electrochemical
stability of electrospun poly(vinylidene fluoride) nanofibrous membrane. The measurement was done with a three-electrode electrochemical cell consisting of a nickel
working electrode, a lithium reference, and a counterelectrode. The measurement was
made at room temperature, the potential was scaned between 2.0 V and 5.0 V and the
scan rate was 1 mV/s. The oxidation peak of the PVDF nanofibrous electrolyte was
observed at about 4.5 V, which is 0.6 V higher than the value of 3.9 V for free liquid
electrolyte, as shown in Fig. 6.41.
Compared with LSV, cyclic voltammetry (CV) takes the experiment a step further. In
a cyclic voltammetry experiment, the working electrode potential is ramped linearly
versus time, as in LSV, and when it reaches a set potential, the working electrode’s
potential ramp is inverted. This inversion can be repeated multiple times during a single
experiment. The current at the working electrode is plotted against the applied voltage
to give the cyclic voltammogram trace. The utility of cyclic voltammetry is highly
dependent on the analyte being studied. The analyte has to be redox active, and is
highly desired for it to be able to display a reversible wave. When an analyte is reduced
or oxidized on a forward scan and is then reoxidized or rereduced in a predictable way
on the return scan, a reversible wave will be obtained.
138
Characterization of nanofibers
Fig. 6.41 Linear-sweep voltammogram of PVDF nanofibrous polymer electrolyte [62].
For a reversible electrochemical reaction the CV recorded has certain well-defined
characteristics.
(I)
The voltage separation between the current peaks is
ΔE ¼ E pa
E pc ¼
59
mV,
n
ð6:21Þ
where, Epa and Epc are, respectively, the oxidation and reduction peak potentials.
(II) The positions of peak voltage do not alter as a function of the voltage scan rate.
(III) The ratio of the peak currents is equal to one
ipa
¼ 1,
ipc
ð6:22Þ
where ipa and ipc are, respectively, the oxidation and reduction peak currents.
(IV) The peak currents are proportional to the square root of the scan rate
pffiffiffiffi
ipa / pffiffiffi
Vffi
ipc / V ,
ð6:23Þ
where V is the scan rate.
Because of the interest in developing supercapacitors from electrospun nanofiber
electrolytes, CV has been actively employed in the characterization of electrochemical
properties of nanofibers [63–66] Lee et al. [67] used CV to evaluate the electrochemical
performance of electrospun carbon nanofiber (CNF) and polypyrrole (Ppy) coated
electrospun CNF with GC/Ppy; the CV curves are as shown in Fig. 6.42. The CV
curves show that the addition of Ppy improves the electrode capacitance dramatically.
6.5 Characterization of other properties
139
Fig. 6.42 CV curves of CNF and CNF/Ppy [67].
6.5.3.2
Chronopotentiometry
Chronopotentiometry (CP) involves the study of voltage transients at an electrode upon
which a constant current is imposed [68]. It is also known as galvanostatic voltammetry.
In this technique, a constant current is applied to an electrode, and its voltage response
indicates the changes in electrode processes occurring at its interface. Considering
the electron transfer reaction O þ e ¼ R, for example. Before the current step, the
concentration of O at the electrode surface is the same as in the bulk solution. The initial
potential is the rest potential or the open circuit potential (Eoc). Once the (reducing)
current step has been applied, O is reduced to R at the electrode surface in order to
support the applied current, and the concentration of O at the electrode surface therefore
decreases. This sets up a concentration gradient for O between the bulk solution and
the electrode surface, and molecules of O diffuse down this concentration gradient
to the electrode surface. The potential is close to the redox potential for O þ e ¼ R,
and its precise value depends upon the Nernst equation:
E ¼ E0 þ
0:059
Cs
log Os ,
n
CR
ð6:24Þ
where CsO and C sR are the surface concentrations of O and R, respectively. These
concentrations vary with time, so the potential also varies with time, which is reflected
in the finite slope of the potential vs. time plot at this stage. Once the concentration
of O at the electrode surface is zero, the applied current can no longer be supported
by this electron transfer reaction, so the potential changes to the redox potential of
another electron transfer reaction. If no other analyte has been added to the solution,
the second electron transfer reaction will involved reduction of the electrolyte; that is,
there is a large change in the potential.
Chronopotentiometry is a common tool in the study of electrochemical storage and
energy conversion devices (e.g. fuel cells, batteries and super capacitors). Lee et al. [67]
conducted a series of CP measurements on electrospun CNF and Ppy coated CNF
electrodes at different current densities. The result is as shown in Fig. 6.43. They also
used the CP results to calculate the specific capacitance of Ppy. The equation used is:
specific capacitance (F/g) ¼ I /m / (dV/dt), where I is the current applied (A), m is the
140
Characterization of nanofibers
Fig. 6.43 CP results of CNF/Ppy [67].
Fig. 6.44 Specific capacitance calculated from CP data vs current density [67].
mass of Ppy (g), and dV/dt is the slope (V/s) of the curves. The calculated specific
capacitance as a function of current density from both electrodes is summarized in
Fig. 6.44. At the same discharge rate of 0.05 A/g, the specific capacitance is 117 F/g for
CNF/Ppy and 132 F/g for GC/Ppy. These capacitances are relatively high compared to
previous studies incorporating Ppy with single (or multi)-walled carbon nanotubes in
aqueous electrolyte. The result also demonstrated that Ppy is able to retain its intrinsic
capacitance in the fibrous form.
6.5.4
Magnetic properties
Apply an external magnetic field, H, to a material, the material will respond to the
magnetic field by generating a flux, i.e. the materials will be magnetized. The relation
between the magenitc field and the flux density, known as induction, B, is described as:
B ¼ μH ¼ μ0 H þ M,
ð6:25Þ
6.5 Characterization of other properties
141
Fig. 6.45 A typical hysteresis loop of a ferromagnetic material.
where μ0 is the permeability of free space, μ is the absolute permeability and M is the
magnetization. And the magnetic susceptibility, χ, is defined to describe the relation
between magnetization and magnetic field
M ¼ χH,
ð6:26Þ
μ ¼ μ0 ð1 þ χÞ:
ð6:27Þ
which gives
In most materials the permeability is very close to unity and typically differs from unity
by a few parts per 105. Materials that have μ < 1 are diamagnetic, and paramagnetic
materials have μ > 1. For ferromagnetic materials, the value of μ can be very large.
Vibrating sample magnetometer (VSM) systems are usually used to measure the
magnetic properties of materials as a function of magnetic field, temperature and time.
This technique involves vibrating the sample at low frequencies (e.g. 80 Hz) in the
gradient region of a copper detection coil and using a lock-in amplifier to detect
the response. These instruments are capable of achieving sensitivities down to about
10 9 Am2 for a second average. Powders, solids, liquids, single crystals and thin films are
all readily accommodated in a VSM. The sensitivity, accuracy, speed and ease of use of
VSMs have made them the most widely used instrument for magnetic characterization
of materials. Generally a hysteresis loop is measured to give the relationship between the
magnetization and the applied magnetic field H. The parameters extracted from the
hysteresis loop are used to characterize the magnetic properties of materials, including
the saturation magnetization Ms, the resonance Mr, the coercivity Hc, the squareness ratio
SQR, μ, defined by the slope of the curve, and the switching field distribution SFD.
Figure 6.45 shows a typical hysteresis loop of a ferromagnetic material. The squareness
ratio is essentially a measure of how square the hysteresis loop is. The formal definition of
the coercivity Hc is the field required to reduce the magnetization to zero after saturation.
Hc is a very complicated parameter for magnetic films and is related to the reversal
142
Characterization of nanofibers
Fig. 6.46 Magnetic properties of the electrospun CoFe2O4 nanoribbons and nanofibers at low
temperature (2 K) [69].
mechanism and the magnetic microstructure, i.e. shape and dimensions of the crystallites,
nature of the boundaries, and also the surface and initial layer properties, etc.
Wang et al. [69] measured the magnetic properties of CoFe2O4 nanofibers calcinated
at different temperatures. The hysteresis loop is as shown in Fig. 6.46. As the figure
suggests, saturation magnetizations of these fibers decrease with decreasing calcination
temperature due to the decrease of CoFe2O4 nanoparticles which determines the
magnetic properties of the fibers. The SQR (Mr/Ms) for nanofibers also increased with
the increase of temperature. The Mr/Ms for nanofibers calcinated at 600℃ is equal to the
theoretical value, and the values for nanofibers calcinated at 700℃ and 900℃ are 0.86
and 0.89, respectively, which are higher than the theoretical value. This indicates that
such 1-D nanostuctures consist of randomly oriented equiaxial particles with cubic
magnetic anisotropy.
With a SQUID (superconducting quantum interference device) magnetometer–
susceptometer (SQUID-MS), both accuracy and sensitivity can be achieved during
measurements of magnetism and magnetic properties of materials. A SQUID-MS
combines several superconducting components, including a SQUID, superconducting
magnet, detection coils, flux transformer and superconducting shields. For a measurement, a sample, typically less than a few millimeters in size, is attached to a sample rod
and scanned through the center of a first- or second-order superconducting gradiometer,
which forms a closed flux transformer that is coupled to a SQUID. The signal from the
SQUID is then recorded as a function of sample position. The shape and magnitude of
the response curve can then be analyzed for a corresponding magnetic moment.
SQUID-MS systems typically offer the ability to measure in applied fields of up to 7
T over a temperature range from above room temperature down to below 2 K [70].
Bayat et al[71, 72] measured the magnetic properties of electrospun Fe3O4/carbon
composite nanofibers using a SQUID-MS (Quantum Design, CA, USA). The magnetic
moment (emu) as a function of applied magnetic field (Oe) at room temperature (300 K)
References
A10F700
A10F900
A10F
Fe3O4
40
20
0
Magnetization (emu /g)
Magnetization (emu /g)
60
143
–20
–40
3
2
1
0
–1
–2
–3
–1000
–60
–500
0
500
1000
Magnetic Field (G)
–20000
–10000
0
10000
20000
Magnetic Field (G)
Fig. 6.47 Magnetic hysteresis loop of Fe3O4 nanoparticles and electrospun Fe3O4/carbon
composite nanofibers [71].
is shown as Fig. 6.47. The Ms of pristine electropun PAN nanofibers with 10 wt% Fe3O4
increased from 5.2 emu/g to 12 emu/g and 16 emu/g after carbonization at 700 C and
900 C, respectively. At the same time, the Hc value increased from 105 Oe for pristine
sample to 500 Oe and 600 Oe, respectively. The inset shown in Fig. 6.47 exhibits the
hysteresis in pristine electrospun sample in comparison to Fe3O4 nanoparticles. This
results proved that the electrospinning method was capable of translating magnetic
behavior of Fe3O4 nanoparticles into the nanofibrous structure due to relatively uniform
distribution of Fe3O4 nanoparticles in the matrix.
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7
Bioactive nanofibers
7.1
The development of biomaterials
A biomaterial has been defined by Hench and Erthridge as a synthetic material used to
replace a part or a function of the body in a safe, reliable, economic and physiologically
acceptable manner. The Celmson University Advisory Board for Biomaterials has
formally defined a biomaterial to be “a systemically and pharmacologically inert
substance designed for implantation within or in a medical device, intended to interact
with biological systems.” Biomaterials have been widely used in many areas, including
replacement of damaged parts (artificial hip), assisting in healing (sutures), improving
biological functions (pacemaker, contact lens), correcting abnormalities (spinal rods),
cosmetics (augmentation mammoplasty), aiding diagnoses (probes) and aiding treating
(catheters). A material is considered biocompatible if it causes no irritation, allergic or
toxic responses when used in a biological system [1] Table 7.1 provides some examples
of biomaterials used in the body.
Biotechnology and nanotechnology are the two of twenty-first century’s most promising technologies. Convergence of these two technologies is expected to create innovations and play a vital role in various biomedical applications. The symposium in 2000
entitled “Nanoscience and Technology: Shaping Biomedical Research” held by the
National Institutes of Health Bioengineering Consortium (BECON) addressed eight
areas of nanoscience and nanotechnology, which include synthesis and use of nanostructures, applications of nanotechnology to therapy, biomimetic and biologic nanostructures, electronic–biology interface, devices for early detection of disease, tools for
the study of single molecules, nanotechnology and tissue engineering [2].
In a living system, the characteristics of a biological tissue are based on a hierarchical
classification system of the levels of its structure – the nanoscale fundamental building
blocks, and the fibrous form of the fundamental building blocks. The convergence of
biomaterial science and nanotechnology elicits specific interactions with cell integrins
and thereby direct cell proliferation, differentiation and extracellular matrix production
and organization [3]. Reducing the fiber diameter can proportionally increase the ratio
of the exposed polymer chains together with its functional groups. Polymer nanofibers
can provide a proper route to emulate or duplicate biosystems – a biomimetic approach.
On the other hand, cells attach and organize well around fibers with diameters smaller
than themselves. Many researchers [4–9] have shown evidence that, apart from surface
chemistry, the nanoscale surface features and topography also have important effects on
147
148
Bioactive nanofibers
Table 7.1 Materials for use in the body
Materials
Advantages
Disadvantages
Examples
Polymers (nylon, silicone
rubber, polyester,
polytetrafuoroethylene, etc.)
Metals (Ti and its alloys,
Co–Cr alloys, stainless steels,
Au, Ag, Pt, etc.)
Ceramics (aluminum oxide,
calcium phosphates including
hydroxyapatite, carbon)
Composites (carbon–carbon,
wire or fiber reinforced bone
cement)
Resilient, easy to
fabricate
Deforms with
time, may
degrade
May corrode,
dense, difficult
to make
Brittle, not
resilient,
difficult to make
Difficult to
make
Sutures, blood vessels, hip
socket, ear, nose, other soft
tissues,
Joint replacements, bone plates
and screws, dental root
implants, pacer and suture wires
Dental; femoral head of hip
replacement, coating of dental
and orthopedic implants
Joint implants, heart valves
Strong, tough,
ductile
Very biocompatible,
inert, strong in
compression
Strong, tailor-made
regulating cell behavior in terms of cell adhesion, activation, proliferation, alignment
and orientation. Recently, electrospinning has been widely studied as an enabling
technology to develop bioactive nanofibers for various biomedical applications including tissue engineering, drug delivery and biosensors, as shown in Fig. 7.1.
7.2
Bioactive nanofibers
7.2.1
Nanofibers for tissue engineering
7.2.1.1
Extracellular matrices for tissue engineering
Over 20 years ago, tissue engineering had already been recognized by the US National
Science Foundation as an emerging area of national importance [10]. Tissue engineering is a promising approach to treat the loss or malfunction of a tissue or organ without
the limitations of current therapies. Tissue engineering typically uses exogenous 3-D
extracellular matrices (ECMs) to engineer new natural tissues from isolated cells. This
approach is based on the following observations [11]. (1) Every tissue undergoes
remodeling. (2) Isolated cells tend to reform the appropriate tissue structure under
appropriate conditions. (3) Although isolated cells have the capacity to form the
appropriate tissue structure, they do so only to a limited degree when placed as a
suspension into tissue. Such cells begin without any intrinsic organization and have no
template to guide restructuring. (4) Tissue cannot be implanted in large volumes; cells
will not survive if they are located more than a few hundred micrometers from the
nearest capillary. Thus, the ECMs are designed so that the scaffold guides cell organization and growth and allows diffusion of nutrients to the transplanted cells. Figure 7.2
shows how the ECMs work as tissue engineering scaffolds.
Synthetic ECMs provide an adhesion substrate for transplanted cells and serve as a
delivery medium into specific sites in the body. Highly porous matrices with a large
surface-area-to-volume ratio are desirable in order to allow delivery of a high density of
7.2 Bioactive nanofibers
149
Fig. 7.1 Biomedical applications of polymer nanofibers.
cells and maintain a potential space for tissue development. The ECMS shoud be able to
promote the ingrowth of blood vessels to facilitate metabolic transport to and from
surrounding native tissue since vascularization of engineered tissues is critical, as cells
more than approximately 200 μm from a blood supply are either metabolically inactive
or necrotic, owing to the limitation of nutrient diffusion (e.g. oxygen). It is possible to
control the vascularization of engineered tissue by controlling the fiber diameter,
porosity and pore size of the synthetic ECM and by incorporating angiogenic factors [2].
7.2.1.2
Nanofiber scaffolds for tissue engineering
The goal in developing new tissue engineering technology is to obtain a biocompatible
3-D platform that can act as a temporary host for tissue cells as they attach, proliferate
and differentiate into the specific tissue that needs to be repaired. Nonwoven nanofiber
mats are widely known for their porous, interconnected, 3-D structures and relatively
large specific surface areas, rendering nanofibers an ideal form of materials to mimic the
natural ECM required for tissue engineering [13]. Figure 7.3 outlines the various aspects
of a tissue engineering nanofibers scaffold system. The nanofiber scaffold can bind cells
and act as the platform for their activity, which is very similar to the extracellular
matrices in native tissue providing cell anchorage through ligands and influencing cell
activity. On top of that, the scaffold can also protect the bioactive molecules. Additionally, the large number of biocompatible polymers ready for nanofiber fabrication is a
driving force behind this combination of nanotechnology and biomedical technology.
Nanofibers are unique in that they mimic the extracellular biological matrices (ECM)
of tissues and organs. Studies of cell–nanofiber interactions have shown that cells adhere
and proliferate well when cultured on polymer nanofibers [15–17]. Khil et al. [18]
150
Bioactive nanofibers
Fig. 7.2 A tissue engineering concept involves seeding cells within porous biomaterial scaffolds
[12]. (a) Cells are isolated from the patient and may be cultivated (b) in vitro on 2-D surfaces for
efficient expansion. (c) Next, the cells are seeded in porous scaffolds together with growth factors,
small molecules, and micro- and/or nanoparticles. The scaffolds serve as a mechanical support
and a shape-determining material, and their porous nature provides high mass transfer and waste
removal. (d) The cell constructs are further cultivated in bioreactors to provide optimal conditions
for organization into a functioning tissue. (e) Once a functioning tissue has been successfully
engineered, the construct is transplanted on the defect to restore function.
fabricated electrospun PCL porous filaments with fiber diameters ranging from 0.5 μm
to 12 μm for 3-D scaffold matrices. Their study indicated that the porous matrices were
beneficial to cell proliferation and these woven fabrics, which consisted of porous
filaments obtained via electrospinning, also provided an environment that supported
tissue proliferation and might be suitable for tissue engineering scaffolds. Figure 7.4
illustrates the difference in the growth kinetics for cells in control and PCL woven
matrix. Venugopal et al.’s study on polycaprolactone-blended collagen nanofibrous
membrane showed the same results for proliferation of fibroblast [19]. They compared
the proliferation result of human dermal fibroblast on PCL, PCL-blended collagen, and
7.2 Bioactive nanofibers
151
Fig. 7.3 Different aspects of a tissue engineering scaffold system [14].
Fig. 7.4 Comparison of the growth kinetics for cells in control and PCL woven matrix [18].
collagen nanofiber scaffolds, at different time lengths, which once again proved the
advantage of nanofibrous scaffolds for tissue engineering, as shown in Fig. 7.5.
A number of biopolymers and biodegradable polymers have been electrospun as
nonwoven membranes used as scaffolds for tissue engineering in recent years. Examples
of materials that have shown potential as skin tissue scaffold include collagen, silk,
chitosan and alginate. The images in Fig. 7.6 show examples of electrospun chitosan and
alginate fabricated by Leung [20]. The resultant nanofibers were cross-linked with
calcium nitrate to enhance structural integrity in aqueous environments. A small amount
of poly(l-lysine) was added into the cross-linking bath to enhance cell adhesion, entrapping the protein in the cross-linked alginate matrix. Fibroblast cells were cultured on
alginate nanofiber scaffolds with and without poly(l-lysine). With the poly(l-lysine), a
Bioactive nanofibers
PCL
2000
Absorbance index
152
1500
PCL-blend collagen
Collagen nanofibrous
scaffolds
*
*
*
*
*
*
1000
500
0
2
4
Days
6
Fig. 7.5 Proliferation of human dermal fibroblast on PCL, PCL-blended collagen, and collagen
nanofiber scaffolds (n ¼ 6) [19].
significant increase in cell attachment compared to the alginate nanofiber scaffold without
the protein was observed. Moreover, fibroblast cells remained viable after 21 days.
Figure 7.7 shows fibroblast cells on the alginate nanofiber scaffold with poly(l-lysine).
Also, nanofibers can be engineered to provide bioactive agents, growth factors,
drugs, therapeutics and genes to stimulate tissue regeneration and further enhancing
the performance [21, 22]. As well as encouraging tissue cells to attach and proliferate, it
is also possible to mimic the layered structure of human skin through pre-seeding
keratinocytes or fibroblasts onto the scaffolds. Keratin is the major protein found in
keratinocytes, which constitute 95% of the cells in the epidermis. Both Yuan et al. and
Li et al. have electrospun PLA nanofibers with keratin derived from wool [23, 24].
Yuan et al. reported superior viability and proliferation of NIH-3T3 fibroblast cells on
PLA–keratin nanofibers compared to plain PLA nanofibers, whereas Li et al. seeded
MC3T3 osteoblast cells and reported an increased number of cells observed on the
PLA–keratin scaffold than the plain nanofiber control.
In bone regeneration, the incorporation of hydroxyapatite has been widely studied due
to its ability to interact with bone cells and resemblance to the apatite found in human
bones. Jose et al. [25] fabricated PLGA-hydroxyapatite nanofibers, and reported that
porosity of the scaffold depends on fiber alignment and the incorporation of hydroxyapatite. Nie et al. [26] also demonstrated the use of PLGA–hydroxyapatite nanofibers, but
containing bone morphogenetic protein-2 (BMP-2) plasmid encapsulated in chitosan.
BMP-2 is known to be able to induce bone healing, whereas hydroxyapatite can enhance
attachment to bone cells. Furthermore, the presence of hydroxyapatite enhanced the
release of the BMP-2 due to its hydrophilicity in a hydrophobic matrix. The authors also
implanted the PLGA scaffolds into mice and reported that the PLGA-hydroxyapatite
composite nanofiber with BMP-2 in chitosan can enhance the healing of bone segmental
defects. Furthermore, transfection efficiency of the BMP-2 was able to be maintained at
7.2 Bioactive nanofibers
153
Fig. 7.6 Electrospun nanofibers of alginate (left) and chitosan (right) [20].
a high level. Besides the PLGA-hydroxyapatite composite nanofiber with BMP-2, Price
et al. [27] has examined the use of carbon nanofibers as a possible scaffold for osteoblast
attachment. They reported that osteoblast cell adhesion on the carbon nanofiber
increases with decreasing carbon fiber size and increasing fiber surface energy, and that
adhesion of competing cells such as fibroblasts can be reduced through adjustment of the
fiber size and surface energy. The osteoblast adhesion is found to be higher than for the
metals used in existing implants [27].
Hepatocytes are liver cells, and a scaffold for growing then will be helpful for liver
regeneration. This may become a promising alternative to liver transplantation, which is
plagued by problems such as donor shortage and hepatic failure [28]. Chu et al. [29]
154
Bioactive nanofibers
Fig. 7.7 Fibroblast cell on alginate nanofibres with poly(l-lysine).
have developed a chitosan nanofiber scaffold for hepatocyte culturing. Compared to a
chitosan cast-film, hepatocyte adhesion on the nanofiber scaffold was superior, and cells
remained viable. To assess the cell activity, they compared hepatocyte functions
through the synthesis of proteins such as albumin, glycogen and urea, and reported that
the nanofiber scaffold outperformed the cast-film control in this respect. Chua et al. [30]
has also reported a hepatocyte nanofibrous scaffold composed of PCLEEP containing
galactose ligand, which can aid cell attachment, spheroid formation and functional
maintenance of hepatocytes.
For neural tissue engineering, Xie et al. [31] studied the effect of embryonic stem cell
differentiation on randomly and uniaxially oriented PCL nanofibers and demonstrated
that neurite outgrowth is higher for the aligned nanofiber scaffold, since the stem cells
tend to elongate parallel to the fibers. The same observation is also made by Yang et al.;
this group reported that neurite outgrowth is higher on aligned PLA fibers [32].
Xu et al. [33] demonstrated that human coronary artery smooth muscle cells can be
cultured on electrospun nanofibrous scaffolds of the copolymer poly-l-lactic acid/polyε-caprolactone (or PLLA/PCL). The study showed reasonable cell proliferation and
normal morphology. The orientation of the nanofiber can lead to guided proliferation of
muscle cells along the aligned nanofibers in a directional manner characterized by the
orientation of the cytoskeletal protein α-actin, as shown in Fig. 7.8.
Vaz et al. demonstrated that the electrospun PLA/PCL bi-layered tube presents
appropriate characteristics to be considered as a candidate scaffold for blood vessel
tissue engineering [34] Figure 7.9 shows the SEM micrographs of electrospun PLA/
PCL constructs after culture with 3T3 mouse fibroblasts at different time lengths.
The morphology of nanofibers can be tailored to deliver the mechanical properties
necessary to support cell growth, proliferation, differentiation and motility. Nanofiber
scaffolds are most commonly applied as a nonwoven fabric, but individual fibers can be
7.2 Bioactive nanofibers
155
Fig. 7.8 Confocal laser scanning micrographs of immunostained α-actin filaments in SMCs after
1 day of culture on aligned nanofibrous scaffold [33].
(a)
(b)
50 mm
Acc.V Spot Magn Det WD Exp
5.00 KV 5.0 500x
BSE 8.3 1
0.4 TorrCYDI
(c)
20 mm
50 mm
Acc.V Spot Magn
7.00 KV 4.0 1000x
Det WD
BSE 8.2 0.4 Torr
20 mm
20 mm
Acc.V Spot Magn Det WD
7.00 KV 4.0 1000x BSE 8.1
20 mm
0.4 Torr
Fig. 7.9 SEM micrographs of electrospun PLA/PCL constructs after culture with 3T3 mouse
fibroblasts for: (a) 1 week; (b) 2 weeks; and (c) 4 weeks [34].
highly aligned depending on the specific application. Furthermore, nanofibers can also
be woven or braided into various structures suitable for biomedical applications [35].
Different woven or braided structures that can be useful for surgical implants are shown
in Fig. 7.10, and are also detailed by Ko [36]. It can be observed that textile technology,
such as weaving and braiding, can be used to fabricate hierarchical fibrous structures
with nanofibers being at the smallest level, which can be a closer structural mimic of the
hierarchical fibrous structures in biological tissues. Such hierarchical structures can
result in a range of size scales and porosities which may be advantageous in certain
surgical applications and in obtaining desired mechanical properties. Examples of yarns
and fabrics suitable for various surgical applications have also been outlined [37].
156
BIAXIAL
WOVEN
WEFT KNIT
WEFT KNIT
Bioactive nanofibers
HIGH MODULUS
WOVEN
MULTILAYER
WOVEN
WEFT KNIT
LAID IN WEFT
WEFT KNIT
LAID IN WARP
WARP KNIT
WEFT INSERTED
LAID IN WARP
WARP KNIT
TRIAXIAL
WOVEN
TUBULAR
BRAID
TUBULAR BRAID
LAID IN WARP
FLAT BRAID
FLAT BRAID
LAID IN WARP
WEFT KNIT
LAID IN WEFT
LAID IN WARP
SWUARE
BRAID
SQUARE BRAID
LAID IN WARP
3–D BRAID
3–D BRAID
LAID IN WARP
WEFT
INSERTED
WARP KNIT
LAID IN WARP
FIBER MAT
STITCHBONDED
LAID IN WARP
BIAXIAL
BONDED
XYZ
LAID IN SYSTEM
Fig. 7.10 Woven and braided structures for surgical implants [14]
7.2.2
7.2.2.1
Nanofibers for drug delivery
Drug delivery systems
The prevalent methods for drug administration, namely oral and intravenous routes,
are not always the most efficient routes for a particular treatment. These conventional
free drug therapy methods have many disadvantages such as the low bioavailability
of the drug at the target site, toxic side effects of the drug on healthy tissues, and
degradation of the drug in the body before reaching the desired site of action. Drug
delivery systems are designed to ensure the drug distribution in a manner such that its
major fraction interacts exclusively with the target tissue at the cellular and subcellular level in addition to providing the desired kinetics for a specific duration [38].
Therefore, problems associated with the administration of free drugs can be overcome
or ameliorated.
The rate of release from a drug carrier is controlled by the properties of the carrier
material, the properties of the drug used and the type of drug carrier system. The
release rate can also be controlled by using external stimulants like pH, ionic
strength, temperature, magnetism and ultrasound, depending on the type of delivery
vehicle used [38]. The mechanism of drug release can be either (a) diffusion of the
drug from the encapsulating carrier, (b) biodegradation of the carrier, (c) swelling of
the carrier followed by diffusion of the drug, or (d) a combination of the aforementioned mechanisms. Through proper drug delievery system design, drugs can be
selectively delivered to the target site in requisite doses without affecting healthy
tissues and organs.
7.2 Bioactive nanofibers
7.2.2.2
157
Nanofibers for drug delivery
Drug delivery with polymer nanofibers is based on the principle that the dissolution
rate of a drug particulate increases with increased surface area of both the drug and the
corresponding carrier if necessary [9]. In addition to the large surface-area-to-volume
ratio, polymer nanofibers have other additional advantages for controlled drug delivery. For example, unlike common encapsulation involving some complicated preparation process, therapeutic compounds can be conveniently incorporated into the
carrier polymers by using electrospinning [9]. The drug release can be easily modulated by changing the morphology, porosity, composition and structure of the nanofibrous membrane. The resulting nanofibrous membrane containing drugs can be
applied topically for skin and wound healing, or post-processed for other kinds of
drug release.
Xie and Wang [39] prepared a formulation consisting of nonwoven electrospun
PLGA mats loaded with the chemotherapeutic agent Paclitaxel for treatment of
gliomas (brain cancer). Implanted nanofiber mats have the advantage that they can
be fabricated or modified to almost any size and therefore represent an attractive form
for local delivery to different target geometries as well as allowing modulation of drug
release rate by controlling fiber diameter. In this study, the main objectives were to
obtain a controllable diameter of PLGA fibers, to encapsulate paclitaxel in electrospun
fibers and to examine the stability of the encapsulated drug as well as the release
kinetics. The prescribed time window in this study correlated well with the period for
polymer degradation, suggesting the possibility of tuned drug release kinetics. Paclitaxel was encapsulated in fibers with an encapsulation efficiency of more than 90%,
and the sustained release could last for more than 2 months in vitro. In vitro cell
viability test results indicated that tumor cell density was much lower after administration of different concentrations of Paclitaxel-loaded PLGA nanofibers, as compared
to control and blank PLGA nanofibers after 72 h. Based on the results of this study,
electrospun micro- and nanofibers may be a good option for achieving the sustained
release of Paclitaxel for treatment of brain tumors. However, one potentially negative
aspect of the formulation used in this study and many other drug delivery studies,
including those discussed above, was the observed initial burst release of drug.
Although the system was able to provide sustained release of about 80% of the
encapsulated drug over 60 days, approximately 15%–20% of the encapsulated drug
was released over the first 24 h. This could be especially problematic in the use of a
powerful chemotherapeutic agent, for example, wherein excessive brain concentrations could cause tissue damage.
Huang et al. [40] demonstrated effective electrospinning of double-layered composite
nanofibers using a high molecular weight polymer (PCL) as the shell and two different
pure drugs (Resveratrol, a kind of antioxidant and Gentamycin Sulfate, an antibiotic) of
low molecular weight as cores. As shown in Fig. 7.11, drug release was smooth in both
the drug-loaded nanofibers with no burst release, indicating that these core–shell
structure drug-loaded PCL nanofibers have potential applications in functional dressings for wound healing.
158
Bioactive nanofibers
Fig. 7.11 In vitro release of drugs from electrospun fibrous membranes that are incubated in
the PBS containing lipase. (a) GS/PCL with 40% GS in the core and (b) RT/PCL with 10% RT
in the core [40].
Fig. 7.12 A 10% release of FITC-BSA from PCL core–shell structured nanofibers against time.
Faster release was observed for nanofibers cultured with HDFs [41].
Ramakrishna et al. [41] developed core–shell structured nanofibers for drugs encapsulation and other therapeutics. A gradual release of fluorescein–isothiocyanateconjugated bovine serum albumin (FITC-BSA) was demonstrated in the release kinetic
studies of the core–shell nanofibers with core-encapsulated FITC-BSA when cultured
with human dermal fibroblasts (HDFs). By contrast, when cells were cultured with
human dermal fibroblasts (HDFs), there was a resulting undesirable burst release profile
(Fig. 7.12). When the nanofibers are cultured with cells, a faster release of FITC-BSA is
observed. One possible explanation for this is that it is a result of a higher polymer
degradation rate in the presence of degradative enzymes secreted by the cells. This
faster release could be desirable for applications like wound dressing, where an initially
higher but sustained release of antibiotics is preferred. This release profile is crucial for
regulating cell growth if nanofiber scaffolds for tissue engineering applications are to
encapsulate bioactive molecules or to allow slow passive delivery in drug delivery
7.2 Bioactive nanofibers
159
Fig. 7.13 The principle of biosensors [43].
apparatuses. Biological and chemical ligands can be conjugated onto nanofibers for cellspecific targeting or in order to produce biosensors, i.e. controlled delivery of insulin, in
response to physiological chances, in diabetic patients.
Nanofiber composites, composed of biocompatible and biodegradable polymers and
magnetite nanoparticles, are of potential interest for various applications in medicine,
especially drug delivery to precise target areas. Tan et al. [42] reported the fabrication of
super-paramagnetic polymer nanofibers intended for drug delivery and therapy. The
electrospinning technique was employed for preparing polymer nanofibers containing
magnetite nanoparticles from commercially available poly-hydroxyethyl methacrylate
(PHEMA) and poly-l-lactide (PLLA).
7.2.3
Nanofibers for biosensors
7.2.3.1
Biosensors
The ability to detect pathogenic and physiologically relevant molecules in the body with
high sensitivity and specificity offers a powerful opportunity in early diagnosis and
treatment of diseases. Early detection and diagnosis can be used to greatly reduce the
cost of patient care associated with advanced stages of many diseases. These costs have
been estimated to be ~$75 billion [1] and ~$90 billion for cancer and diabetes,
respectively. Currently, cancer can be detected by monitoring the concentration of
certain antigens present in the bloodstream or other bodily fluids, or through tissue
examinations. Correspondingly, diabetes is monitored by determining the glucose
concentrations in the blood over time [38].
A biosensor is commonly defined as an analytical device that uses a biological
recognition system to target molecules or macromolecules, which is generally coupled
to a physiochemical transducer that converts the recognition to a detectable output
signal [43], as shown in Fig. 7.13. A typical biosensor consists of three components
[38]: (1) the detector, which identifies the stimulus; (2) the transducer, which converts
the stimulus to a useful output; and (3) the output system, which amplifies and displays
the output in an appropriate format.
Bioactive nanofibers
0.12
0.12
0.10
0.10
0.08
0.08
i/ m A
i/ m A
160
0.06
0.04
0.06
0.04
0.02
0.02
Casting Film:
Response Time = 5.2s
0.00
0.00
14
(a)
Electrospun Membrane:
Response Time = 1s
15
16
17
t/s
18
19
20
52.4
(b)
52.6
52.8
53.0
53.2
53.4
53.6
53.8
t/s
Fig. 7.14 Response time of the immobilized enzyme electrode modified by casting film (a) and
electrospun membrane (b), respectively, with the glucose concentration of 0.5 mm [47].
7.2.3.2
Nanofiber biosensors
Sensitivity, selectivity and fast response and recovery times are crucial to a sensor.
Nanosensors have the advantage of increasing the number of available sensing sites
without increasing the amount of overall sample required through their increased
surface area of the detector substrates [44, 45]. Nanofiber-based sensors give the
advantage of fast response times and high sensitivity due to the higher surface-areato-volume ratio compared to common sensors.
The successful incorporation of biotin in PLA nanofibers through electrospinning for
biosensors based on biotin–streptavidin specific binding was reported by Lu et al. [46].
A biotinylated DNA probe was captured by immobilized streptavidin in a preliminary
biosensor assay by using this PLA nanofiber membrane substrate. Biosensor assay
experiments confirmed that the PLA nanofiber membranes could successfully transport
analyte solutions via wicking.
A glucose biosensor can be fabricated with electrospun glucose oxidase (GOD) with
PVA nanofibers membrane [47]. The glucose biosensors have a rapid response time
(1 s) and a higher output current (microampere level) to glucose than in both normal and
diabetic conditions. The linear response ranging from 1 mmol/l to 10 mmol/l and the
lower detection limit (0.05 mmol/l) of the sensor can meet the demands for detection of
glucose in medical diagnosis. Figure 7.14 shows the response time of the immobilized
enzyme electrode modified by casting film and electrospun nanofiber membrane.
A novel gas sensor composed of electrospun nanofibrous membranes and a quartz
crystal microbalance (QCM) has been demonstrated [48]. The electrospun fibers with
diameters of 100–400 nm were deposited on the surface of QCM by electrospinning
homogenous blend solutions of cross-linkable PAA and PVA. The sensitivity of the
nanofibrous membrane-coated QCM sensor was much higher than that of a continuous
film-coated QCM sensor.
Microsensors fabricated from four different cross-selective electrospun polymer
composite fiber arrays for detection and identification of volatile organic compounds
have also been reported [49]. The four organic compounds could be discriminated based
7.3 Assessment of nanofiber bioactivity
161
1.0
0.8
0.6
0.4
0.2
PECH
PEO
PIB
PVP
0.0
Trichloroethylene
Toluene vapor vapor
Methanel vapor
1,5
Dichloropentene
vapor
Fig. 7.15 Three-dimensional graph of sensor array response pattern to four different analyte vapors
[49]: 1,5-dichloropentane; methanol; toluene; and trichloroethylene.
on the unique response pattern from the sensor array. Figure 7.15 compares the sensor
array response pattern to four different analyte vapors.
Single-molecule detection could be achieved by using the nanochannels that can
provide zeptoliter detection volume. Wang et al. [50] fabricated silica nanochannels
with inner diameter as small as 20nm using scanned coaxial electrospinning.
A fluorescent dye was injected by capillary force into the individual nanochannels
and single-molecule detection was performed by monitoring the photon signals from
5-iodoacetamidofluorescein (5-IAF). Figure 7.16 shows (a) a fluorescent micrograph of
aligned nanochannels filled with IAF, (b) a schematic diagram of single molecule
detection, (c) photon counts of a blank nanochannel, and (d) photon counts of a
nanochannel filled with 4.9 μm/l of IAF solution.
Nanofibers offer a higher sensitivity than thin films as chemiresistors. Flueckiger
et al. [51] fabricated a polymer blend PANi/PEO nanofiber-based sensor as well as a
metal oxide TiO2 nanofiber-based sensor using electrospinning. The conductivity of
those fibers is highly sensitive to the chemical environment and is modified through the
absorption of different species.
7.3
Assessment of nanofiber bioactivity
Bioactivity is the key of biomaterials. To evaluate the bioactivity of a biomaterial,
biocompatibility and biodegradation are usually tested and assessed. In general,
the evaluations are conducted first under in vitro conditions prior to animal tests or in
vivo tests.
162
Bioactive nanofibers
Fig. 7.16 (a) Fluorescent micrograph of aligned nanochannels filled with IAF. The scale
bar is 10 μm. (b) Schematic diagram of single molecule detection. (c) Photon counts of a
blank nanochannel. (d) Photon counts of a nanochannel filled with a 4.9 μm/l of IAF
solution [50].
References
7.3.1
163
Assessment of tissue compatibility
A toxic material is defined as a material that releases a chemical in sufficient quantities
to kill cells either directly or indirectly through inhibition of key metabolic pathways.
The number of cells that are affected is an indication of the dose and potency of the
chemical. Biocompatibility is generally evaluated by using three primary cell culture
assays: direct contact, agar diffusion and elution (also known as extract dilution). The
outcomes are measured by observations of changes in the morphology of the cells.
Assessment of tissue compatibility is the same for nanofiber biomaterials as for the
common biomaterials. The choice of method varies with the characteristics of the test
material, the rationale for doing the test and the application of the data for evaluating
biocompatibility. Detailed precedures of the compatibility tests can be found in the
literature.
7.3.2
Assessment of degradation
Assessment of the degradation behavior of porous scaffolds plays an important role in
the engineering process of a new tissue. The degradation rate of porous scaffolds affects
cell vitality, cell growth and even host response. Upon implantation, a scaffold should
maintain mechanical properties and structural integrity until the loaded cells adapt to the
environment and excrete a sufficient amount of extracellular matrix; after the mission is
accomplished, completely degradation and absorption of the scaffold are desired. It is
believed that the ideal in vivo degradation rate may be similar or slightly less than the
rate of tissue formation. The details for assessment of scaffold degradation can be found
in Refs. [52, 53].
It should be noted that synergistic pathways should be considered to understand the
biological degradation of implant materials[54]. For example, swelling and water
uptake can similarly increase the number of site for reaction. Degradation products
can alter the local pH, stimulating further reaction. Hydrolysis of polymers can generate
more hydrophilic species, leading to polymer swelling and the entry of degrading
species into the bulk of the polymer. Cracks might also serve as sites initiating
calcification.
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49. R. Kessick, and G. Tepper, “Electrospun polymer composite fiber arrays for the detection and
identification of volatile organic compounds,” Sensors and Actuators B: Chemical, vol. 117
(1), pp. 205–210, 2006.
50. M. Wang, et al., “Electrospinning of silica nanochannels for single molecule detection,”
Applied Physics Letters, vol. 88, p. 033106, 2006.
51. J. Flueckiger, F. K. Ko, and K. C. Cheung, “Electrospun electroactive polymer and metal
oxide nanofibers for chemical sensor applications,” in Proceedings of the ASME 2010
International Mechanical Engineering Congress & Exposition, 2010.
52. L. Wu, and J. Ding, “In vitro degradation of three-dimensional porous poly(,-lactide-coglycolide) scaffolds for tissue engineering,” Biomaterials, vol. 25(27), pp. 5821–5830, 2004.
53. K. Kim, et al., “Control of degradation rate and hydrophilicity in electrospun non-woven poly
(d, l-lactide) nanofiber scaffolds for biomedical applications,” Biomaterials, vol. 24(27),
pp. 4977–4985, 2003.
54. B. Ratner, et al., Biomaterials Science: an Introduction to Materials in Medicine. Elsevier
Academic Press, 2004.
8
Electroactive nanofibers
8.1
Introduction
With the rapid advances of materials used in science and technology, various intelligent
materials that can sense variations in the environment, process the information, and
respond accordingly are being developed at a fast pace. Shape-memory alloys, piezoelectric materials, etc., fall into this category of intelligent materials. Polymers have
attractive properties compared to inorganic materials. They are lightweight, inexpensive, fracture tolerant, pliable and easily processed and manufactured [1]. An organic
polymer that possesses the electrical, electronic, magnetic and optical properties of a
metal while retaining the mechanical properties and processability, etc., commonly
associated with a conventional polymer, is termed an “intrinsically conducting polymer” (ICP), or more commonly, a “synthetic metal” [2]. The unique properties of these
materials are highly attractive for a wide range of applications such as actuators,
supercapacitors, batteries, etc. With the development of nanotechnology, these materials
can be engineered to develop a variety of multifunctional active materials for intelligent
applications that were previously imaginable only in science fiction. Figure 8.1 shows
an artistic interpretation of the Grand Challenge for EAP actuated robotics.
8.2
Conductive nanofibers
8.2.1
Conductive polymers and fibers
Conductive polymers pave the way for smart applications due to their flexibility, light
weight and favorable metallic and electrical properties. The polymers listed in Table 8.1
are some of the ICP polymers. Among them, polyaniline is the most commercially
available and the one that has been studied in a wide range of applications from
actuators to rechargeable batteries[4–6]. To facilitate manipulation and application,
conductive polymer products are mainly formed as fibers or cables.
Most commercially available conductive fibers are composed of a blend of nonconductive polymer and conductive particles, such as carbon black, carbon nanotubes,
metallic particles, metal clad aramid fibers, etc. Conductive fibers containing metallic
components are prone to damage due to excessive exposure to moisture and continuous
fatigue. Furthermore, the high stiffness of metallic wires reduces the flexibility of
167
168
Electroactive nanofibers
Table 8.1 Conductivities of conducting polymers
Polymer
Conductivity (Siemens/cm)
Polymer
Conductivity (Siemens/cm)
Polyacetylene
Polypyrrole
Polythiophene
Poly(3alkylthiophene)
Polyphenylene Sulfide
Polyphenylene-vinylene
10 000a
500–7500
1000
1000–10 000a
500
10 000a
Polythienylenevinylene
polyphenylene
Polyisothianapthene
Polyazulene
Polyfuran
Polyaniline
2700a
1000
50
1
100
200a
Advanced Polymer Courses, VIIIth International Seminar on the Technology of Inherently Conductive
Polymers, June 18–20, 2001.
a
Fig. 8.1 An artistic interpretation of the Grand Challenge for EAP actuated robotics [3].
conductive fiber assemblies, causing decreased mobility, maintenance and durability.
Conductive fibers can be integrated into a base fabric device by means of embroidering,
weaving or knitting. This technique has recently been studied in wearable electronics
and wearable computers [7]. Table 8.2 lists some of the commercially available
conductive fibers, provides a brief description, and shows their conductivity values.
Disadvantages of these conductive fibers include corrosion and fatigue, because the
fibers contain metallic components. Metallics are known to have low flexibility and
poor fatigue properties. Continuous bending and rubbing of wires during use might
result in wire failure due to fatigue, hence disabling the function of the device.
However, the disadvantages will be overcome; the electroactivity and mechanical
properties of the conductive particles, metallic fibers and polymers will be thoroughly
changed when the materials are made in nanosize, because matter at the nanoscle no
longer follows Newtonian physics but rather quantum mechanics.
8.2 Conductive nanofibers
169
Table 8.2 Commercially available conductive fibers[7]
Type
Conductivity (Ohms/cm)
BK 50/2 – Bekaert Fiber Technologies
(Steel/polyester spun yarn)
Bekintex – Bekaert Fiber
(Continuous cold drawn stainless steel fibers)
Bekintex 15/2 – Bekaert Fiber
(Stainless spun fibers)
VN 140 nyl/35 x 3
(Nylon core wrapped with continuous stainless steel wires)
Aracon – DuPont
(Metal clad aramid fiber, Core: Kevlar; Cladding metals:
Ag, Ni, Cu, Au, Sn)
50
1
1
10
0.001
Fig. 8.2 Metals form ohmic contacts [8].
8.2.2
Fundamental principle for superior electrical conductivity
Unusual current conduction properties arise when the size of (nanometer in diameter)
wires are reduced below a certain critical thickness [8]. Furthermore, the rectifying
contact to a wire will work better if the wire size is reduced below such a critical
thickness. Hence, it is expected that new wires and contacts (the diodes that form the
building blocks of electronics) will behave more favorably when forced to operate
under reduced dimensional regimes.
It is also well know that electronics, by definition, is the study of electron conduction.
This conduction, or transport, is affected by the properties of the medium of conduction
and its physical dimensions. Unusual properties arise when electrons become spatially
confined, that is, when they are transported in a nanoscale media. The following
paragraphs will identify the expected properties arising from confinement and contact
between nanoscale wires and macro (3-D) systems.
Three types of contact to a nanowire exist [8]: ohmic, rectifying and tunneling.
Ohmic contact is an ideal contact in which the electron wave is not reflected from the
wire. Current conduction is then the property of the wire, not the contact, and is by
definition the product of cross-sectional area A, charge q, carrier concentration n, and
carrier velocity v (I ¼ A q n v). However, below a critical thickness of A, carrier
concentration changes due to the change in density of states and, more importantly, the
velocity of carriers is altered due to differences in scattering. This is because the
electrons in a 1-D system can scatter only by completely reversing their direction,
whereas in 2-D and 3-D systems they can scatter by changing the angle of motion [9].
170
Electroactive nanofibers
Fig. 8.3 Metals form rectifying Schottky contacts [8].
Hence, it is concluded that current conduction should increase below a critical diameter.
This is schematically shown in Fig. 8.2, where it is proposed that I (A1) þ I (A2) > I (A3)
even though A3 ¼ A1 þ A2.
Rectifying (or Schottky) contacts are produced in systems that have different electron
affinities [8]. Current transport in these systems is based on kinetic energy requirements
for motion from one system to another. Thermionic emission of carriers is the typical
source of conduction in these systems. In a 3D–1D contact, calculation of themionic
emission arises from carriers that have enough kinetic energy to overcome the barrier
between the two systems. However, both carrier concentration and velocity change, due
to carrier confinement in a wire as a result of (a) the changing density of electronic states
and (b) the fact that electron energy is quantized. As a result, electrons that are emitted
from a metal to a nanowire have to occupy higher states, causing a barrier height
increase. A step-by-step mathematical derivation [10] reports that a metal has a much
higher barrier and a different thermal signature than bulk material. Hence, below critical
thickness the system rectifies better. This is schematically shown in Fig. 8.3, where it is
proposed that I (A1) þ I (A2) > I (A3) even though A3 ¼ A1 þ A2.
In a 1-D system the electrons are collectively excited, producing what is known as a
Luttinger (as opposed to Fermi) liquid [11]. This property then alters the tunneling
probabilities compared to 3-D systems. On the basis of the previous discussion, it is
proposed that by reducing the size of a wire beyond a certain dimension, it can be
expected simultaneously to achieve both better rectification properties and superior
transport in a nanowire.
8.2.3
Electroactive nanofibers
Using these conductive polymers in the form of nanofibrous assemblies offers us two
major advantages. First, the fibrous form gives the opportunity of having electronic
textiles and of obtaining tactile properties for different applications. Second, nanoscale
fibers provide the fundamental building blocks for construction of devices and structures.
Electroactive nanofibers are fibers of less than 100 nm diameter that are electrically
conductive (in terms of electrical, ionic and photoelectric conductivity). Polymer classes
that are suitable for fabricating electroactive nanofibers include, but are not limited to,
polyimides, polyamides, vinyl polymers, polyurethanes, polyureas, polythioureas, polyacrylates, polyesters and biopolymers [12]. Polyacetylene, polypyrrole, polythiophene
and polyaniline are the most widely studied intrinsically conducting polymers [13].
Electroactive polymer can be doped with inclusions, such as piezoceramic powders,
nanotubes and nanofibers, for dielectric enhancement. Fibers and fibrous nonwoven mat
8.2 Conductive nanofibers
171
Fig. 8.4 A 50 wt% nanofiber blend of PAn.HCSA fabricated from 2 wt% PAn.HCSA and 2 wt%
PEO from chloroform solution at 25 000 V (anode/cathode separation 25 cm) [19].
can be fabricated through electrospinning. It is anticipated that electroactive nanofibers
will be used in the fabrication of tiny electronic devices such as sensors, actuators and
Schottky junctions. Electroactive nanofibrous membranes are appropriate for use as
porous electrodes in high-performance batteries because of the well-established fact that
the rate of electrochemical reactions is directly proportional to the surface area of the
electrode [14, 15]. The high specific surface area makes nanofibers an ideal candidate
for making high-performance electronic devices. Electroactive fibrous membranes are
potentially useful for applications including electrostatic dissipation, corrosion protection, electromagnetic interference shielding and photovoltaic devices[16, 17].
Electrospinning of nanofibers from pure electronic polymers (in their semiconducting
and metallic regimes) or their blends in conventional organic polymers for the purpose
of ascertaining their applicability in the fabrication of nano-electronic devices has already
been widely explored. Norris et al. [14] fabricated the first conducting polymer fibers
(diameter ~950 nm to 2100 nm) of polyaniline doped with d,l-camphorsulfonic acid
(PAn.HCSA) as a blend in polyethylene oxide (PEO) (see Fig. 8.4). The chemical and
physical structure of polyaniline was found preserved. The obtained fiber has a diameter
ranging from 500 nm to 1600 nm. With an appropriate substrate – glass slide, silicon
wafer or loop of copper wire, etc. – held between the anode and cathode at a position
close to the cathode, individual fibers could be collected for certain electrical studies [18].
For a single 419 nm fiber of a blend of 50 wt% PAn.HCSA and polyethylene oxide, the
conductivity was measured with two probes to be ~ 10 1 S/cm, as given in Fig. 8.5.
Nonlinear I/V curves may be obtained from some polyaniline samples, possibly caused
by the presence of defect sites induced by imperfections or impurities in the polyaniline.
Such imperfections are expected to be more apparent in thin fibers since there are fewer
molecular pathways by which charge carriers can bypass the defect sites[19].
By using the same method for producing polyaniline fibers [14], highly conducting
sulfuric acid-doped polyaniline fibers (average, 139 nm; maximum, 275 nm; minimum,
96 nm) were obtained by placing a ~20 wt% solution of polyaniline (Versicon™, Allied
Signal) in 98% sulfuric acid in a glass pipette with the tip ~3 cm above the surface
172
Electroactive nanofibers
Fig. 8.5 Current/voltage curves of 50 wt% PAn.HCSA/PEO blend nanofiber [18].
of a copper cathode immersed in pure water at a potential difference of 5000 V [19].
As expected, the conductivity of the obtained fiber was ~ 0.1 S/cm (since partial fiber
de-doping occurred in the water cathode). The diameter and length of the fibers appear
to be sensitive to the nature of the polyaniline used [19]. No great difficulty was
foreseen in producing fibers with a diameter less than 100 nm diameter.
The (reversible) conductivity/temperature relationship between 295 K and 77 K for a
single 1320 nm fiber (72 wt% PAn.HCSA in PEO) spun from chloroform solution is
shown in Fig. 8.6. To minimize heating effects the applied voltage was held constant at
10 mV, at which value the current is very small. The conductivity (~33 S/cm at 295 K)
was unexpectedly large for a blend since the conductivity of a spun film of the pure
polymer cast from chloroform solution is only ~10 1 S/cm [20]. The results reveals that
the significance of the high surface-to-volume ratio provided by the electrospun fibers
was evident from the greater than first-order magnitude increase in the rate of the vapor
phase de-doping and at least two orders of magnitude increase in the rate of spontaneous
re-doping, compared to the cast film. Figure 8.7 illustrates the electrical conductivity of
the PAn.HCSA/PEO blend electrospun fibers and cast films prepared from the same
solution [19] This suggests there may be significant alignment of polymer chains in the
fiber [21, 22].
Fabrication of poly(3,4-ethylenedioxythiophene)(PEDOT)/poly(styrenesulfonate)
(PSS) blend fibers were also proved to be feasible by electrospinning as shown in
Fig. 8.8a. Good alignment was observed due to the phenomenon of yarn self assembly
during electrospinning. It is evident as seen in Fig. 8.8b that the addition of PEDOT to
the PAN system affects the diameter value of the fibers. The fibers became finer by
increasing the amount of the PEDOT, but one should be cautious, as increasing
the PEDOT amount to higher concentration can cause the appearance of beads, which
is not required.
A 4-probe device has been used to indirectly measure the conductivity of electrospun
fibers [18]. This was done by electrospinning the PEDT/PAN polymer directly to the
8.2 Conductive nanofibers
173
Fig. 8.6 Conductivity/temperature relationship for a 72 wt% blend fiber of PAn.HCSA in PEO [18].
Fig. 8.7 Electrical conductivity of the PAn.HCSA/PEO blend electrospun fibers and cast films
prepared from the same solution [14, 18].
Si wafer. The I/V curves of the Si wafer with and without the fibers were drawn to get
the resistance of the wafer and wafer with fibers. By considering the Si wafer and the
fiber mat as resistors in parallel connection, the resistance of the electrospun fibers can
be calculated by applying the relationship of
174
Electroactive nanofibers
Fig. 8.8 (a) SEM of PEDOT/PSS fibers. (b) Fiber diameter vs PEDOT/PAN concentrations.
V ¼ IR,
ð8:1Þ
ρ ¼ RA=L,
ð8:2Þ
where V is the applied electrical potential, I is the current, R is the resistance of the
material, A is the cross-sectional area perpendicular to the direction of the current, L is
the distance between the two points at which the voltage is measured and ρ is the
resistivity of the material. The conductivity of the material can be calculated from the
reciprocal of resistivity. Figure 8.9 shows an I/V curve of a tested material.
It can be seen that as the fiber diameter decreases, the electrical conductivity
increases, which supports the hypothesis that elimination of small-angle scattering
due to size confinement can increase conductivity, as introduced in Chapter 1.
8.2 Conductive nanofibers
175
Fig. 8.9 An I/V curve of a tested nanofiber material [18].
Fig. 8.10 The effect of increasing the concentration of PEDOT on the fibers’ conductivity [18].
The increase of PEDOT concentration in the fibril matrix increases the electrical
conductivity of the yarn [18], as shown in Fig. 8.10. The electrical conductivity of
the PEDOT/PAN yarn was 0.001– 0.012 S/cm.
To accomplish the goal of obtaining conductive nanofibers with a diameter <100 nm,
an 8 wt% solution of polystyrene (Mw 212 400; Aldrich Co.) in THF (glass pipette
orifice: 1.2 mm) was electrospun [19]. The diameters of the polystyrene nanofibers were
176
Electroactive nanofibers
consistently <100 nm. It should be noted that a 16 nm fiber is ~30 polystyrene
molecules wide, therefore dimensions of this size were expected to greatly affect the
kinetics, as well as possibly the thermodynamics, of the polymer. It is also of interest to
note that a 16 nm fiber such as the one mentioned above lies well within the ~4–30 nm
diameter range of multi-walled carbon nanotubes [19, 23].
Waters et al. [24] reported the use of nanofibers in the development of a liquid crystal
device (LCD) optical shutter, which is switchable under an electric field between
substantially transparent and opaque states. This LCD mainly consisted of a layer of
nanofibers permeated with a liquid crystal material, having a thickness of only a few
tens of microns. The nanofibers were located between two electrodes, by means of
which an electric field could be applied across the layer to vary the transmissivity of the
liquid crystal/nanofiber composite. It is the fiber size used that determines the sensitivities of refractive index differences between liquid crystal material and the fibers, and
consequently governs the transmissivity of the device. Evidently nanoscale polymer
fibers are necessary in this kind of devices.
Li et al. [13] reported that the doping of gelatin with a few percent of PANi leads to
an alteration of the physicochemical properties of gelatin. The electroactive PANi–
gelatin fibers were used as a fibrous matrix for supporting cell growth, where H9c2 rat
cardiac myoblast cells were cultured on glass cover slips coated with the electrospun
fibers. The results indicated that all PANi–gelatin blend fibers supported H9c2 cell
attachment and proliferation to a similar degree as the control tissue culture-treated
plastic (TCP) and smooth glass substrates.
Pawlowski et al. [25] used electrospinning to fabricate nanofibers from a piezoelectric copolymer of PVDF and trifluoroethylene (TrFE) solution to create lightweight,
electrically responsive wing skins for micro-air vehicle (MAV) wing frame designs. In
preliminary attempts at fiber actuation, the fiber-coated wing frames exhibited a perceptible vibration upon excitation with a 2 kV (peak-to-peak) sine wave at 6.7 Hz. These
results confirmed the feasibility of utilizing electrospinning to process novel electroactives for fabrication of lightweight, responsive MAV wings. Figure 8.11 illustrates the
TrF1 fibers electrospun onto MAV wing frames.
PAN and PAN-based graphite nanofibers were obtained by electrospinning and
subsequent pyrolysis [26]. The graphitization of the PAN nanofiber leads to a sharp
increase in conductivity to around 490 S/m. Figure 8.12 illustrates the I/V curves of
PAN-based graphite fiber under small (a) and large (b) signals.
The incorporation of carbon nanotubes into nanofibers was shown to improve
the electroactivity of the nanofibers [27–31]. Single-walled carbon nanotubes
(SWCNTs) and conductive poly-3,4-ethylene dioxythiophene/poly-styrene sulfonate
(PEDT/PSS) with polyacrylonitrile, polyacrylonitrile, Bombyx mori silk, etc., have
been co-elctrospun to form conducting nanofibers. The inclusion of SWCNTs
showed a further increase in the electrical conductivity of nanofibrous yarn, as well
as in the tensile strength.
Hwang reported the successful electrospinning of a composite fiber containing
carbon black (CB) and polyurethane (PU) [32]. The effects of introducing CB into
the PU-matrix in the form of fiber web were investigated with respect to mechanical,
8.2 Conductive nanofibers
177
Fig. 8.11 TrF1 fibers electrospun onto MAV wing frames. (a) Double wing configuration,
approximate dimensions are 15.2 cm 5.1 cm; (b) single wing configuration, approximate
dimensions are 10.2 cm 6.4 cm [25].
15
Data point
Linear Fitting line
200
10
100
I (nA)
I (nA)
5
0
–5
–100
–10
–200
–40
0
–20
0
20
40
–15
–2
V (V)
–1
0
V (V)
(a)
(b)
1
2
Fig. 8.12 I/V curves of PAN-based graphite fiber under small (a) and large (b) signals [26].
electrical and thermal properties. The highest conductivity of the fiber web was experimentally determined to be approximately 10 2 S/cm for 8.03 vol% of CB filler content,
which is deemed useful for several applications including electrodes in polymer
actuators.
Lee et al. [34] synthesized copolymer poly(acrylonitrile-co-acrylamide) (P(AN-AM))
and electrospun it into nanofibers for conversion into conductive carbon fibers. To
improve the electrochemical properties of CNT fibers, Ppy was deposited on CNT
fibers, and the CNT/Ppy fibers are compared with glassy carbon(GC)/Ppy. The CV
178
Electroactive nanofibers
Fig. 8.13 CV of CNF/Ppy [33].
Fig. 8.14 CV of GC/Ppy [33].
Fig. 8.15 CP of GC/Ppy [33].
curves in Fig. 8.13 and Fig. 8.14 showed that both CNF/Ppy and GC/Ppy composites
were able to maintain a better capacitive behavior at lower scan rates, represented by
more rectangular shaped curves. When the scan rate increased, the CV curve became
oval shaped, indicating higher resistance from both electrodes. As shown in Fig. 6.43
and Fig. 8.15, at the same discharge rate of 0.05 A/g, the specific capacitance is 117 F/g
for CNF/Ppy and 132 F/g for GC/Ppy. These capacitances are relatively high compared
with previous studies incorporating Ppy with single (or multi)-walled carbon nanotubes
in aqueous electrolyte. The result also demonstrated that Ppy is able to retain its intrinsic
8.3 Magnetic nanofibers
179
Table 8.3 Critical particle diameter for supermagnetism of various metals
Metal
Critical particle diameter (nm)
Metal
Critical particle diameter (nm)
Co
Fe
70
14
Ni
Fe3O4
55
128
capacitance in the fibrous form. At higher scan rates, the specific capacitance of CNF/
Ppy was in general ~15F/g lower than that of GC/Ppy. One contrasting result is shown
in the discharge curve of Fig. 6.44. When the scan rate increased to 0.37 F/g, the
discharge capacitance of GC/Ppy decreased to 29 F/g, lower than that of CNF/Ppy at a
higher scan rate of 0.5 A/g. However, the capacitance of CNF/Ppy, sharing a similar
trend with GC/Ppy, dropped 20% and 58% when the current density increased to 0.1 A/g
and 0.5 A/g, respectively. Improving the conductivity of CNF is essential to improve
the capacitance of the composite electrode at higher scan rates. The specific capacitance
of polypyrrole deposited on the nanofibers was measured to be 117 F/g at 0.05 A/g in
propylene carbonate. This number was fairly close to the specific capacitance of pure
polypyrrole film (132 F/g at 0.05 A/g), implying that the capacitance of polypyrrole was
retained in the fibrous form.
8.3
Magnetic nanofibers
8.3.1
Supermagnetism
The term magnetism describes how materials respond at the microscopic level to an
applied magnetic field. Two well-known forms of magnetism are ferromagnetism, by
which materials such as iron form permanent magnets or are attracted to magnets, and
paramagnetism, whereby the paramagnetic materials are attracted only when an externally magnetic field is applied. Paramagnetic materials are attracted to magnetic field
and hence have a relative magnetic permeability 1. Unlike ferromagnets, paramagnets do not retain any magnetization in the absence of an externally applied
magnetic field. Thus the total magnetization will drop to zero when the applied field
is removed.
Superparamagnetism is a form of magnetism that occurs in small ferromagnetic or
ferrimagnetic nanoparticles. In small enough nanoparticles, magnetization can randomly flip direction. The typical time between two flips is called the Néel relaxation
time. In the absence of an external magnetic field, when the time used to measure the
magnetization of the nanoparticles is much longer than the Néel relaxation time,
the magnetization appears to be on average zero and the particles are said to be in the
superparamagnetic state. In this state, an external magnetic field can magnetize the
nanoparticles, similarly to a paramagnet. However, their magnetic susceptibility is
much larger than the one of paramagnets. Table 8.3 shows the critical particle diameter
of various metals.
180
Electroactive nanofibers
Fig. 8.16 TEM image of Fe3 O4-filled CNFs with 40 wt% FeAc2 [34].
8.3.2
Supermagnetic nanofibers
Magnetic composite fibers, in which magnetic nanoparticles are embedded in a polymeric fiber, are anticipated to possess interesting properties such as supermagnetic
and magnetostrictive characteristics. These distinctive properties allow them to be
applied in many areas including EMI shielding materials, medical diagnosis and
electromagnetic devices, etc.
The ability to incorporate nanoparticles to fabricate electrospun nanofibers by electrospinning has inspired many studies relating to electrospun supermagnetic composites
nanofibers. Zhu et al. [34] successfully prepared conductive and magnetic Fe3O4-filled
CNFs with super-hydrophobic properties by electrospinning and low-temperature carbonization, as shown in Fig. 8.16. Magnetic properties of the Fe3O4-filled CNFs were
investigated via a superconducting interference device (SQUID) magnetometer at 300 K.
As shown in Fig. 8.17, the Fe3O4-filled CNFs with 40 wt% Fe3Ac2 are ferromagnetic,
displaying a distinct hysteresis loop with a saturation magnetization (MS) and coercivity
(HC) of 55.62 emu/g1 and 150.72 Oe, respectively. shows the elationship between the
FeAc2 content of carbonized nanofibers and the saturation magnetization (Ms) of Fe3O4filled CNFs.
Song et al. [35] reported that composite nanofibers of poly-caprolactone (PCL) with
iron–platinum (FePt) nanoparticles were successfully fabricated via coaxial electrospinning. Figure 8.18 shows a typical TEM image of the core–shell structured composite
nanofibers of FePt/PCL composite nanofiber. The magnetic behaviour of FePt/PCL
composite nanofibers was investigated by alternating gradient magnetometer at room
temperature. Figure 8.19 illustrates the hysteresis loop of the FePt/PCL composite
nanofibers. The hysteresis loop shows superparamagnetic behavior of the FePt/PCL
8.3 Magnetic nanofibers
181
Fig. 8.17 Relationship between the content of carbonized nanofibers and the saturation
magnetization (Ms) of FeAc2-filled CNFs [34].
nanofibers which is ascribed that the as-synthesized FePt nanoparticles in the PCL
nanofibers have a face-center cubic (fcc) phase structure and are superparamagnetic.
Chen et al. [36] fabricated cobalt ferrite (CoFe2O4)-embedded polyacrylonitrile
(PAN) nanofibers by electrospinning. The magnetic properties of the CoFe2O4,
CoFe2O4/PAN and CoFe2O4/carbon nanofibers were determined by using a vibrating
sample magnetometry (VSM) with applied fields from 11 000 to 11000 Oe at 300 K,
showing that the saturation magnetization of the CoFe2O4/PAN nanofibers was 45 emu/g
and that the fibers demonstrated superparamagnetic behavior.
Bayat et al. [37, 38] produced multifunctional magnetite (Fe3O4) nanoparticle
reinforced composite nanofibres using polyacrylonitrile (PAN) as the matrix followed
by heat treatment. Various amounts of magnetite (Fe3O4) nanoparticles ranging from
1 wt% to 10 wt% were incorporated in the fibers. The Fe3O4 nanoparticles were successfully transferred into the as-spun Fe3O4/PAN composite nanofibrous structure, as shown
in Fig. 8.20a. Electrically conductive and magnetically permeable composite carbon
nanofibres were obtained by thermally treating the pristine composite nanofibres; see
Fig. 8.20b,c. The electrical conductivity of carbonized samples increases with increasing the carbonization temperature ss shown in Fig. 8.21. The electrical conductivity
increases from 0.0550.004 S/cm to 2.60.9 S/cm with increasing the carbonization
temperature from 700 C to 900 C. This increase was attributed to increasing the
degree of graphitization resulted from the higher pyrolysis temperature, as revealed by
the XRD and Raman spectra. The magnetism hysteresis loops of as-spun nanofibers are
almost at the same level of Fe3O4 nanoparticles. Carbonized composite nanofibers show
larger Ms as shown in Fig. 6.47. The sample has Ms of 5.2 emu/g, which has been
182
Electroactive nanofibers
Fig. 8.18 The coaxial electrospun FePt/PCL composite nanofibers observed under transmission
electron microscope [36].
increased to 12 emu/g and 16 emu/g after carbonization at 700 C and 900 C
respectively. Simultaneously, the Hc value increased from 105 Oe to 500 Oe and
600 Oe the same coercivity value as bulk magnetite respectively. The magnetic properties and electrical conductivity of the composite materials were able to be tuned by
adjusting the amount of Fe3O4 nanoparticles in the matrix and carbonization process.
8.4
Photonic nanofibers
8.4.1
Polymer photonics
Owing to structural flexibility, easy processing and fabrication capabilities, polymers
are becoming increasingly attractive for a variety of photonic applications including
telecommunications, optical interconnects, high-density data storage, optical processing, electro-optic (EO) modulation, switching and displays. The advantages of polymers
over the other materials for the fabrication of optical devices include the following [39].
(1) Polymers can provide a wider bandwidth of amplification if an appropriate gain
mechanism is identified. (2) The microstructure can be easily engineered at low cost to
provide desired optical parameters such as bandwidth of transparency, high EO
8.4 Photonic nanofibers
183
Fig. 8.19 The hysteresis loop of the FePt/PCL composite nanofibers [35].
Fig. 8.20 TEM micrographs of (a) as spun Fe3O4/PAN nanofiber, (b) Fe3O4/PAN nanofiber
calcinated at 700 C and (c) Fe3O4/PAN nanofiber calcinated at 900 C [37].
coefficient values, and temperature stability for specific applications. (3) The thermooptic (TO) coefficient (Dn/DT) of polymeric material is one order of magnitude, more
than that of SiO2; as a result a polymer-based thermal optical switch can potentially
perform both switching and variable attenuation functions simultaneously. (4) Unlike
any of the inorganic materials that cannot be transferred to other substrates, the
polymeric passive and active devices proposed herein can be easily integrated on any
surface of interest.
Polymer photonic fiber technology has advanced rapidly in recent years, with the
expectation that a polymer fiber will remain mechanically flexible, which, in combination with easy and inexpensive connectivity of the fibers during manipulation, is in
contrast to an inorganic photonic material. Owing to the capability of producing
photonic nanofibers in a simple way, electrospinning has attracted much attention in
recent years.
Electroactive nanofibers
3.70
Electrical conductivity (S/cm)
14
700°C
900°C
12
3.65
d(002) -900°C
10
3.60
8
3.55
6
4
d(002) (nm)
184
3.50
2
3.45
0
3.40
0
2
6
4
Fe3O4 (wt.%)
8
10
12
Fig. 8.21 Effect of Fe3O4 content on electrical conductivity and planar spacing of (002) plane
(d(002)) of carbonized samples [37].
8.4.2
Fluorescent nanofibers
Fluorescent nanofibers have potential applicability for nano-optoelectronic devices such
as photoluminescent (PL), electroluminescent (EL) and non-linear optical devices, flat
panel displays and photocathodes for solar cells.
Sui et al. [40] successfully prepared polyethylene oxide/ZnO (PEO/ZnO) composite
fibers via the electrospinning technique. It was shown that PEO passivated the interface
defects and quenched the visible emission of ZnO quantum dots by forming O–Zn
bonds with ZnO nanoparticles and that the passivation effect of PEO could be enhanced
by increasing the electrospinning voltage. Furthermore, the fibers prepared at higher
voltages exhibited more intense ultra-violet emission. Figure 8.22 shows that the fibers
electrospun at higher voltage exhibit more intense ultra-violet emission with small
FWHM and large IU/ID.
Wang et al. reported that 1-D TiO2 nanoparticles within polyvinylpyrrolidone (PVP)
fiber matrices were prepared via coupling sol–gel method and electrospinning [41].
TEM analysis indicated that a TiO2 nanoparticle chain was in the middle of the PVP
fiber and was dispersed linearly along the fiber direction, which originated from the
effect of polarization and orientation caused by a high electric field. With the increase of
the concentration of sol–gel precursor in the electrospinning solution, the diameter of
nanoparticles in resulting fibers decreased from 60 nm to 13 nm. Figure 8.23 shows the
TEM images of PVP/TiO2 composite fibers with different TBT concentrations at 10.5 wt%
(left) and 14.0 wt% (right). Figure 8.24 shows ultra-violet–visible spectra of various
PVP/TiO2 composite fibers with different TBT concentrations. This composite nanofiber
is anticipated to be applied in PL, electroluminescent (EL) and nonlinear optical devices,
and photocathodes for solar.
Yang [42] co-electrospun cadmium sulphide (CdS) quantum dots with polyethylene
oxide for the production of a fluorescent nanofiber composite. The results suggested
8.4 Photonic nanofibers
185
Fig. 8.22 (a) PL spectra of ZnO/PEO electrospun fibers under voltages of 12, 14, 16 and 18 kV
at room temperature. (b) Inset is the PL spectrum of ZnO composite fibers. The ultra-violet
emission peak IU/ID of ZnO/PEO electrospun fibers is shown as a function of the electrospinning
voltages [40].
Fig. 8.23 TEM images of PVP/TiO2 composite fibers with different TBT concentrations at 10.5 wt%
(a) and 14.0 wt% (b) [41].
successful transfer of optical properties from CdS quantum dots to polyethylene oxide
nanofibers, which demonstrated stable fluorescence properties and characterized the
photoluminescence. Figure 8.25 shows the photoluminescence of CdS–PEO electrospun composite fibers.
186
Electroactive nanofibers
Fig. 8.24 Ultra-violet–visible spectra of various PVP/TiO2 composite fibers with different TBT
concentrations: (a) PVP fibers, (b) 0.0, (c) 1.4, (d) 5.6, (e) 10.5 and (f) 14.0. Inset is PL spectrum
of TiO2 in PVP fibers [41].
Fig. 8.25 Photoluminescence of CdS–PEO electrospun composite fibers [43].
8.4.3
Photo-catalytic nanofibers
Photo-catalytic nanofibers have potential applications in highly oriented, wire-like
assemblies of metal nanoparticles for chemical or biological sensing, as antimicrobials
and as solar energy collectors, etc. TiO2 is one of the well-known oxide semiconductors
for photocatalysis. With the irradiation of ultra-violet light (>3.2 eV), the absorption of
photons by TiO2 promotes electrons from the valence band into the empty conduction
band, thus generating electron–hole pairs. The photogenerated holes can oxidize water
8.4 Photonic nanofibers
(a)
(b)
(c)
(d)
187
−200 nm
Fig. 8.26 SEM images of TiO2 nanofibers that have been irradiated by ultra-violet light in a solution
containing 1.0 10 4 m of HAuCl4 and different capping reagents at various concentrations for
1 h: (a) poly-vinyl pyrrolidone (PVP), 5.0 10 6 m; (b) PVP, 0.1 m; (c) poly-vinyl alcohol
(PVA), 5.0 10 6 m; and (d) PVA, 0.1 m. The scale bar in the insets is 50 nm [43].
to produce hydroxyl radicals, which can further oxidize organic contaminants. The
excited electrons have the ability to reduce some metal ions. Therefore TiO2 has been
intensively involved in research on photo-catalytic nanofibers.
Li et al. [43] demonstrated that gold nanostructures could be selectively deposited on
electrospun titania nanofibers through the photocatalytic reduction of HAuCl4 in the
presence of an organic capping reagent. Depending on the type and concentration of
capping reagent used, gold nanoparticles, fractal nanosheets or nanowires could be
obtained. The experiment results showed that thephotocatalytic reduction is well-suited
for decorating TiO2 nanofibers with gold nanoparticles. Figure. 8.26 shows the SEM
images of TiO2 nanofibers that have been irradiated by ultra-violet light in a solution
that contained 1.0 10 4 m of HAuCl4 and different capping reagents at various
concentrations for 1 h. The selective deposition scheme is expected to provide a new
route to the fabrication of highly oriented, wire-like assemblies of metal nanoparticles
for chemical or biological sensing.
Zhan et al. [44] reported fabricated mesoporous TiO2/SiO2 composite nanofibers
with a diameter of 100–200 nm and silica shell thickness of 5–50 nm by a sol–gel
188
Electroactive nanofibers
Fig. 8.27 Effect of irradiation time of white light upon the decomposition rate of methylene blue
by the photocatalysis of TiO2 nanotubes calcined at various temperatures and commercially
available TiO2 nanoparticles (ST-21) [45].
combined two-capillary co-electrospinning method. The selective photocatalytic activities of the composite nanofibers were investigated by photo-oxidation of methylene
blue (MB) and disperse red S-3GFL (DR) dyes. The absorptions at 664 nm and 228 nm
are proportional to the concentrations of MB and DR, respectively, and the concentrations of these two dyes are measured as a function of irradiation time. Using the
nanofibrous cores after dissolution of SiO2 as a photocatalyst, the absorptions of both
MB and DR almost completely disappear after irradiation for 50 min. However, using
the composite nanofibers as the photocatalyst, only the absorption peaks of MB disappear and the DR absorption at 228 nm hardly changes. The selective photocatalytic
activity of this nanofiber might open new potential fields of applications such as
separation processes.
Nakane et al. [45] demonstrated that photocatalysis of the TiO2 nanotubes was
superior to that of commercially available anatase type TiO2 nanoparticles (ST-21).
TiO2 hollow nanofibers (nanotubes) were fabricated by using electrospun poly-vinyl
alcohol (PVA) nanofibers as precursor nanofibers. The precursor nanofibers of the
PVA–Ti alkoxide hybrid were formed by the impregnation of Ti alkoxide into PVA
nanofibers obtained by electrospinning. The photocatalytic reaction using the TiO2
nanotubes formed was investigated. The nanotubes obtained had a high specific
surface area and the existence of mesopores on the nanotube wall was indicated by
the nitrogen adsorption isotherms. Figure 8.27 shows the relationship between the
decomposition rate of methylene blue and the irradiation time of white light for each
TiO2. The photocatalysis of the anatase type TiO2 nanotube (calcination at 500 C
and 600 C) is higher than that of ST-21, and the nanotube (calcination at 600 C)
shows the highest photocatalysis in this experiment. The specific surface area of
ST-21 is higher than that of TiO2 nanotubes, but the TiO2 nanotubes perform better
References
189
in terms of photocatalysis, which could be due to the mesoporous walls of the
nanotubes that lead to the efficient photocatalysis of TiO2 nanotubes.
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9
Nanocomposite fibers
A nanocomposite is a materials of which the matrix contains reinforcement materials
having at least one dimension in the nanoscale (<100 nm), wherein the small size offers
some level of controllable performance that is expected to be better than in conventional
composites. In another words, these nanocomposites should show great promise either in
terms of superior mechanical properties, or in terms of superior thermal, electrical, optical
and other properties, and in general, at relatively low-reinforcement volume fractions [1, 2].
The principle properties for such reinforcement effects are that (1) the properties of nanoreinfrocements are considerabley higher than the reinforcing materials in use and (2) the
ratio of their surface area to volume is very high, which provides a greater interfacial
interaction with the matrix [1]. Table 9.1 shows the geometries, types and surface-tovolume relations of reinforcements and their arrangement modes in fiber composites.
Table 9.2 lists the typical functional nanoparticles and matrices that have been used
for the composites. Among all the nano-reinforcements, carbon nanotubes (CNTs),
nanoclay, graphene and nanofibers are the most usually involved materials for the
structural nanocomposites that are introduced in this chapter.
9.1
Carbon nanotubes
Single-wall carbon nanotubes (SWCNT) and multi-wall carbon nanotubes (MWCNT)
are the two major types of nanotubes popular as reinforcement materials. The SWCNT
is made of a single graphene sheet rolled seamlessly into a cylinder with a diameter on
the order of 1 nm and a length that varies in the range of around 100 nm to the
centimeter scale [3–6]. The MWCNTs range from 2 nm to 100 nm in diameter and
can be tens of centimeters in length [7, 8]. The most common synthesis methods for
CNTs include laser ablation [9], chemical vapour deposition [10], arc discharge [11]
and gas catalytic decomposition [12, 13].
9.1.1
Structure and properties
9.1.1.1
Structure
Carbon nanotubes, composed of honeycomb lattices, are built from sp2 carbon units and
are structurally seamless. The SWCNT consists of a single seamlessly rolled graphene
sheet with a typical diameter on the order of 1 nm, while the MWCNT is an array of
192
9.1 Carbon nanotubes
193
Table 9.1 Reinforcement geometries and their arrangement mode in fiber composites
Dimension
Type
Surface/Volume relation
0-D
(3 Nano Axis)
Nanoparticle
Nanosphere
3
r
1-D
(2 Nano Axis)
Nanotube
Nanowire
Nanowhisker
Nanofiber
2 2
þ
r l
2-D
(1 Nano Axis)
Nanoplatelet
2 4
þ
t l
Arrangement mode
Table 9.2 Typical functional nanoparticles and matrices
Function
Nanoparticles
Polymer
Mechanical
Thermal
Electrical
Magnetic
Optical
Biological/ antimicrobial
Hydrophillicity
Photocatalytic
CNT, Graphene
Clay
CNT
Fe3O4
CdS
Ag, TiO2
SiO2
TiO2
PAN, PVDF, Cellulose
PAN
PEDT, SBS
PVDF, PCL PAN PEO
PEO PMMA
PCL, PLGA
PAN, PA66
PU
concentric SWCNT cylinders with a 0.35 nm separation between each two walls, which
is identical to the basal plane separation in graphite [5]. Also, MWCNTs tend to form in
bundles which are parallel in contact and consist of tens to hundreds of nanotubes [14].
Figure 9.1 illustrates a TEM image of SWCNTs, and Fig. 9.2 shows a TEM image of an
MWCNT.
The manner in which the graphene walls of the nanotube are rolled together can lead
to the formation of an armchair, a zigzag or a chiral structure [17] Figure 9.3 illustrates
the atomic structure of (a) an armchair and (b) a zigzag nanotube.
These groups are distinguished by their unit cells, which are determined by the chiral
vector given by the equation: Ch ¼ nâ1 þ mâ2, where â1 and â2 are unit vectors in the
2-D hexagonal lattice, and n and m are integers. Another important parameter is the
chiral angle, which is the angle between Ch and â1 [17]. Figure 9.4 shows a schematic
diagram of the way a hexagonal sheet of graphite is “rolled” to form a CNT.
194
Nanocomposite fibers
Fig. 9.1 TEM image of SWCNTs [15].
Fig. 9.2 TEM image of an MWCNT [16].
9.1 Carbon nanotubes
(a)
195
(b)
Fig. 9.3 Illustrations of the atomic structure of (a) an armchair and (b) a zigzag nanotube [17].
Armchair
Zig-Zag
or
ect
V
iral
Ch
Ch
a2
ma2
q. Chiral Angle
a1
na1
Fig. 9.4 Schematic diagram showing how a hexagonal sheet of graphite is “rolled” to form a
CNT [17].
196
Nanocomposite fibers
The nanotube is of the armchair type when n ¼ m and the chiral angle is 30o. When m
or n is zero and the chiral angle is equal to zero, the nanotube is known as a zigzag
nanotube. When the chiral angles are between 0o and 30o, chiral nanotubes are formed.
The chiral angles and tube diameter determine the properties of the nanotube. The CNTs
can be semiconducting or metallic. The metallic behavior occurs when n m¼3L and
L ¼ 0, resulting in the highest occupied molecular orbital and lowest unoccupied
molecular orbital (HOMO–LUMO), the fundamental gap being 0 eV. The electronic
properties are a result of the electrons being normal to the nanotube axis. While acting as
a semiconductor, the fundamental gap was found to be 0.5 eV, and a function of the
diameter which causes them to exist as ropes in their native state [18]. The energy gap is
determined by the equation Egap ¼ 2y0acc/d, where y0 is the C–C tight bonding overlap
energy (2.7 0.1 eV), acc is the nearest neighbour C–C distance (0.142 nm), and d is the
diameter. It was shown that a small gap would exist because of π/σ bonding orbital and
π*/σ* anti-bonding orbital at the Fermi level. The Fermi energy is the highest occupied
orbital, and has neighboring finite density carbon atoms in the flat sheet. The phase
difference is known to be at the 2π levels for a metallic tube and zero for a semiconductor. The density state occurs at sharp peaks as the energy level is increased.[19]
9.1.1.2
Mechanical properties
The interatomic bonding strength determines the mechanical properties of solid matter.
Based on the C–C sp2 bonding energy, CNTs are predicted to have high stiffness and
axial strength [20]. CNTs were reported have an elastic modulus greater than 1 TPa, and
a tensile strength of up to 150 GPa according to both experimental and theoretical
results [17, 21, 22]. Unlike carbon fibers, which easily fracture, nanotubes are found
very flexible. They can be elongated, twisted, flattened, or bent into circles before
fracturing, and regain their original shape up to a very large bending angle [23–25]. The
“kink-like” ridges allow the structure to relax elastically while under compression [26].
Nanotubes were also found to have an extremely large breaking strain, which decreased
with temperature [27]. The elastic modulus, Poisson’s ratio and the bulk modulus were
all found to be directly affected by the tube’s radius.
9.1.1.3
Electrical properties
It was shown that metallic conduction in CNTs can occur in the absence of the doping
effects. The I-tight-binding model within the zone folding scheme elucidated that one
third of CNTs were found to be metallic while two thirds were semiconducting, depending
on their chirality [20]. Semiconducting nanotubes have chirality-dependent band gaps
that are inversely proportional to the diameter, ranging from approximately 1.8 eV
for very small diameter tubes, to 0.18 eV for the widest possible stable SWCNT [28].
High-purity CNTs are extremely conductive. Owing to their 1-D nature and ensuing
quantum confinement effects, charge carriers can travel through nanotubes without
scattering, resulting in ballistic transport. The absence of scattering means that Joule
heating is minimized such that nanotubes can carry very large current densities, of up to
100 MA/cm2 [29]. In addition, carrier mobilities as high as 105 cm2/Vs have been reported
in semiconducting nanotubes [30]. Superconductivity has also been observed in
9.1 Carbon nanotubes
197
SWCNTs, albeit with transition temperatures of 5 K [31]. Theoretical and experimental results showed superior electrical properties of CNTs. They can produce electric
current carrying capacity 1000 times higher than copper wires [32]. The electronic
capabilities possessed by CNTs are seen to arise predominately from interlayer interactions, rather than from interactions between multilayers within a single CNT or
between different CNTs [33]. These optical properties have proved to be especially
unique, and confer either metallic or semiconducting capabilities, depending on tubule
diameter and chiral angle.
9.1.1.4
Thermal properties
Phonons determine the thermal properties (specific heat capacity and thermal conductivity) of CNTs [20]. A phonon is a quantum of acoustic energy identical to that of a
photon. Phonons are a result of lattice vibrations observed in the Raman spectra [33].
The phonon contribution to these quantities dominates and is due in particular to
acoustic phonons at low temperatures. Nanotubes have been found to be very conductive for phonons. Theory predicts that CNTs can have a room-temperature thermal
conductivity of up to 6000 W/m K [34]. While this has not yet been attained in practice,
values around 200 W/m K have been measured [35].
9.1.2
Dispersion of carbaon nanotubes
Owing to the nanoscale diameter, strong attractions present among CNTs make agglomeration the main challenge to uniform distribution of CNTs in CNT nanocomposites.
Therefore a great deal of effort has been focused on CNT dispersion when it comes to
CNT nanocompsites. Approaches for dispersion of CNTs have been proposed, including mechanical or chemical methods or a combination of these.
9.1.2.1
Purification
As-grown CNTs assemble as ropes or bundles with the presence of some catalyst
residuals, amorphous carbon, polyhedron graphite nanoparticles and other forms of
impurities. Thus, purification is necessary prior to any processes having CNTs involved
in applications.
Purification of CNTs usually involves the following methods: gas- or vapor-phase
oxidation [36], wet chemical oxidation [37], filtration [38, 39] or centrifugation [40].
Gas- or vapor-phase oxidation purification is typically conducted by annealing raw
CNTs at a high temperature ranging from 225 C to 760 C under an oxidizing
atmosphere such as air, a mixture of Cl2, H2O and HCl, a mixture of Ar, O2 and
H2O [41], a mixture of O2, SF6 and C2H2F4, H2S and O2, etc. [42].
Figure 9.5 compares the raw CNTs with oxidized CNTs at 750 C for 30 min, which
evidently shows the success of the oxidation method in removing impurities. However,
gas- and vapor-phase oxidation purifications are usually found to result in very low
yields. It should also be noted that metal particles present in raw material catalyze CNTs
in the presence of oxygen and other oxidizing gases. Therefore it is necessary to remove
the metal particles before running an oxidation removal of carbonaceous impurities [43].
198
Nanocomposite fibers
(a)
(b)
100 nm
Fig. 9.5 TEM images of CNTs: (a) raw CNTs produced by arc-charge and (b) CNTs oxidized at
750 C for 30 min [36].
Unlike gas- or vapor-oxidation, wet chemical oxidation using oxidative ions and acid
ions can simultaneously remove both amorphous carbon and the metal catalyst. Therefore selection of oxidant type and precise control of treatment condition can produce
high-purity CNTs in a high yield. The commonly used oxidants for wet chemical
oxidation include HNO3, H2O2 or a mixture of H2O2 and HCl, a mixture of H2SO4,
HNO3, KMnO4 and NaOH, and KMnO4 [42]. Nitric acid is the most commonly used
reagent for wet chemical oxidation purification for its mild oxidation ability, inexpensive and nontoxic nature [36].
Ultimately, the purification method consists of refluxing CNTs in 2.6 m nitric acid
and resuspending the nanotubes in pH 10 water with the aid of surfactant. The purified
CNTs are then filtrated with a cross-flow filtration system [44]. A further purification
process involving cutting the CNTs could be followed. [44]: 10 mg of the CNTs are
suspended in 40 ml of a 3:1 mixture of concentrated H2SO4/HNO3 in a 100 ml test tube
and sonicated in a water bath (55 kHz) for 24 h at 35 C to 40 C. The resultant
suspension is then diluted with 200 ml of water, collected on a 100 nm pore filter
membrane and washed with 10 mm NaOH solution. The tubes are then further polished
by suspension in a 4:1 mixture of concentrated H2SO4: 30% aqueous H2O2 with stirring
at 70 C for 30 min, followed by filtration and washing. However, strong oxidation and
9.1 Carbon nanotubes
199
Fig. 9.6 A combination process for purification of CNTs.
acid refluxing affect the structure of CNTs. These CNTs can be opened, shortened and
thinned. Wet chemical oxidation gives rise to functional groups on the nanotube surface
as well [43], which bring considerably property alterations for CNT.
Unlike oxidation, filtration works based on the differences of physical size, aspect
ratio and solubility between CNTs and impurities. Small size and low aspect ratio
particles and soluble matter will be filtered out along with the solution while CNTs will
remain. The filtration process is quite straightforward. The raw CNTs are dispersed in a
solution with or without surfactant, followed with repeated filtration or microfiltration
until a desired result is achieved. The advantage of filtration is that it avoids strong
chemical reactions, and thus leaves CNT undamaged. The disadvantage is that this
procedure is time consuming and ineffective in removing impurities that are stuck on
CNTs. Moreover, the purification result relies on the dispersion quality to a large extent.
Similar to filtration, centrifugation has been used to physically separate carbon
nanoparticles (CNPs) from CNTs at suitable concentrations based on the observation
that well-dispersed CNTs remains stable in the solution while CNPs precipitate as
sediment and therefore can be decanted, due to the difference of net surface charge [40].
Basically centrifugation is considered a supplementary mean to the other methods since it
can only separate graphenes from CNTs on the condition of a well dispersion.
Except filtration and centrifugation, another mean worthy of note is electrophoresis
[45–47] which is also a mild process for CNT purification by applying an electric field
to surfactant-stabilized single-walled CNT (SWCNT) suspensions and induces separation of metallic particles from CNTs relying on their different electric field-induced
polarizabilities. As a matter of fact, purification of CNTs is a combination of these
above methods in most cases, following the sequence with possible repetition of some
of the methods, as shown in Fig. 9.6. In some cases, gas- or vapor-phase oxidation may
be conducted before wet chemical oxidation.
9.1.2.2
Mechanical dispersion
CNTs can be mechanically dispersed in a solution or melt by means of ultrasonication,
high-shear-stress separation methods and ball milling.
Ultrasonication is an extremely common tool used to break up nanotube aggregates in
solution processing of CNTs because it is convenience and practical. Ultrasonication
dispersion is primarily done through a cavitation mechanism: bubbles nucleate at the
CNT surface and expand rapidly, pushing CNTs apart; CNTs remain separated after
implosion of the bubbles, hence the CNTs are dispersed [48, 49]. There are two major
200
Nanocomposite fibers
methods for delivering ultrasonic energy into liquids and hence into CNT bundles: the
ultrasonic bath and the ultrasonic horn. The frequency of the ultrasound determines the
maximum bubble size in the fluid. Low frequencies (about 20 kHz) produce large
bubbles and generate high energy forces as bubbles collapse. Increasing frequency
reduces bubble size and nucleation, as well as cavitation. An ultrasonication bath does
not produce a defined cavitation zone as does a horn, and the energy seems to be more
uniformly dispersed through the liquid phase [50] However, CNTs are usually found
damaged and shortened as a side effect of the involvement of high energy [51].
Buckling and compression defects, as well as stripping and fracture of CNT walls,
were observed. In practice, ultrasonication dispersion is an essential approach widely
involved with almost all the other mechanical methods and all the chemical methods.
The principle of high-shear-stress dispersion is the shear stress exerted on CNTs
during mixing that overcomes electrostatic and van der Waals’ interactions between
CNTs and results in the breakup of agglomerates [52]. High-shear-stress dispersion of
CNTs can be conducted with different apparatus including a high-shear homogenizer
[53, 54], single [55] or twin-screw extruder [56], injection-molding machine [57] and
microfluidic channels [58].
Ball milling is usually used for the opening [59] and shortening [60, 61] of CNTs, or
producing nanoparticles [62] from material sources including CNTs. As strong mechanical stresses shorten the length of CNTs, the entanglements of CNTs are subsequently
broken, which makes ball milling one of the solutions for the dispersion of CNTs in
solution or matrix.
Experimental results from Wang et al. [63] showed that most of the mechanical
methods cannot completely separate CNTs from entanglements but can decrease the
size of the CNT agglomerates for one or two orders when applied, respectively. With
the aid of a surfactant and a combination of ultrasonication, high-shear stress generated
from a homogenizer and a microfluidic system, SWCNT dispersion in water was found
to be significantly improved, as well as the stability [58].
9.1.2.3
Chemical dispersion
In order to achieve a uniform dispersion of CNTs, chemical methods are employed in
most cases. The CNTs are to be chemically functionalized either by cutting the entangled CNTs into short pieces, or by modifying the surfaces to improve the solubility of
CNTs. These chemical approaches are generally classified as covalent and noncovalent
functionalization, according to their structural effects on CNTs.
9.1.2.3.1 Noncovalent functionalization
Noncovalent functionalization is a very straightforward strategy for solubilizing CNTs
simply by noncovalent interactions, such as enclosing, π-stacking, wrapping and electron donor–acceptor complexation without causing structural damage to CNTs. Generally noncovalent functionalization of CNTs is fulfilled by mechanically dispersing
CNTs and chemical agents in a certain solvent to have CNTs uniformly coated with
the chemicals.
9.1 Carbon nanotubes
201
One type of the most common chemicals used for coating CNTs is a surfactant.
Surfactants are attracted to the surface of CNTs by hydrophobic interactions and induce
electrostatic or steric repulsions that can counterbalance van der Waals attractions
between CNTs. Some commonly used surfactants are sodium dodecyl sulfate (SDS),
Triton X-100 (TX-100), lithium dodecyl sulfate (LDS), cetyltrimethyl ammonium
chloride (CTAC) and dodecyltrimethyl ammonium bromide (DTAB). Polymers are
another type of chemical for coating CNTs. Amphiphilic polymers such as poly(styrene)-block-poly(acrylic acid) copolymers (PS49-b-PAA54, PS159-b-PAA58, or PS106-bPAA16) [64], polystyrene-block-poly(4-vinylpyridine) copolymers (PS-b-P4VP20,
PS-b-P4VP40, PS-b-P4VP70) [65], pyrene-containing copolymers ((1-pyrene)methyl
2-methyl-2-propenoate (PyMMP)-co-poly(MMA) and poly(ethylene-co-butylene)-bpoly(MMA-co-PyMMP)) [66] were found to be capable of enclosing CNTs and
forming soluble CNT micelles. Proteins [67–71] and peptides [72] were also found to
be excellent intermedia for separation of CNTs in solution.
Some chemicals can keep CNTs separated in solvents by wrapping them up. Linear
short-chain polymers such as polyvinyl pyrrolidone (PVP), polystyrene sulfonate (PSS),
poly((5-alkoxy-m-phenylenevinylene)-co-((2,5-dioctyloxy-p-phenylene)vinylene))
(PAmPV) derivatives [73], pentaoxoethylenedodecyl ether (C12E5), polyoxyethylene
lauryl(C12EO8) [74], amylase [75], porphyrin [76], polysaccharide (dextrin) and gum
arabic (GA) [77] were found gaining dispersion of CNTs with this mechanism, as
shown in Fig. 9.7 [78].
We may distinguish π-stacking from the wrapping approach by solubilizing CNTs via
short, rigid conjugated polymers. The rigid backbone of such polymers cannot wrap
around CNTs and thus the major interaction between polymer backbone and nanotube is
most likely to be π-stacking, which allows control over the distance between functional
groups and the CNT surface, through variation of the polymer backbone and side
chains [79]. Most polymers having π-stacking with CNTs are amphiphilic, therefore
the hydrophobic interaction is another major mechanism. Polymers that have been
explored are mostly polyelectrolytes, including poly(aryleneethynylene)s (PPE) [79],
fluorescein polyethylene glycol (Fluor-PEG) [80], 1-pyrenebutanoic acid [81], sodium
lignosulfonate (SLS) [82], oligothiophene-terminated poly(ethylene glycol) (TN-PEG)
[83] and humic acid (HA) [84].
CNTs are reported to behave amphoterically (in a Lewis acid/base sense), i.e. they
interact strongly with both electron donors and electron acceptors [85]. Alkali metals, small
molecules such as NH3 and H2, and larger molecules such as aromatic amines [86] have
been studied as electron donors. Halogens, small molecules like NO2 and O2, and organic
amine have been investigated as electron acceptors [85]. The solubility of SWCNTs in
anilines is up to 8 mg/ml, which is believed to be among the highest standard [86].
9.1.2.3.2 Covalent functionalization
Covalent functionalization is distinguished from the noncovalent approach by breaking
some carbon–carbon bonds of CNTs and making new bonds through chemical reactions. Covalently functionalized CNTs are expected to have a better solubility and can
be more easily integrated into inorganic, organic and biological systems.
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Nanocomposite fibers
Fig. 9.7 Some possible wrapping arrangements of PVP on an SWCNT [78].
As we have mentioned in the previous section, oxidation will cut CNTs into small
pieces and leave their ends open. In effect, these oxidative processes are capable of
generating a variety of oxygenated functional groups, such as aldehydic, ketonic,
esteric, alcoholic and carboxylic moieties [87–89], at the nanotube ends or along the
tube walls [90–92]. In particular, the predominance of carboxylic acid groups on CNTs
has been the basis for many of the subsequent functionalizations, thus far. Oxidized
SWCNTs have been solubilized by reaction with long-chain amines, by mixing with
polymers, through coordinate complexation using metal complexes, and even by
attaching titanium oxide nanocrystals and quantum dots using carbodiimide chemistry.
Fluorination is one of the popular approaches for CNT functionalization. CNTs can
be fluorinated by elemental fluorine at between room temperature and 600 C [93–97].
But the best results were achieved at temperatures between 150 C and 400 C [98].
Fluorinated SWCNTs are found to be appreciably soluble in alcoholic solvents with
evidence showing CNTs are separted from exfoliation of CNT ropes and bundles.
9.1 Carbon nanotubes
203
However, fluorination was found to cut the CNTs to an average length of less than
50 nm when applied to small diameter HipCO-SWCNT [99], and dramatically to impair
the electrical conductivity of SWCNTs. A sample of untreated SWCNTs was reported
to have a resistance of 10–15 Ω, while a fluorinated SWCNT sample had a resistance of
>20 MΩ [100]. The fluorine substituents on SWCNTs can be displaced by treatment
with strong nucleophiles such as Grignard and alkyllithium reagents [101–103] and
metal alkoxides [100]. Alkylated CNTs can disperse in common organic solvents like
THF and can be completely dealkylated upon heating at 500 C in an inert atmosphere,
to recover pristine CNT. Diamines [104, 105] and OH group-terminated moieties [106]
can also react with fluoronanotubes via nucleophilic substitution reactions by refluxing
fluoro-nanotubes in the corresponding diamine for 3 h in the presence of a catalytic
amount of pyridine. Aminoalkylated CNTs are soluble in dilute acids and water, while
hydroxyl CNTs form stable suspension solutions in polar solvents such as water,
ethanol and dimethylformamide. Organic peroxide functionalizations for phenyl and
undecyl sidewall attached CNTs from fluoro-nanotubes were also reported [107, 108].
Cycloaddition is another important type of chemical modification method of CNTs.
Cycloaddition of CNTs with dichlorocarbene was first reported by Haddon’s group [90,
109–111]. The cycloadditions were generated with potassium hydroxide from chloroform [90], and later from phenyl(bromodichloromethyl)mercury [110, 111], as shown
in Fig. 9.8. However, the functionalization degree in these cases was low. Sidewall
functionalization of CNTs via nitrene cycloadditions [90, 109–114] allows for covalent
binding of a variety of different groups such as alkyl chains, aromatic groups, dendrimers, crown ethers and oligoethylene glycol units, and leads to a considerable increase
Fig. 9.8 Dissolution and dichlorocarbene reaction of SWCNTs [111].
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Nanocomposite fibers
Fig. 9.9 Schematic depiction of oxidated SWCNTs followed by treatment with thionyl chloride,
and subsequent amidation.
in the CNT solubility in organic solvents such as 1,1,2,2-tetrachloroethane (TCE),
dimethyl sulfoxide (DMSO) and 1,2-dichlorobenzene (ODCB). The highest solubilities
of 1.2 mg/ml were found for SWCNT adducts with nitrenes containing crown ether of
oligoethylene glycol moieties in DMSO and TCE, respectively [114].
Grafting polymers with CNTs is a method that plays an important role in producing
CNT nanocomposites. Through “grafting to” or “grafting from” methods, oligomers
and polymers are covalently linked to the defect sites of CNTs formed during purification and oxidization, resulting in pastes containing well distributed CNTs, which then
go through a series of composite manufacturing process and are formed into CNT
composites.
The “grafting to” method means the readymade polymers react with the functional
groups on the surface of CNTs. Most reported “grafting to” procedures involve with
initial treatment of the oxidized CNTs with thionyl chloride for acryl chloride activation.
A typical example is as follows (Fig. 9.9). (1) Mix CNTs in SOCl2/dimethylformamide
(DMF) (volume ratio is 20:1), stir the solution at 70 C for 24 h, followed by centrifugation and washing the CNTs with anhydrous tetrahydrofuran (THF). (2) Heat the mixture
of the resulting CNTs and long-chain molecule octadecylamine (ODA) at 90 C to
100 C for 96 h, followed by removal of the excess ODA by washing in ethanol.
(3) Dissolve the remaining solid in dichloromethane, filter and dry. The yield of the
grafted SWCNTs is usually >60% (based on shortened SWCNTs) [109]. The obtained
SWCNTs are substantial soluble in chloroform, dichloromethane, aromatic solvents
(benzene, toluene, chlorobenzene, 1,2-dichlorobenzene) and carbon disulfide (CS2).
Polymers terminated with amino or hydroxyl moieties are commonly involved in amidation and esterification grafting reactions with the functionalized CNTs [115, 116]. So far
poly(styrene-cohydroxymethylstyrene) (PSA) [109], poly(styrene-co-p-(4-(40 -vinylphenyl)-3-oxabutanol)) (PSV) [117], poly(vinyl alcohol) (PVA) [118], poly(vinyl acetateco-vinyl alcohol) (PVA-VA) [119] and poly[3-(2-hydroxyethyl)-2,5-thienylene]
(PHET) [120], functionalized porphyrin(5-p-hydroxyphenyl-10,15,20-tritolylporphyrin,
por-OH) [121], oligomeric species containing derivatized pyrenes(3-decyloxy-5-pyrenyloxyphenylmethan-1-ol, IPy) [122] and lipophilic and hydrophilic dendra which are
terminated with long alkyl chains and oligomeric poly(ethylene glycol) moieties [123]
have been grafted to CNTs via ester linkages; poly(propionylethylenimine-coethylenimine) (PPEI-EI) [124], poly(styrene-co-aminomethylstyrene) (PSN) [125],
poly(amic acid) containing bithiazole ring [126], poly(propionylethylenimine-coethylenimine) (PPEI-EI) [127–129], poly(m-aminobenzene sulfonic acid) (PABS) [130],
monoamine-terminated poly(ethylene oxide) (PEO) [131] and glucosamine [132] have
been successfully grafted to acryl chloride activated CNTs via amidation reaction.
9.1 Carbon nanotubes
205
The “grafting from” method, also known as in situ synthesis, means the monomers
were covalently attached to CNTs and then polymerized for CNT containing polymers.
Polymers such as polystyrene (PS) [133], poly(sodium 4-styrenesulfonate) (PSS) [134],
poly(4-vinylpyridine) [135], polyamide (PA), and poly(methylmethacrylate)-blockpolystyrene (PMMA-b-PS) [136] have been radically grafted onto CNTs. Xu et al.
[137] constructed amphiphilic polymer brushes with a hard core of multi-walled
CNTs (MWCNTs) and a relatively soft shell of polystyrene-block-poly
(N-isopropylacrylamide) (PS-b-PNIPAM) by in situ surface reversible addition–
fragmentation chain transfer (RAFT) polymerization of styrene and N-isopropylacrylamide
on the modified convex surfaces of MWCNTs (MWCNT-PS). In situ atom transfer radical
polymerization (ATRP) is a novel approach for functionalization of CNTs for its capability
of controlling the thickness of functional layers by control the molecular weight of graft
polymer chains. Kong et al. [138] summarized the general four-step strategy for grafting
polymers from the CNTs via ATRP: (1) functionalization of CNT (CNT-COCl) with
carbonyl chloride groups, (2) introduction of hydroxyl groups onto the surface of CNT
by reaction of CNT-COCl with glycol, generating CNT-OH, (3) initiation sites (CNT-Br)
for ATRP by reacting CNT-OH with 2-bromo-2-methylpropionyl bromide, and (4)
grafting polymerization of methyl methacrylate (MMA) from MWCNT-Br by means of
in situ ATRP. The resulted MWCNT-PMMA following this procedure showed a relatively
good solubility in non- or weakly polar solvents such as THF and CHCl3, but a poor
solubility in strong polar solvents such as DMF and DMSO [138]. In situ nitroxide
mediated polymerization (NMP) is a very attractive method for CNT/polymer composites
with controllable polymer architecture. This method does not involve any indispensable
CNT pre-treatment and can be used for polymerization of hydrophilic polymers, such as
acrylic acid. Using NMP methods, Datsyuk et al. [139] successfully polymerized amphiphilic block copolymers (a hydrophilic block (polyacrylic acid PAA) and a hydrophobic
block (polystyrene PS)) with double-walled CNTs(DWCNTs).
9.1.3
Alignment of carbon nanotubes
To take full advantage of the anisotropic nature of nanotubes, it is important to have
CNTs aligned in a polymer matrix. The aligment of CNTs in a matrix can be fulfilled
before, during or after process the composite fabrication process. Before or during the
process, an external force field, such as a magnetic or electric field, or a shear force, is
usually applied to the fluid containing CNTs to align CNTs. After the process, shear
stress mechanically applied through slicing, rubbing or stretching becomes a effective
method for CNT alignment.
9.1.3.1
Alignment of carbon nanotubes in solution
A high magnetic field has been shown to be an efficient and direct means to align CNTs.
Fujiwara et al. [140] susessfully aligned arc-grown MWCNTs by applying a high
magnetic field of 7 T to an MWCNT dispersion in methanol. The MWCNTs were
found aligned parallel to the field. The result was explained by the difference between
the diamagnetic susceptibilities parallel and perpendicular to the tube axis. If the parallel
206
Nanocomposite fibers
Fig. 9.10 TEM of a 100 nm thin film of 1 wt% MWCNT–polyester composite [141]. The
composite was sliced (a) parallel and (b) perpendicular to the applied magnetic field.
diamagnetic susceptibility is larger than the perpendicular diamagnetic susceptibility,
MWCNTs tend to align parallel to the magnetic field by overcoming the thermal energy.
Based on the work done by Fujiwara et al., Kimura et al. [141] demonstrated that this
technique is helpful for fabricating aligned nanotube reinforced polymer composites
possessing anisotropic electrical and mechanical properties. It can be seen from
Fig. 9.10 that MWCNTs parallel to the direction of the magnetic field are relatively
longer than those in a perpendicular direction.
9.1.3.1.1 Electric field
Application of an electric field during nanocomposite curing can induce the formation
of aligned conductive nanotube networks between the electrodes. Through this method,
Martin et al. [142] achieved well-aligned CVD-grown MWCNTs as electrically conductive fillers in an epoxy system based on a bisphenol-A resin and an amine hardener.
Figure 9.11 shows a transmission optical micrograph of the MWCNT containing epoxy
composites.
9.1.3.1.2 Liquid crystalline phase
Liquid crystal phase can force CNTs aligned to a great extent. The cooperative reorientation of liquid crystals and the overall direction of the nanotube alignment can be
further controlled both statically and dynamically by the application of external electric,
magnetic, mechanic, or even optic in nature fields [143]. When dispersed in superacids
at low concentrations, SWCNTs were found to dissolve as individual tubes behaving as
Brownian rods. At higher concentrations, SWCNTs formed an unusual nematic phase
consisting of spaghetti-like self-assembled supermolecular strands of mobile, solvated
tubes in equilibrium with a dilute isotropic phase. At even higher concentrations, the
spaghetti strands self-assemble into a polydomain nematic liquid crystal. The liquid
9.1 Carbon nanotubes
207
Fig. 9.11 Transmission optical micrographs of epoxy composites containing well-aligned 0.01
wt% multi-wall carbon nanotubes during curing at 80 oC in a D.C. field of 100 V/cm [142].
crystal phase separated into needle-shaped strands (20 μm long) of highly aligned
SWCNTs, termed alewives, upon the introduction of small amounts of water [144].
9.1.3.2
Alignment of carbon nanotubes in matrix
9.1.3.2.1 Composite slicing
Ajayan et al. reported a CNT alignment method in a polymer matrix by cutting thin
slices (50–200 nm) of a CNT–polymer composite [145]. The nanotube-reinforced
composite demonstrated a parallel and well-separated arrangement of nanotubes. Nanotube alignment by composite slicing showed the nature of rheology on nanometer scales
in composites and flow-induced anisotropy, produced by the cutting process.
9.1.3.2.2 Film rubbing
DeHeer et al. [148] reported that aligned nanotube-reinforced composites could be obtained
by rubbing the surface of a randomly aligned nanotube film with a thin Teflon sheet or
aluminum foil. The nanotube film was first prepared by drawing a nanotube suspension
through a 0.2 μm pore ceramic filter, which left a uniform black deposit on the filter. The
deposited material was then transferred onto a plastic surface (Delrin or Teflon) by pressing
the tube-coated side of the filter onto the polymer. The surface facing the filter was exposed
by lifting it from the filter. A remarkable transformation was observed when this surface
was lightly rubbed with a thin Teflon sheet or aluminum foil [146]. The surface became
silvery in appearance and was found to be densely covered with nanotubes.
9.1.3.2.3 Mechanical stretching
Jin et al. demonstrated nanotube alignment in a polymer matrix by mechanical stretching [147]. The composites were fabricated by casting a suspension of CNTs in a
solution of a thermoplastic polymer and chloroform. These composites were uniaxially
stretched at 100 C and were found to remain elongated after removal of the load at
room temperature. The orientation and the degree of alignment were determined by
208
Nanocomposite fibers
Fig. 9.12 TEM image of an internal fracture surface of a composite after being sliced parallel to the
stretching direction by a microtome [147].
X-ray diffraction. Figure 9.12 shows a TEM of an internal fracture surface of a
composite after being sliced parallel to the stretching direction by a microtome. The
sample thickness is about 90 nm. It shows fiber pull-out. The nanotubes are aligned
parallel to the stretching direction. In some areas, nanotubes bridge the microvoids (or
microcracks) in the matrix and presumably enhance the strength of the composite.
Haggenmueller et al. [148] applied a high draw ratios (λ ¼ 20–3600) stretching to
melting spun SWCNT/PMMA fibers. Raman spectra of the fibers at the indicated fiber
angles with respect to the incident laser polarization axis showed the mosaic distribution
width at half maximum (FWHM) was as small as 4 , which indicates the good
alignment of SWCNTs along the fiber axis. Owing to the alignment of the SWCNTs,
these PMMA/SWCNT nanocomposite fibers show improved mechanical properties and
the nanocomposite films show increased electrical conductivity.
9.1.3.2.4 Electrospinning
Alignment of nanotubes in a polymer matrix can also be achieved through electrospinning. It was shown that electrospinning is an effective and efficient technique to induce
nanotube alignment in a polymer composite [149–152]. During co-electrospinning of
9.1 Carbon nanotubes
209
Fig. 9.13 “Logs in a river” analogy (LIRA).
CNT and polymer, alignment of the CNT is induced by the flow of the polymer
solution, the presence of electrostatic charge, and nanometer diameter confinement.
Flow induced alignment is analogous to “logs in a river,” as shown in Fig. 9.13, where
the SWCNT tends to orient parallel to the fiber axis. The presence of electrostatic charge
further orients the CNT along the fiber due to the stretching of the polymer jet.
Stretching of the polymer jet also induces molecular orientation. Lastly, diameter
confinement is due to the fact that the produced fibers have diameters on the nanoscale
and the SWCNTs are 1–2 μm in length. Normally, the SWCNTs appear in bundles of
relatively longer length. Therefore, orientation of the SWCNT ropes along the fiber
direction is confined by the diameter of the nanofiber.
9.1.4
Carbon nanotube nanocomposite fibers
The desire to translate the outstanding mechanical, thermal and electrical properties of
individual CNTs into bulk structures has motivated the search for continuous fabrication
methods for nanotube assemblies. In order to exploit the remarkable mechanical properties
of an individual nanotube, continuous CNT fibers (collection of CNT) and yarns (collection
or linear assembly of CNT fibres) purely or largely comprising CNTs have been fabricated
by several methods. In this section the key methods for producing continuous CNT
composite fibers and yarns are reviewed. To demonstrate the feasibility of manufacturing
nanotube-reinforced multifunctional continuous yarns, the structure and properties of
SWCNT-filled continuous PAN yarns produced by electrospinning and MWCNTreinforced continuous cellulosic yarns by liquid crystal electropinning are presented.
A number of processes are dedicated to the production of CNT based fiber and
fibrous assemblies. Each of these processes has its own unique features and thus
potential applications. Table 9.3 provides a summary of the structural features of
various CNT fiber/yarn produced by different processes and their corresponding
strength and modulus. One obvious observation is that none of these CNT based
structures comes close to reaching the property potential of the CNT. Similar to staple
textile yarn structures, the mechanical properties of CNT assemblies are governed by
the cohesive/interaction forces between CNTs. Therefore the critical factor for making a
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Nanocomposite fibers
Table 9.3 Properties of CNT fibers and yarns [154]
Fiber
Method
Neat
SWCNT
fibers
CVD [155]
Dry spinnning
[156, 157]
Liquid crystal
spinning (Smalley)
[158, 159]
Liquid crystal
spinning (10%
SWCNT/PBO) [160]
Wet spinning
[161–163]
Electrospinning [152]
Composite
CNT fibers
Gel spinning
(5% MWCNT/
UHMWPE)
Volume
fraction
Alignment
fraction
Content
(%)
Diameter
(μm)
Modulus
(GPa)
Strength
(GPa)
< 0.48
0.53
0.85
twist angel
21
0.9 mosaic
angle 31
95
720
210
360
330
4(9)
3.3
100
/
120
0.1
10 wt%
/
10
25
167
4.2
/
Alignment
angle 9
/
60
50
80
3.2
4
/
207
/
/
5
/
136.8
4.2
0.70
0.028
(4 wt%)
/
strong CNT assembly is related to the perfection of the structural features or fiber
architecture, which depends on the packing density and orientation of the CNTs in the
fiber and yarn structures. Therefore a hybrid process combining an ability to orient
CNTs and a defect-free matrix to transfer the load would provide the pathway to
connect the outstanding properties of CNTs to the fiber and yarn structures. Owing to
the alignment ability and nanosize, electrospinning is expected to achieve nanotube
nanocomposites with a high load transfer ratio [153, 154].
9.1.4.1
Methods for producing carbon nanotube fibers
In the decade since the discovery of CNTs, several methods have been developed to
manufacture fibres and yarns from CNT of discrete length. These methods include:
chemical vapor deposition (CVD) process [164–169], solid-state processes [156, 157,
170, 171], electrophoretic proceses [172], liquid crystal spinning [158, 173], wet
spinning [163, 174–182], electrospinning [183–189] and traditional spinning, which
includes traditional wet spinning and melt spinning. The first three processes produce
pure CNT fibers, whereas the latter three produce CNT composite fibers. The liquid
crystal spinning process can produce pure CNT fibers as well as composite fibers. Each
method has its advantages and disadvantages. Invariably, these methods are still in the
laboratory development stage. There is a strong desire to transition these methods to
robust manufacturing processes.
9.1.4.2
Chemical vapor deposition
Strands of CNTs have been produced by variations of the chemical vapor deposition
(CVD) process [164–167]. Alan Windle’s group showed the possibility of obtaining
continuous fibers without an apparent limit to the length by mechanically drawing the
9.1 Carbon nanotubes
211
Fig. 9.14 CVD synthesis and spinning set-up for the fabrication of continuous CNT yarns. (a)
Schematic diagram of the synthesis and spinning set-up. (b) A photograph showing a layered
CNT sock formed in the gas flow, and spinning of the finished fiber on the final spool (c) [192].
CNTs from the chemical vapor deposition (CVD) synthesis zone of a furnace [168,
169]. In the MWCNT fibers, the nanotube diameters were 30 nm, with an aspect ratio of
1000. They contained 5 wt%–10 wt% iron but no extraneous carbon particles. The
quality of alignment of the nanotubes measured from transforms of scanning electron
microscope images showed the full width (at half maximum) of the inter-nanotube
interference peak measured around the azimuthal circle was 11 . There are indications
that the degree of alignment can be improved if greater tension is applied to the fiber
during processing. The SWCNT fibers contained more impurities than the MWCNT
fibers, with the proportion of SWCNTs estimated from transmission electron microscope observations as being 50 vol%. The SWCNTs had diameters between 1.6 nm and
3.5 nm and they were organized in bundles with lateral dimensions of 30 nm. The
highest strength reported for direct spun CNT fiber was 2.2 N/tex, and the stiffness
was 160 N/tex (equivalent to 4.4 GPa and 320 GPa respectively, assuming a density of
2.0 g/cc) These values are within the range of properties of typical carbon fibers [190].
The electrical conductivity measured along a fiber was 8.3 105 Ω1m1, which is
slightly higher than the typical value for carbon fibers [191]. Li’s group [192] further
developed a water-densification and spinning process for this technique, which allows
fabrication of continuous CNT yarns with a length of over several kilometers, as
shown in Fig. 9.14. The yarn consists of multiple monolayers of CNTs concentrically
assembled in seamless tubules along the yarn axis. The yarn quality close to conventional textile yarns. This direct spinning process allows one-step production of nanotube
fibers, ribbons and coatings with potentially excellent properties and wide-range applications [191].
The CVD process has currently been scaled up in 2004 by Nanocomp Technologies,
as shown in Fig. 9.15. It has been reported that Nanocomp Technologies has produced
and delivered 10 km of CTex™ CNT yarn to aerospace customers, and it was claimed
that they are capable of delivering 4 8 square foot CNT mats [193]. Taking advantage
of the outstanding properties of the CNT products, value added components such as
conductive cables, thermal straps, EMI shielding “skins,” and high strength sheets or
yarns for incorporation into final end-user products are also being developed.
212
Nanocomposite fibers
Fig. 9.15 Scale-up CVD process [193]. Equipment in Nanocomp Technologies Inc: (a) CVD setup;
(b) CTex™ CNT yarn; (c) CNT mats.
9.1.4.3
Dry spinning
Dry spinning is a method by which the carbon nanotube yarns are drawn from an asgrown nanotube forest [170]. This process is quite similar to that used in the conversion
of card webs to slivers and subsequently twisted yarns in textile manufacturing. As
shown in Fig. 9.16, during spinning a collection of carbon naotubes from a CNT forest
are being drawn-twisted to form a linear twisted CNT yarn assembly. The strength
development of the CNT forest derives from the cohesion of the CNT assembly and the
entanglement of CNTs as in the staple yarn. As the CNTs at the edge of the forest are
pulled away from the forest, the CNTs cling together and form a continuous strand.
Because of high inter-fiber contact area per yarn volume as a result of the very high
surface-to-volume ratio of the MWCNTs, the yarns can be twisted, knitted and knotted
to form higher-order structures [171]. The highest tensile strength of CNT fibers
attained is 3.3 GPa [156], spun from a 1 mm array, which is much higher than that of
CNT fibers from the 0.65 mm array (1.91 GPa) [157]. The highest reported Young’s
modulus is up to 330 GPa [157].
9.1.4.4
Liquid crystal spinning
Analogous to the formation of conventional rod-like polymers such as poly(pphenylene benzobisoxazole) (PBO) and poly(p-phenylene benzobisthiazole) (PBZT),
Smalley’s group developed a liquid crystal spinning process for pure CNT yarns [158,
194]. A dispersion of purified SWCNTs (5 wt%–10 wt%) in 102% sulfuric acid (2 wt%
excess SO3) were prepared in a nitrogen-purged dry box. The mixture was manually
mixed and then transferred to the mixing apparatus via a stainless steel syringe. The
material was subsequently extruded through a small capillary tube into a coagulation
bath. Continuous lengths of macroscopic neat SWCNT fibers were obtained. The neat
SWCNT fibers possess a Young’s modulus of 12010 GPa, at a tensile strength of
11610 MPa. The relatively low strength is attributed to the presence of localized
defects and voids [158].
To take advantage of the extraordinary mechanical, electrical and thermal properties
of CNTs, studies have been done to reinforce polymers and other matrix systems with
CNTs. Kumar et al. [160] synthesized PBO in poly(phosphoric acid) (PPA) in the
9.1 Carbon nanotubes
213
Fig. 9.16 Dry spinning of CNT yarns from CNT array: (a) overview; (b) close-up of self-assembly
of CNTs; (c) detail of twist insertion; and (d) detail of yarn structure. [171].
presence of purified HiPco SWCNTs with 5 wt% and 10 wt% concentration. The solution
was dry-jet wet spun into water coagulant with an air gap of 10 cm. For 10 wt%
SWCNT solutions, the modulus, tensile strength and elongation at break of obtained
fibers were improved by 20%, 60% and 40% respectively for (90/10) PBO fiber.
9.1.4.5
Wet spinning
Vigolo’s group has successfully coagulated SWCNT-reinforced poly(vinyl acetate)
(PVA) fibers [163, 174–179] by using the wet spinning process by injecting SWCNTs
aqueous solution into a rotating bath of PVA solution to obtain gel-like composite fibers
[174, 180]. Although strong nanotube composite fibers were obtained after wash and
post-processes, the condensed gel-like fibers are difficult to handle thus limiting chance
to scale up the process.
To improve the mechanical properties and solve the handling problem, Baughman’s
group modified the Vigolo process and enabled it to produce continuous SWCNT/PVA
composite fibers [181, 182] as shown in Fig. 9.17. The gel fibers were assembled by
manual pulling from the coagulation bath and mechanically drawn using a range of
draw ratios. The composite fibres were subsequently dried in air (with or without further
washing) [182]. For the treated fibers, the density-normalized modulus and strength
values were measured to be 56 GPa cm3/g and 1.30 GPa cm3/g1 respectively. From the
dried fiber density of 1.4 g/cm3 (measured by flotation), these density-normalized
parameters translate to modulus and strengths of 78 GPa and 1.8 GPa, respectively.
While these fibers contain about 60% SWCNTs by weight (from thermogravimetric
analysis), the measured properties fall far short of the theoretical value of individual
SWCNTs. The fibers reported here have comparable properties to PVA-SWCNT fibers
214
Nanocomposite fibers
Fig. 9.17 Schematic diagram of the Baughman continuous spinning system [182].
prepared by a similar spinning method that are post-processed by hot drawing. Moreover, the properties achieved are comparable to other high-performance natural such as
spider silk and synthetic fibers such as Kevlar.
9.1.4.6
Traditional spinning
CNT-reinforced polymer composite fibers have also been obtained through traditional
spinning. SWCNT-reinforced polyacrylonitrile (PAN) and PVA fibers, and MWCNT
cellulose composite fibers were produced by traditional wet spinning process
[195–198]. The highest reported Young’s modulus is 80 GPa at a tensile strength of
3.2 GPa [162, 163]. The first melt spun composite SWCNT fibers were produced by
Andrews et al. using carbon pitch as the main component [199]. The results highlight
the potential that exists for developing a spectrum of material properties through the
control of the matrix, nanotube dispersion, alignment and interfacial bonding [199].
SWCNT functionalization and surfactant stabilization improved the nanotube dispersion in solvents, but only functionalization was shown to be capable of improving the
dispersion in composites. However, functionalization induced nanotube length decrease
and larger nanotube separation resulted from functional groups limited mechanical and
electrical properties of the composites containing these nanotubes [200]. Gao et al.
reported a chemical processing method that allows the continuous spinning of singlewalled CNTs (SWCNTs)–nylon 6 (PA6) fibers by the in situ polymerization of caprolactam in the presence of 0.1 wt%–1.5 wt% SWCNTs. This process simultaneously
optimizes the morphology of the composite [201].
9.1.4.7
Electrospinning
CNTs have been co-electrospun with a wide range of polymers including polyethylene
oxide (PEO) [183, 184], polyacrylonitrile (PAN) [184, 188, 202], polyvinyl alcohol
(PVA) [184], polymethyl methacrylate (PMMA) [187], and Bombyx mori silk [189,
203–205]. By encapsulation of CNT in a polymeric nanofiber matrix the CNTs are
protected from direct contact and facilitaes stress transfer from the CNT to the linear
composite assembly. CNT composite nanofibers have been shown not only to enhance
the mechanical properties but also improve the electrical conductivities and thermal
conductivities of the matrix.
Through continuously electrospun SWCNTs into PAN nanofibers, Ko et al. [152]
demonstrated the feasibility of incorporation of CNTs in nanofibers through
9.2 Nanoclay
215
electrospinning and proved the alignment effects on CNTs of this technique. SWCNTs
were shown to maintain their straight shape and were parallel to the axis direction of the
PAN fibres indicating that a good alignment of SWCNTs has been achieved. The
improved orientation also resulted in a better distribution – nearly every investigated
section of the polymer fibers contained at least some SWCNTs. The elastic modulus of
the fiber was calculated to be around 207 GPa using the linear portion of the load–
deformation curve at small deformation and under low forces (<100 nN).
9.2
Nanoclay
9.2.1
Structure and properties
Nanoclays, from the smectite family, have a unique morphology. They form platelets of
about 1 nm thick and 100 nm in diameter. The essential nanoclay raw material is
montmorillonite (MMT), a two-to-one layered smectite clay mineral with a platey
structure. Individual platelet thickness of the MMT is just 1 nm, but the lateral
dimensions of these layers may range from 200 nm to 2000 nm, resulting in an
unusually high aspect ratio. The crystal structure of each layer contains two outer
tetrahedral sheets, filled mainly with Si, and a central octahedral sheet of alumina or
magnesia (Fig. 9.18). The layers are separated by a very small gap, called the interlayer
or the gallery. The negative charge, generated by isomorphic substitution of Al3þ with
Mg2þ or Mg2þ with Liþ within the layers, is counterbalanced by the presence of
hydrated alkaline cations, such as Na or Ca, in the interlayer. Since the forces that hold
the layers together are relatively weak, it is possible to intercalate small organic
molecules between the layers. The surface area of each nanoclay particle is around
750 m2/g and the aspect ratio is larger than 50 [1, 206].
Fig. 9.18 Crystal structure of smectite clay [206].
216
Nanocomposite fibers
9.2.2
Clay nanocomposites
Clay can be incorporated with polymers by dispersing with polymeric species or with
the monomers, which will be subsequently be polymerised in situ to give the corresponding polymer–clay nanocomposite. The second method is the more successful
approach to date. Both thermosets and thermoplastics have been used for clay nanocomposites, including nylons, polyolefins (e.g. polypropylene), polystyrene, ethylenevinyl acetate (EVA) copolymer, epoxy resins, polyurethanes, polyimides and poly
(ethylene terephthalate) (PET).
One important consequence of the charged nature of the clays is that they are
generally highly hydrophilic species and thus naturally incompatible with a wide range
of polymer types. A necessary pre-requisite for successful formation of polymer–clay
nanocomposites is therefore alteration of the clay polarity to make the clay “organophilic.” An organophilic clay can be produced from a normally hydrophilic clay by ion
exchange with an organic cation such as an alkylammonium ion.
Depending on the nature of the components used (layered silicate, organic cation and
polymer matrix) and the method of preparation, three main types of composites may be
obtained when layered clay is associated with a polymer [1, 207], as shown in Fig. 9.19.
(1) The phase-separated dispersion, in which the polymer is unable to intercalate the
silicate sheets and the silicate particles are dispersed as phase-separated domains, called
tactoids. (2) The intercalated dispersion, in which one or more polymer molecules are
intercalated between the silicate layers. The resulting material has a well-ordered
multilayered morphology of alternating polymer and silicate layers. The spacing
between the silicate layers is between 2 and 3 nm. (3) The exfoliated dispersion, in
which the silicate layers are completely delaminated and are uniformly dispersed in the
polymer matrix. The spacing between the silicate layers is between 8 nm and 10 nm.
This is the most desirable dispersion for improved properties.
The intercalation of the polymer chains usually increases the interlayer spacing, in
comparison with the spacing of the organoclay used, leading to a shift of the diffraction
peak towards lower angle values in XRD pattern [207]. Figure 9.20 shows the typical
XRD patterns for different intercalated structures of polymer/silicate hybrids.
Transmission electronic spectroscopy (TEM) can be used to characterize the nanocomposite morphology [208]. Figure 9.21 shows the TEM micrographs obtained for the
three types of dispersions. Besides intercalation and exfoliation, other intermediate
arrangements can exist presenting both intercalation and exfoliation. In this case, a
broadening of the diffraction peak is often observed by XRD and one must rely on TEM
observation to define the overall structure [207].
Incorporation of nanoclay in polymer composites can significantly improve a variety
of properties of the composites, including mechanical properties, gas barrier properties
and fire retardant properties. The mechanical analysis conducted by Fischer’s group
indicates an increase in elastic modulus and the stiffness of monomorillonite–polyamide
nanocomposites compared to the pure polymer matrix by 100%–150%; the yield at
break has been increased by 20% [209]. Similar mechanical property improvements
were presented for polymethyl methacrylate–clay hybrids [210]. The gaseous barrier
9.2 Nanoclay
217
Fig. 9.19 Three main types of clay dispersion in composites [207].
Fig. 9.20 Typical XRD patterns from polymer/silicate hybrids. (a) XRD obtained from an
“immiscible” system (here polyethylene/C18FH), and is identical with the XRD of the neat or
gano-silicate (C18FH). For intercalated hybrids the d-spacing shifts to a higher value (b) as the
gallery expands to accommodate the intercalating polymer (here polystyrene/ C18FH); second
and third order reflections – as shown here – are very common and sometimes intercalated hybrids
can have up to 13 order reflections, manifesting a remarkable long-range registry. (c) Typical
XRD of an exfoliated/delaminated structure or a disordered system (here a siloxane/C18FH
delaminated hybrid) [207].
property improvement can result from incorporation of relatively small quantities of
nanoclay materials. The oxygen and carbon dioxide barrier properties of the PP/EPDM
blend demonstrated about two-fold improvement with the addition of only 1.5 vol%
organoclay. Both the amount of clay incorporated in the polymer and the aspect ratio of
the filler contribute to the overall barrier performance. In particular, the aspect ratio is
shown to have a major effect: the higher the ratio, the more dramatic the enhancement of
gaseous barrier properties. Such excellent barrier characteristics have resulted in
218
Nanocomposite fibers
Fig. 9.21 TEM micrographs of nanoplatelet in epoxy matrices. (a) Phase-separated dispersion;
(b) intercalated dispersion; and (c) exfoliated dispersion [208].
considerable interest in nanoclay composites in food packaging applications, both
flexible and rigid [211]. Nazaré et al. studied the effects of nanoclay on the flammability
properties of unsaturated polyester resin by using cone calorimetry. The results suggested that incorporation of nanoclays (5% w/w) reduces peak heat release rate (PHRR)
by 23%–27% and total heat release (THR) values by 4%–11%. The fire growth rate
index (FIGRA) is also reduced by 23%–30% following nanoclay inclusion [212].
9.2.3
Nanoclay nanocomposite fibers
Since the mechanical properties of fibers in general improve substantially with decreasing fiber diameter, there is considerable interest in nanoclay-reinforced electrospun
nanofibers.
The crystalline strucuture of electrospun nanofibers was shown to be significant
improved by addition of nanoclay. Lincoln et al. [213] reported that the degree of
crystallinity of nylon-6 annealed at 205 C increased substantially with the addition of
MMT, implying that the silicate layers could act as nucleating agents and/or growth
accelerators. Fong et al. showed a very similar overall degree increase of the crystallinity for electrospun nylon-6 and nylon-6/cloisite-30B nanocomposite fibers containing
7.5 wt% of O-MMT layers [214]. Fornes and Paul [215] found that O-MMT layers
could serve as nucleating agents at the low concentration of 3% in a nylon-6/O-MMT
nanocomposite, but retarded the crystallization of nylon-6 at high concentration of
around 7%. Li et al. [216] demonstrated the increase of both crystallinity and the
crystallite size of nylon-6 nanofibers reinforced by organically modified montmorillonite (O-MMT), cloisite-30B.
Crystal polymorphism was found in electrospun nanoclay-reinforced poly(vinylidene
fluoride) (PVDF) fibers [217]. Both of the nanoclays, Lucentite™ STN and SWN,
induced more extended PVDF chain conformers in the beta and gamma phases but
reduced the alpha phase conformers in electrospun PVDF/nanoclay composite nanofibers. With the attached organic modifier, even a small amount of STN can totally
eliminate the non-polar alpha crystal conformers while SWN cannot. The ionic organic
modifier makes STN much more effective than SWN in causing crystallization of the
polar beta and gamma phases of PVDF. The behavior was explained by an ion–dipole
interaction mechanism, as suggested by Ramasundaram et al. [218]: the partially
9.2 Nanoclay
219
Fig. 9.22 The possible interaction between hectorite and PVDF chain in the electrospun composite
nanofiber: partially positive C–H bond is attracted by the negatively charged silicate layer in
STN and SWN due to the static electric force [217].
positive CH2 dipoles on the PVDF chains could have an ion–dipole interaction with the
negatively charged nanoclay platelets and make the polymer chains align on the surface
of hectorite, which will enhance the extended TTTT conformers and result in formation
of the beta crystallographic phase. The tiny amount of TTTGTTTG’ conformers
existing in the nanofiber is likely to be due to the gauche effects formed from local
internal chain rotation. Figure 9.22 shows the possible interaction between hectorite and
PVDF chain in the electrospun composite nanofiber: the partially positive C–H bond is
attracted by the negatively charged silicate layer in STN and SWN due to the static
electric force. Similar behaviour was shown by Liu et al. [219], who worked on
electrospun organically modified montmorillonite reinforced PVDF nanofibers.
The thermal behavior of electrospun nanofibers was found to be enhanced by
nanoclays. Adanur reported that when the Laponite® nanoclay was added in PVA, both
the glass transition and the melting point temperatures increased [220]. Park et al. [221]
found that higher thermal stability of PVA nanofibers with embedded high content rate
of MMT was achieved, and this was attributed to the higher chain compactness resulting
from the interaction between the PVA and the clay. However, the thermal behavior
differs with nanoclay-reinforced PA6 nanofibers [222]. Under pyrolytic conditions,
PA6 nanofibers and PA6/ Fe–OMT composite nanofibers degraded in one step. The
onset thermal stability of the PA6/Fe–OMT composite nanofibers was not enhanced
relative to that of pure PA6 nanofiber. The onset temperature of the degradation (5 wt%
weight loss) for the PA6/Fe–OMT composite nanofibers was reduced from 390.3 C for
the pure PA6 nanfiber to 373.6 C and 327.6 C, respectively. The reasons were
attributed to (1) the degradation of instable alkylammonium cations; (2) the decomposition of alkyl ammonium cations in Fe–OMT following the Hofmann elimination
reaction and the reaction product catalyzed the degradation of PA6 materials; and
220
Nanocomposite fibers
(3) the possibility that the clay itself could also catalyze the degradation of polymer
materials. However, the yield of charred residue at 700 C for the PA6/Fe–OMT
composite nanofibers increased from 4.94% for the pure PA6 nanofiber to 8.23% and
9.78%, respectively. The reasons might be that the cations of some transition metals
(e.g., Fe3þ) promoted the molecular cross-linking and so increased the charred residue.
It may also attributed to the nanosized silicate clay layers, which could presumably
facilitate the reassembly of lamellas to form 3-D char that might occur on the surface of
the composite nanofibers and create a physical protective barrier. Meanwhile, the
silicate clay layers could act as a superior insulator and mass-transport barrier and
subsequently mitigate the escape of volatile products generated during the thermal
decomposition. The increased amount of charred residue contributed to the improved
thermal stability of the PA6 composite nanofibers [222].
Flammability tests were also conducted on Laponite® filled PVA nanofiber
webs [223]. When the pure PVA web burned, it cringed and turned into a greenish blue
solid gel in some parts where the PVA was exposed to heat more than flame. The PVA/
Laponite® web cringed and turned into a gold-brown brittle substance, while the parts
highly exposed to the flame were turned into black ash. The burning rate of pure PVA was
0.01 g/s while 5 wt% Laponite® in PVA decreased it to 0.005 14 g/s indicating 48.63%
reduction of burning rate. Good flame retardation was achieved by adding Laponite®
nanoclay in electrospun PVA webs. The Laponite® in the polymer may not delay the
ignition but it probably acts as a char promoter, delaying the chain scission and forming a
protective layer, thereby slowing down the decompositionso that burning rate decreases.
By measuring the tensile properties of the fiber mat, Li et al. [216] found the Young’s
modulus and ultimate strength of the nanocomposite mats electrospun from 15% NC5
(Nylon-6/O-MMT composite with 5 wt% of O-MMT) solution were increased by 70%
and 30%, respectively. Park et al. [221] noticed that MMT has a reinforcement effect on
MMT-reinforced PVA nanofibers. But the tensile strength reaches a maximum value at
a clay concentration of 5 wt%. Further increase in the clay concentration results in a
decrease in the tensile strength due to the aggregation of a large amount of MMT
particles in the polymer matrix at high concentration conditions.
Contact angles and the time required for water penetration were also found to be
increased for most of the PA/nanoclay composites relative to the uncoated substrate [224].
9.3
Graphite graphenes
9.3.1
Structure and properties
Natural flake graphite (NFG) is composed of layered nanosheets, where carbon atoms
positioned on the NFG layer are held tightly by covalent bonds, while those positioned
in adjacent planes are bound by much weaker van der Waals forces. The weak interplanar forces allow for certain atoms, molecules and ions to intercalate into the interplanar spaces of the graphite. The interplanar spacing is thus increased. As it does not
bear any net charge, intercalation of graphite cannot be carried out by ion exchange
9.3 Graphite graphenes
221
Fig. 9.23 SEM images of (a) intercalated, (b) expanded, (c) sonicated (exfoliated) and (d) graphite
nanosheet [229].
reactions into galleries like layered silicates. Expanded graphite (EG) can be easily
prepared by rapid heating of an intercalation compound (GIC), which is initially
prepared from an NFG [225]. The original graphite flakes with a thickness of 0.4–
60 mm may expand up to 2–20 000 mm in length [226]. These sheets/layers get
separated down to 1 nm thickness, forming a high aspect ratio (200–1500) and high
modulus (1 TPa) graphene. When dispersed in the matrix, graphene exposes an
enormous interface surface area (2630 m2/g) and plays a key role in the improvement
of both the physical and mechanical properties of the resultant nanocomposite [227].
Basically, EG is a loose and porous vermicular product. Its structure is essentially parallel
boards, which collapse and deform desultorily. This collapse and deformation forms many
pores of different sizes ranging from 10 nm to 10 μm. Because of the high expansion ratio
(generally 200–300) of EG in the c-axis, the interplanar spacing is larger than that of the
nontreated graphite. Thus galleries of EG can be easily intercalated by suitable monomer
molecules and/or catalysts through physical adsorption because of the porous native of
EG and the polar interaction between monomer and the –OH and –COOH groups on EG
sheets. These polar groups result from chemical oxidation of the double bonds of graphite
sheets during the preparation of GIC [225]. Figure 9.23 shows the SEM images of
graphite before and after being expended. Graphite is also a good electrical conductor
with an electrical conductivity of 10 4 S/cm at room temperature [228].
222
Nanocomposite fibers
9.3.2
Graphene nanocomposites
Graphene–polymer composites possess outstanding physical, chemical, mechanical and
membrane-like properties, which perfectly meet the essential requirement for the construction of advanced magnetic, electronic, optical and electro-optical devices and sensors.
Improvement in a polymer’s electrical conductivity can be achieved by adding
graphenes to a polymer. Addition of graphenes immediately transforms the polymer
from an electrical insulator to an electrical semiconductor. The percolation threshold
value of the conducting composite (with 1.8 wt% of expanded graphite) was much
lower than that of conventional composites [226].
Yasmin and Daniel [230] fabricated graphite/epoxy and clay/epoxy nanocomposites
respectively to investigate the reinforcement performance of the two types of particles.
They found that graphite/epoxy has a higher (16% for 2 wt% particle content) elastic
modulus compared to that of clay/epoxy with the same particle content.
Incorporation of the graphene shows an pronounced improvement in composite
thermal stability. The incorporation of 2.5 wt% and 5 wt% graphene in pure epoxy
matrix increased the 5% decomposition temperature of pure epoxy by 13 C and 25 C,
respectively. The composites also showed higher char content and reduced weight loss
at 800 oC as the graphene content increased. The homogeneous distribution of graphite
particles and the tortuous path in the composites hinders the diffusion of volatile
decomposition products in composites compared to that in pure epoxy [230]. A low
thermal expansion coefficient (CTE) is often desirable for a composite to achieve
dimensional stability. The CTE of epoxy was reduced from 60 106/ C to
36–41 106/ C as well by the addition of 2.5 wt% graphene as the fine dispersion
and rigidity of graphite platelets in the epoxy matrix inhibits the expansion of polymer
chains with increasing temperature [230].
9.3.3
Graphene nanocomposite nanofibers
To develop linear, planar and 3-D assemblies of graphite nanocomposite fibrils for
macroscopic composites, electrospinning has been employed to produce graphene
reinforced nanofibers [231].
Graphite was synthesized by using known graphite-intercalation chemistry [232] and
then exfoliated for graphenes. Figure 9.24 shows the simplified schematic of the intercalation and exfoliation process. The first-stage is intercalation: the intercalation compound,
KC8, is readily formed by heating graphite powder with potassium metal under vacuum at
200 C. Addition of an aqueous solvent, in this case ethanol, caused exfoliation of
graphite. Ethanol solvates the potassium, producing potassium ethoxide, and hydrogen
gas evolution aided in separating the graphitic layers into nanoplatelets, as shown below:
KC8 þ CH3 CH2 OH ! 8C þ KOCH2 CH3 þ 1=2H2 :
ð9:1Þ
PAN was used as the polymer matrix. The electrospun graphene/PAN nanofibers have a
fairly uniform size distribution. The fiber diameters range from 5 nm to 500 nm, with an
9.4 Carbon nanofibers
223
Fig. 9.24 Schematic showing intercalation of pristine graphite with potassium metal, followed by
exfoliation with ethanol to form graphenes [231].
average diameter of 300 nm. TEM graphs proved the presence of graphenes within the
polymer fiber. While the PAN nanofibers are amorphous, the graphenes showed some
degree of crystallinity, as shown in Fig. 9.25. The hexagonal polycrystalline spot pattern
was indexed to the graphene crystal structure, a ¼ 2.49 Å, c ¼ 6.67 Å [231].
The addition of graphenes leaded to a modest improvement in the thermal stability of the
composite nanofibers. At 50% weight loss, the incorporation of 2 wt% and 4 wt% graphenes
increased the thermal stability to oxidation by 10 C and 25 C, respectively [231].
The elastic moduli of the nanocomposite fibers were measured to be 77, 113, 121 and
133 GPa, respectively, for nanoplatelet contents of 1, 2, 3 and 4 wt%. The trend shows an
increase in Young’s modulus with increasing content of graphenes. The two-fold increase
in reinforcement for the 4 wt% graphene/PAN nanocomposite is noteworthy. The
significant stiffening attributes to the high stiffness and high aspect ratio of the incorporated graphenes and is comparable to result seen for CNT/PAN nanofibers [231, 152].
9.4
Carbon nanofibers
Carbon nanofibers (CNF) are a unique form filling the gap in physical properties
between conventional carbon fiber (5–10 μm) and CNTs. CNF are produced either in
vapor-grown form or by electrospinning. They are typically 20–200 nm in diameter.
The reduced diameter of the nanofiber provides a larger surface area with surface
functionalities in the fiber.
9.4.1
Vapor-grown carbon nanofibers
Vapor-grown carbon nanofibers (VGCNF) are produced in the vapor phase by decomposing carbon-containing gases such as methane (CH4), ethane (C2H6), acetylene
(C2H2), carbon monoxide (CO), benzene or coal gas in the presence of floating metal
224
Nanocomposite fibers
Fig. 9.25 TEM micrograph of a 1wt% graphene/PAN nanofiber (the inset is a selected-area
electron-diffraction pattern taken from the circled area of the nanofiber). [231].
catalyst particles inside a high-temperature reactor [1]. The most common structure of
CNFs is truncated cones, but the internal structure of carbon nanofibers varies and is
composed of various configurations of modified graphene sheets (cone, stacked coins,
etc). The conical structures of CNTs range from a few to hundreds of nanometers in
diameter, and from less than a micron to millimetres in length [233]. The stacked cone
structure is usually referred to as a herringbone (or fishbone) formation because the
cross-sectional TEM images resemble a fish skeleton (Fig. 9.26a), while the stacked
cups structure is mostly referred to as bamboo type, resembling the compartmentalized
structure of a bamboo stem (Fig. 9.26b) [234]. Currently there is no strict classification
of nanofiber structures. The main distinguishing characteristic of nanofibers from
nanotubes is the stacking of graphene sheets of varying shapes.
Mechanical properties such as strength, stiffness, fracture behaviour and toughness of
materials depend on the microstructure of the fiber. An important material characteristic
9.4 Carbon nanofibers
225
Fig. 9.26 (a) STEM image of a herringbone CNF produced by dc C-PECVD with Ni catalyst;
(b) TEM image of a bamboo-type CNF grown under the same conditions with Fe catalyst [233].
of a small-diameter fiber is that it confers much higher strength than the bulk material due
to its reduced probability of containing critical structure flaws with decreasing specimen
volume [235]. CNFs contain polycrystalline and amorphous carbon in structure, therefore their tensile strengths are on an order of magnitude lower than CNTs, ranging from
1.5 GPa to 4.8 GPa. Their Young’s modulus is between 228 GPa and 724 GPa [236]. The
tensile modulus and strength of CNFs will not be affected by water, solvents, acids or
bases at room temperature. Even though their mechanical properties are not as astounding
as those of CNTs, CNFs are more economical and have higher manufacturability
compared. Table 9.4 lists the properties of a commercial carbon nanofiber (Pyrograf III).
Lee et al. [237] investigated the electrical characteristics of individual CNFs by
measuring the I–V characteristics of suspended nanofiber bridges. At low positive and
negative applied voltages, the nanofibers exhibited linear I–V characteristics. The
measured section of the nanofiber had a resistance of 622 Ω, which corresponds to
1 kΩµm of nanofiber length. The resistivity was estimated varying between 106 and
105 Ωm based on an assumption that conduction was through the entire cylindrical
cross-sectional area of the nanofiber. Generally, the resistivities of CNFsare comparable
to those of arc-grown MWCNTs and ropes of SWCNTs whose resistivities are approximately 106 Ωm [238, 239].
9.4.2
Electrospun carbon nanofibers
Traditional vapor-growth methods and plasma-enhanced chemical vapor depositing
methods involve complicated processes and high costs. On the contrary, stabilization
and carbonization of electrospun nanofiber precursors is a straightforward and low-cost
226
Nanocomposite fibers
Table 9.4 Properties of a commercial vapor-grown CNF (Pyrograf III, produced by Applied
Sciences, Inc.)
Properties
CNF-(pyrotically stripped)
Diameter (nm)
Density (g/cm3)
Tensile Modulus (GPa)
Tensile Strength (GPa)
Coefficient of thermal expansion(10 6/ C)
Electrical resistivity (mV cm)
60–200
1.8
600
7
1.0
55
Table 9.5 Carbon nanofiber precursors and their properties
Polymer
Ref.
Diameter
(nm)
d002
ID/IG
Temperature
( C)
Conductivity
(S/m)
Poly(acrylonitrile)
(PAN)
[240]
/
[241]
[242]
/
/
[243]
/
[244]
[245]
[246]
200–600
/
50–400
200–300
/
/
90–300
/
11040
1000–2000
80–200
0.36
/
/
/
/
/
0.37
0.34
0.368
/
/
2.73
1.39
2.93–1.67
3.81
3.41
1.31
/
/
0.9250.004
/
/
700
1000
873–1473
700
1000
2000
900
1300
1100
1000
600
6.8 103
1.96
/
/
2.193
55.41
/
/
/
2.5
3.7 101
[247]
/
2000–6000
/
/
2200
1000
1200
5.3
6.3
8.3
poly(amic acid) (PAA)
Polyvinyl alcohol
(PVA)
Pitch
approach for fabricating CNFs. The CNFs based on electrospun nanofibers also have an
advantage in making multifunctional nanocomposite carbon nanofibers. Simply by
adding functional nanoparticles to the electrospun precursors, multifunctional nanocomposite CNFs can be achieved. Table 9.5 lists the polymers that have been used as
the precursors for CNFs and their properties.
9.4.3
Carbon nanofiber composites
Similar to graphene and CNT reinforced composites, the performance of CNFincorporated polymer matrix can be significantly enhanced.
Finegan et al. [248] conducted studies on the tensile properties of CNF/PP specimens
prepared by using melt processing (injection molding) with 15 vol% CNF loaded. In all
cases, they observed an increase in both tensile modulus and strength compared with
pure PP. The tensile modulus and strength of the composite with CNFs treated in a CO2
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atmosphere at 850 C were 4 GPa and 70 MPa, respectively; both were three times
greater than the corresponding values for pure PP. Tibbetts and McHugh’s study also
showed a notable improvement in the mechanical properties of VGCNF/PP. The tensile
strength was doubled and the Young’s modulus was more than tripled [248, 249].
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epoxy by nearly doubling the flexural modulus and increasing the flexural strength for
about 36%.
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as low as 0.15 Ω/cm can be obtained with about 20 vol% fiber loading. They also
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200 MHz for 15 wt% high temperature heat-treated VGCNF in a vinyl ester matrix.
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W/(m K) to 2.8 W/(m K) for epoxy resin with 20 wt% VGCNF.
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10
Future opportunities and challenges
of electrospinning
10.1
Past, present and future of nanotechnology
In 2000, Roco et al. [1] estimated that there would be two million nanotechnology
workers worldwide (800 000 in the United States) and the product value would reach
$1 trillion, of which $800 billion would be in the United States, by 2015, with a 25%
rate growth. The initial estimation for the quasi-exponential growth in the nanotechnology workforce and the product value held up to 2008, as shown in Fig.10.1. The market
is doubling every three years as a result of the successive introduction of new products,
and new generations of nanotechnology products are expected to enter the market
within the next few years [1]. So the estimated value of 2015 for both workforce and
product value would have been realized as the 25% growth rate is expected to continue.
Nanotechnology is evolving toward new scientific and engineering challenges in
areas such as assembly of nanosystems, nanobiotechnology and nanobiomedicine,
development of advanced tools, environmental preservation and protection, and pursuit
of societal implication studies. Key areas of emphasis in nanotechnology [1] over the
next decade are as follows.
Integration of knowledge at the nanoscale and of nanocomponents in nanosystems with deterministic and complex behavior, aiming toward creating fundamentally new products.
Better control of molecular self-assembly, quantum behavior, creation of new
molecules, and interaction of nanostructures with external fields in order to build
materials, devices and systems by modeling and computational design.
Understanding of biological processes and of nanobio interfaces with abiotic
materials, and their biomedical and health/safety applications, and nanotechnology solutions for sustainable natural resources and nanomanufacturing.
Nanotechnology is already creating a strong impact in the development of many
scientific and engineering areas, ranging from electronics to textiles; by 2020, it is
likely to become a broad-based technology, covering multidisciplinary areas, seamlessly integrated with most technologies and applications, driven by economics and by
the impetus of achieving innovative solutions in medicine, productivity, sustainable
development, and human quality of life [1] Figure 10.2 shows the development
timelines of nanotechnology and possible mass application areas of nanotechnology
after around 2020.
240
10.2 Global challenges and nanotechnology
241
Fig. 10.1 Past and estimated worldwide number of workforce and final product value with a
quasi-exponential growth (data from Ref. [1]).
Fig. 10.2 Development timeline of nanotechnology and possible mass applications of
nanotechnology after ~2020 (referred and modified from Ref. [1]).
10.2
Global challenges and nanotechnology
As referred to in his talk “Future global energy prosperity: the terawatt challenge” [2],
Smalley describes energy, water, environment and disease as four out of the top ten
242
Future opportunities and challenges of electrospinning
global challenges that need to be faced and solved as soon as possible. Still, as Smalley
said, “Innovations in nanotechnology and other advances in materials science would
make it possible to transform our vision of plentiful, low-cost energy into a reality.”[2]
Nanotechnology is also the possible solution to the challenges of “water, environment
and disease.”
10.2.1
Nanofibers in energy
It is estimated that, in order to give all people on the planet the level of energy prosperity
those in the developed world are used to, a couple of kilowatt-hours per person, 60
terawatts around the planet need to be generated. And it is rather obvious that burning
up the limited energy resources on Earth cannot solve the global energy problems; but
will cause greater environmental problems by the emission of large volumes of carbon
dioxide. Therefore we will require not only new sustainable energy resources but also
innovations in collection, storage and transmission of energy.
10.2.1.1
Electrolytes in fuel cells or batteries
Fuel cells are devices that have been widely used for automobile propulsion and as
portable electronic devices, such as personal digital assistants (PDAs), cell phones and
notebook computers. With high accessible surface area and good electrical conductivity, nanofiber membranes may lower the cost of fuel cells as the anode catalyst or the
anode catalyst supporting materials, and provide conducting pathways and surface
interactions between nanoparticles and the electrolyte [3]. Electrospun nanofiber membranes are also good candidates for solid electrolytes in fuel cells. Experimental results
indicate that the electrical performance of fuel cells is proportional to the surface area of
the fibrous anodes [4]. Electrospun carbon nanofiber membrane (CFMs) supported
on Pt anodes exhibit higher electrocatalytic activity, more stability, larger exchange
current and smaller charge transfer resistance than that on commercial carbon papers
(CPs) [5–7].
Given that nanofiber membranes having large surface area fibers for incorporation of
large amounts of liquid/gel electrolyte and fully interconnected pore structures offering
a good ion conduction channel, which assures the nanofiber electrolytes are capable of
attaining of high ionic conductivity at room temperature [8, 9], electrospun nanofiber
membranes are attractive as polymer electrolytes for lithium ion batteries and fuels cells.
10.2.1.2
Supercapacitors
The capacitance of an electrical capacitor is a function of the specific surface area, pore
volume and resistivity of the sample, which indicates that electrospun nanofibers can be
innovative materials for supercapacitors. Experimental studies have proved that electrospun nanofibers possess outstanding capacitance. The specific capacitance behavior of
polyaniline nanofibers was characterized by using cyclic voltammetry, exhibiting the
highest specific capacitance of 298 F/g [10]. The specific capacitance from the PBI
precusor activated carbon nanofibers ranged from 125 F/g to 178 F/g depending on the
activation temparatures [11].
10.2 Global challenges and nanotechnology
10.2.1.3
243
Dye-sensitized solar cells
Researchers have realized that in dye-sensitized solar cells grain boundaries between
nanoparticles diminish the efficiency of electron conduction in the matrix, and lead to
charge–carrier recombination. One-dimensional morphology of the metal oxide fibers
can contribute to better charge conduction due to their continuity compared with that of
sintered nanoparticles, and also provides high specific surface area for increased
adsorption of dye sensitizers. Photonic nanofibers can be easily produced [12–15] and
have shown excellent photovoltaic properties – achieving an energy conversion efficiency of 6.2%[16].
10.2.1.4
Power transmission lines
New transmission line materials that are of lighter weight, are stronger and have lower
loss than copper are needed to address the global demand for power. In his testimony to
the Senate Committee on Energy and Natural Resources, Nobel Prize winner Professor
Richard Smalley predicted massive electrical power transmission over continental
distances [2]. Electrical power transmission is a superb way of moving energy from
one place to another and, at least on a small scale, electrical power can be stored. He
envisaged that nanotechnology in the form of single-walled carbon nanotubes, forming
what he called the Armchair Quantum Wire, may play a big role in this new electrical
transmission system [2]. Electrospinning, which has demonstrated its power in manufacturing multifunctional continuous fibers with nanotubes incorporated and well
aligned, can be a useful tool in producing lighter weight, stronger, lower cost and more
efficient power transmission lines.
10.2.2
Nanofibers in filtration
Concerning the global environmental issues such as water and air purification, membranes and filtration systems are believed to be among the most-used tools. The
technical requirements for filters are a balance of the three major parameters of filter
performance: filter efficiency, pressure drop and filter lifetime [17]. An improvement
in one category generally means a corresponding sacrifice in another. It was shown
that proper use of nanofibers provided marked improvements in both filtration efficiency and lifetime, while keeping a minimal impact on pressure drop. The large
surface area of nanofiber webs allows rapid and effective adsorption of impurities and
other particles such as micro-organisms or viruses from the air or water as well as
hazardous molecules. Beside fineness and the resulting large specific surface area of
nanofiber membranes, the high porosity and small pore size contribute further to their
high adsorption and filtration efficiency [17]. Furthermore, electrospun nanofiber
membranes can offer removal of impurities from the environment at lower energy
and hence lower cost at high efficiency. Owing to the similar mechanism, nanofiber
filtration systems for personal protection such as respiratory and clothes are also under
development for efficient protection of people from biological and chemical elements
[18, 19].
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Future opportunities and challenges of electrospinning
The application of nanofiber membranes for filtration started in the early 1980s. Since
then, utilizing nanofibers as filter media has spread worldwide. More and more companies such as Donaldson, Toray and Nanoval GmBH have already used nanofibers in
their commercial filter products.
Using synthetic or natural polymers bearing special functionalities or specific addons to the spinning solution, chemical and biological functionality can be achieved. For
example the incorporation of silver nanoparticles into nanofibers offers synergies of two
nanotechnologies [20].
10.2.3
Nanofibers in biomedical engineering
The power of nanotechnology in biomedical engineering lies in its ability to operate on
the same small scale as all the intimate biochemical functions involved in the growth,
development and aging of the human body. Nanotechnology is expected to provide a
new framework for diagnosing, treating and preventing disease [21]. Characterized by
nano- to microsize, a wide range of pore size distribution, high porosity, effective
mechanical properties and specific biochemical properties, nanofiber membranes are
morphologically and chemically similar to the extracellular matrix of natural tissue.
Therefore it is hypothesized that the intrinsic properties of nanofiber membranes will
facilitate the attachment and proliferation of cells, and control of their cellular functions.
This has resulted in recent research on the application of polymer nanofibers in the field
of biomedicine and biotechnology such as scaffolds used in tissue engineering, drug
delivery, wound dressing, artificial organs and vascular grafts.
10.3
Challenges
While electrospinning has been accepted as a versatile nanofiber processing technique
for a variety of applications, its true potential has yet to be realized. It is very
important to gain a better fundamental understanding of electrospinning process.
Therefore, better control of this process can be achieved. Effort should be made to
control processing variables so as to obtain defect-free and uniform diameter nanofibers. For effective applications exploring different functionalied nanofibers by posttreatment, incorporation of functional particles or developing novel structures are
generally required. Finally, to bridge the gap between scientific research and practical
application, investigation on the scale-up of electrospinning and tecnniques for
assembling these nanosized fibers into micro- or even higher-scaled structures are
inevitable.
10.3.1
Mechanism analysis
Revealing the fundamental mechanisms of a technique is the basic way and powerful
tool for one to understanding and gain control of it. It is widely accepted that the
whipping/bending instability of the jet is the reason fibers we formed with a diameter in
10.3 Challenges
245
the nanoscale. The whipping instability was explained by two possible mechanisms
[22]: (1) small lateral fluctuations in the centerline of the jet result in the induction of a
dipolar charge distribution and the dipoles interaction with the external electric field
producing a torque that further bends the jet; and (2) mutual repulsion of surface charges
carried by the jet causes the centerline to bend. Although understanding how these
mechanisms work on the jet and induce the instablility is vital for controling this
electrospinning, a detailed explaination is still unavailable.
As modeling plays a very important role in understanding a process, different models
have been proposed for electrospinning. Some of the representative models are Spivak–
Dzenis model [23, 24], Rutledge’s model [25, 26] and the Wan–Guo–Pan model [27,
28]. However, applications of these models in analyzing electrospinning were conducted
under very ideal and simple conditions. How to balance the complicated conditions with
a more realistic analysis based on a precise model needs to be mapped out.
10.3.2
Quality control
Instead of the beauty of small size and high specific surface area, electrospun nanofibers
have their own drawbacks, including non-uniform diameter distribution along a single
fiber or between fibers results from the whipping instability, weak mechanical properties
due to the lack of crystalline regions and defects caused by the evaporation of solvents
after the fiber forming, or the spinnability of solution. Therefore efforts need to be made
in investigating controling or manipulating the whipping motion of the jet, adjusting the
solution properties for better spinnability and controlling of process variables for a
designed fiber properties. Post-treatment of nanofiber products for high mechanical
properties might also need to be conducted, considering that most of commercial fibers
gain their mechanical properties during post-processes such as drawing and annealing.
10.3.3
Scale-up manufacturing
Apart from the challenges in process and property controlling, the huge challenge that
hinders the commercialization of electrospun nanofibers is the productivity and scaleup of electrospinning devices. Conventional electrospinning setups are based on needle
systems: the polymer solution or melt is conducted to a spinneret attached to a syringe,
a Taylor cone, from which a jet is ejected if the applied electronic force surpasses the
surface tension of the cone. When it forms at the tip of the spinneret, the jet then
solidifies into a nanofiber when it moves towards the collector. In such a system, the
throughput of a single needle is typically 0.1–1.0 g/h by fibre weight or 1.0–5.0 ml/h
by flow rate, depending on the polymer solution [29, 30], which is much too low to
meet the strong market requirement. Therefore many efforts have been centered on
increasing output by multiplexing the electrospinning system either by stacking an
array of capillaries/syringes, or by operating a multiple-jet mode.
Linear (Fig. 10.3a) [31–33], matrix (Fig. 10.3b) [31, 33, 34], hexagonal (Fig. 10.3c)
[35], elliptic (Fig. 10.3e) and concentric (Fig. 10. 3d) [36] jet arrays have been proposed
for scale-up electrospinning. Many companies such as Espin, MECC Co. Ltd and
246
Future opportunities and challenges of electrospinning
Fig. 10.3 Multiple-jet arrays: (a) linear array 2–26; (b) matrix array; (c) hexagonal array;
(d) concentric array; and (e) elliptic array.
Fig. 10.4 Structure of an industrial electrospinning machine.
Electrospunra have even commercialized industrial electrospinning setups. Figure 10.4
shows the basic structure of an industrialized electrospinning machine. Although
significant improvements in output can be made, there are barriers for these systems
to implement robustly. One of the biggest issues is the interference among jets caused
by electrostatic repulsion between each needles due to the applied electrostatic field, as
shown in Fig. 10.5. As a consequence, the inference will distort jets resulting in nonuniform diameter and un-controllable deposition issues, as shown in Fig. 10.6. Needle/
pore clogging is another common issue caused by the quick evaporation of solvents
from the surface of Taylor tone. Moreover, fluid dripping and nozzle cleaning beset the
multiple-jet electrospinning setup all the time.
To solve the electrostatic repulsion problem, Kim et al. [37] applied a cylindrical
electrode to a five-nozzle setup, as shown in Fig. 10.7. However, obviously this could
be effective only for a small nozzle array. A bottom-up multiple-jet system can avoid
solution dripping problems [38], but nozzle clogging and cleaning are still challenges.
10.3 Challenges
247
Fig. 10.5 A 3 3 matrix jet array [34].
Fig. 10.6 Fibers collected from multiple-jet array at varying the solution applied voltage and the
working distance [33]: (a) þ20 kV, 17 cm, (b) þ25 kV, 17 cm, (c) þ30 kV, 17 cm, (d) þ30 kV,
21 cm, (e) þ30 kV, 25 cm, and setup 5: (f) þ30 kV, 21 cm.
The invention of the rotary porous tube [39](Fig. 10.8a), rotary cone [40]
(Fig.10. 8b), and conical wire [41] (Fig. 10.9) break the barriers of needle electrospinning, and have shown their promising in high productivity. However, for the rotary
systems, the rotation aggravates the diameter distribution of nanofibers. For the conical
wire spinneret, relatively high solution viscosity is required for the wire to be able to
hold the solution from dripping, wherein large fiber diameter is inevitable.
To avoid the needle clogging and cleaning, and solution dripping problems, bottom-up
needleless systems, including multiple-spike ([42] (Fig. 10.10a), multiple-cleft [43]
248
Future opportunities and challenges of electrospinning
Fig. 10.7 Cylindrical electrode [37].
Fig. 10.8 (a) Rotary porous tube and (b) rotary cone.
(Fig. 10.10b), bubble [44](Fig. 10.10c) and rotary drum (nanospider, Fig. 10.11)
[45] electrospinning systems have been proposed. In contrast to the needle-based
multiple-jet electrospinning systems, there are no needle arrays in these setups. The
mechanism is based on electrohydrodynamic (EHD) induced fluctuations on top of free
surfaces from a thin layer of solution to create artificial liquid jets for electrospinning [43,
46]. Based on a detailed analysis of a dispersion law, Lukas et al. [43] made a hypothesis
to explain the needleless electrospinning mechanism: above a certain critical value of
applied electric field intensity/field strength the system starts to be self-organized in the
mesocopic scale due to the mechanism of the “fastest forming instability,” according to
which there is a particular wave with a characteristic wavelength whose amplitude
10.3 Challenges
249
Fig. 10.9 Conical wire electrospinning [41].
Fig. 10.10 (a) Multiple spikes; (b) mulitiple clefts; and (c) bubble spinning.
boundlessly grows faster than the others, resulting in a fastest growing stationary wave.
From the free liquid surface of this wave, crests jets originate and nanofibers are
obtained. In multiple-spike setup, a magnetic field and a metal-spike complex is involved
to help jet origionation. A multiple-cleft setup has a setup of clefts as well. Rotary drum
and bubble system are, on the contrary, trying to initiate the “wave jets” through the drum
or by blowing bubbles. Compared with needle systems, needleless electrospinning does
not have the issues brought by needles. However, these systems have their own problems. The involvement of free surface enlarges the diameter variation and the evaporation of solvent in the container is accelerated since all the systems are open.
Apart from the spinning setup, collecting uniform nanofiber nonwovens or aligned
continuous nanofiber assemblies are challenges that have to be conquered before electrospun nanofiber products can be industrially applied. Owing to the intrinsic instability of
the electrospinning process and the interference among jets, it is very hard to control the
deposition of nanofibers, much less a uniform membrane or an aligned nanofiber assemblies. Two main routes have been followed for controllable fiber collection: control of the
instability of jet by replacing D.C. potentials with an A.C. one [47], or using an A.C.
biased D.C. potential [48] (Fig. 10.12a), applying one [37, 49, 50] or more auxiliary
electric fields [51] (Fig. 10.12b) or magnetic field [52, 53] (Fig. 10.12c) to the jet; or
250
Future opportunities and challenges of electrospinning
Fig. 10.11 Scale-up needleless electrospinning system – Nanospider: (a) the Nanospider™
electrospinning line and (b) a Nanospider machine [45].
shrink the jet depositing spot to a sharp edge or point with specially designed collectors.
Reduced jet instability means decreased stretching on the jet and therefore increased fiber
diameters, which is not favorable for nanofibers, although they do show their potential in
gaining stable deposition. High-speed rotary collectors [54, 55] or multiple-electrode
array [56–58] collectors, as shown in Fig. 10.13, have demonstrated their capability of
collecting aligned fiber bundles. So far, the high-speed disk [59], as an extreme example
of high-speed rotary collectors, is believed to be the most effective device for aligned
nanofiber bundles. However, a high-speed disk can only collect a very limited amount of
aligned nanofibers. As a matter of fact, most of the collection designs except the rotary
collectors with large width are designated to one needle electrospinning setup, which
means a scale-up of electrospinning equipment would be beyond their working scope.
10.3 Challenges
251
Fig. 10.12 (a) A.C. biased D.C. potentials, (b) electric field auxiliary collection, and (c) magnetic
field assisted collection.
Fig. 10.13 (a) High-speed drum; (b) high-speed frame; (c) high-speed disk; (d) parallel electrodes;
and (e) counter electrodes.
Thus collectors that are not only good at dealing with jet instablilty but suitable for
collection of large amount of fibers need to be designed.
No matter whether they are aligned or not, fibers obtained from these collectors
mentioned above are in the form of membranes unless the collection time is very short.
There is strong demand for continuous aligned fiber bundles for further manufacturing.
Ko’s group specifically addressed the implementation of a modified drum electrospinning process for production of continuous nanofiber assemblies, as shown in Fig. 10.14
[60]. A continuous aluminium slit was wrapped around the drum as a collector. After a
period of accumulation, the slit was unwrapped and fibers bundles were obtained by
being peeled off the slit. Fiber bundles with length up to 2.5 m were produced using this
process. These fiber bundles was shown can be twisted up to 3.48 twist per inch (tpi).
After twisting, the alignment of the nanofibers in the yarn was greatly increased, as
252
Future opportunities and challenges of electrospinning
Fig. 10.14 Modified drum electrospinning process for continuous CNT yarns [60].
Fig. 10.15 Composite ESEM image of continuous yarn: (a) 700 untwisted yarn, and (b) 350 denier
3.48 tpi yarn [60].
shown in Fig. 10.15. The disadvantage of this method is the arbitrary separation of
the fiber bundles. Fibers may be broken if they are deposited upon more than one turn of
the slit.
From the point of view of the textile industry, electrospinning is two-free-end
spinning. People have no control on the initiation of the jet or on the deposition. The
10.3 Challenges
253
Fig. 10.16 (a) Liquid and (b) water vortex collection of continuous uniaxial electrospun nanofibers.
“invisible” size of the fiber from electrospinning makes people feel uncertain. A visible
and manipulatable medium that could be of help in controlling the motion of nanofibers
is desired, which stands out from a liquid support system. Smit et al. [61] and Khil et al.
[62] demonstrated that by spinning nanofibers onto a water or coagulant reservoir and
drawing the resulting nonwoven web of fibers across the liquid, continuous uniaxial
fiber bundles of yarns can be obtained, as shown in Fig. 10.16a. In a further step, a water
vortex [63] was employed for a continous twisted uniaxial fiber yarn, as shown in
Fig. 10.16b. A theoretical production rate of such a collector is 180 m of yarn per hour
for a single needle electrospinning setup, which is very promising. However, the yarn
diameter variation is very large and fibers could adhere and coalesce if they are bundled
before they are fully solidified.
10.3.4
Structural property improvement
In order to pursue novel structured nanofibers, core–shell spinning, side-by-side spinning and polymer-blend electrospinning techniques have been invented. Posttreatments, aiming at altering the nanofiber web structure, such as cross-linking, surface
chemical functionalization, coating and heat treatment, have been applied. These
techniques not only pave the way for brilliant multifunctional nanofibers but also
diversify the types of nanofibers and broaden the applicability of elelctrospun
nanofibers.
However, weakness is one of the key problems facing electrospun nanofiber products
and hindering further development of electrospinning, and we are waiting for this to be
solved. As previously mentioned, electrospinning cannot be applied to highly viscous
solutions because it is impossibile for the weak electric force to overcome the high
surface tension; the highly unstable whipping motion of the electrospinning jet is
inevitable due to the uneven distribution of electrical charge in and on the surface of
the jet. These two characteristics destin electrospun nanofibers to have a loose
254
Future opportunities and challenges of electrospinning
Fig. 10.17 The concept of intelligent clothes.
molecular chain arrangement in the structure, which has barely any crystallines and lack
of orientation, and hence high proportion of defects and fragility of the fibers. To
address the problem ultrasonic-vibration-electrospinning has been invented. Using this
setup, electrospinning of high viscous polymer solutions was achieved [64] and a hint of
improvement of the crystallization structure of nanofibers was also found [65]. Aside
from this progress, no further report relating to significant improvement of mechincal
properties of nanofibers has been published.
Therefore, to conquer the bottleneck of electrospinning and propel practical applications of electrospun nanofibers, both theoretical analysis and practical studies on
structure improvement of electrospun nanofibers need to be extensively conducted
and untimately solved.
10.4
New frontiers
Developments in electrospinning technology as well as all types of electrospun nanofibers provide fresh opportunities for the development of innovative nanostructured
materials. Combination of nanofibers and nanotechnology has brought the novel concepts of “smart skin,” or “intelligent clothes,” which could perform with comprehensive
functions such as being lightweight, protective, smart and adaptive. Figure 10.17
illustrates the concept of intelligent clothes.
References
255
Nanofibers and nanofiber-reinforced nanocomposites possess the advantange of
being lightweight since they are composed of polymers, perform with nanoeffects and
require less content of functional particles.
Nanofiber products are protective and wearable since the nanoscale dimension endow
the nanofibers with high surface-to-volume aspect ratio, high porosity, high surface
activity and high flexibility. High surface-to-volume aspect ratio, high porosity and
surface activity provide effective and efficient protection from nuclear, biological or
chemical (NBC) threats. Small diameter and abundant small pores allow the passage of
water vapor. High flexibility makes the products feel comfortable and allows then to
move with one’s motion.
Functionalized nanofibers are intelligent and can act as gas, pressure, light, electromagnetic wave and thermal sensors. When the sensors are attached to, or emebedded in,
electroconductive nanofiber networks, the environmental or one’s body signal can be
collected and sent away for monitoring. Clothes equipped with such intelligent components will be smart in signal capture and transmittance.
When phase-change materials are used during fabrication of nanofibers, those nanofibers can be adapted for changing conditions. For instance, when the air temperature
becomes lower than the standardized one, the thermal sensor “feels” cold and sends a
signal to the adaptive components; then the energy-storage phase-change material in the
nanofiber will be activated, melt and hence heat one’s body up. If blood-sensitive
material and blood sealant are present in nanofibers, then the nanofibers will automatically seal a wound when blood immerses the nanofibers.
Furthermore, nanofibers can be designed as solar cells or motion power generators,
which means clothes armed with such energy-generating components will be selfsustained, and the smart components will run with no restriction of power.
All the components involved with this “smart skin”concept have been expolored.
Astonishing achievements have proved that electrospinning is a versatile and strong
technique. And it is apparent that there will be many more possibilities beyond image.
Therefore exploration and investigation should be continued to exploit more techniques,
more products and hence wider applications.
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Appendix I
Terms and unit conversion
Terms for structure character
Cotton count (Ne) The number of pieces of fibers/yarns, 840 yards long needed to
make up 1 lb weight. Hence 10 count cotton means that 10 840
yards weighs 1lb.
Denier
A unit measuring the linear mass density of fibers/yarns. It is
defined as the mass in grams per 9000 m. 1 denier ¼ 1 g/9000 m.
Dtex
decitex, the mass in grams per 10 000 m.
Engineering
A measure of the average force per unit area of a surface.
stress
Metric count (Nm) The number of pieces of fibers/yarns of 1 m long to make up 1 g
weight.
Specific area
Area per unit bulk volume. For a fiber, Asp (cm2) ¼ Denier/
(Asp)
(9 105 ρ).
Specific stress
The force per unit linear density. It is more useful for fibres and is
(σsp)
the usual mode in the textile community: σsp ¼ N/tex.
Specific stress for The fore force per areal density divided by width.
textile
Tenacity
the length of a fiber/yarn falling under its own weight.
Tex
A unit measuring the linear mass density of fibers/yarns. It is
defined as the mass in grams per 1000 m.
pffiffiffiffiffiffiffiffiffiffiffi
TPI ¼ TM count , where TM is the twist multiplier, also
known as K or the twist factor. This twist multiplier is an
empirical parameter that has been established by experiments
and practice that the maximum strength of a yarn is obtained for
a definite value of K.
Twist
Measures the twist degree of a yarn, the number of twists applied
to a yarn in one unit length(1 inch, tpi/1 meter, m 1).
Unit conversion
dTex ¼ 10 Tex ¼ 1.11 Denier
Tex ¼ 0.11 Denier ¼ 0.1 Tex ¼ 590.5 / Ne ¼ 1000 / Nm
Denier ¼ 9 Tex ¼ 0.9 dTex ¼ 5315 / Ne ¼ 9000 / Nm
259
260
Appendix I
Twist factor ¼ Turns/cm (Tex)1/2 ¼ 9.57 TPI / (Ne)1/2
1 N/tex ¼ 1 GPa / ρ (g/cm3)
1 N/tex ¼ 1 kJ.g ¼ 1(km/s)2 ¼ 10.2 g/dtex ¼ 11.3 g/denier
1 g/denier ¼ 0.88 N/tex
1 N/tex ¼ 102 km ¼ 3.94 106 inch¼ 145000 psi/(g/cm3)
1 GPa ¼ 102 kg/mm2 ¼ 145,000 psi ¼ 104 bar ¼ 10000 kg/cm2
1psi ¼ g/denier 12800 ρ
Diameter and linear density
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
4
Denier
dðmÞ ¼
π 9 109 ρ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
4
Denier
dðcmÞ ¼
π 9 105 ρ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4
Denier
dðinÞ ¼
π 58 105 ρ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4
Denier
dðmilÞ ¼
π 58 1011 ρ
Appendix II Abbreviation of polymers
Abbreviation
Polymer
DNA
PA
PAA
PAI
PAEK
PAK
PAL
PAN
PANI
PARA
PAS
PB
PBAN
PBD
PBI
PBN
PBS
PBT
PC
PC/ABS
PCL
PCT
PCT-G
PCTFE
PDLA
PE
PEBA
PEEK
PEI
PEK
PEKEKK
PEKK
PEN
PEO
PES
PET
PET-G
Deoxyribonucleic acid
polyamide (nylon)
poly acetic acid
polyamide-imide
polyaryletherketone
polyester alkyd
polyanaline
polyacrylonitrile
polyaniline
polyaryl amide
polyarylsulfone
polubutylene
polybutadiene acrylonitrile
polybutadine
polybenzimidazole
polybutylene napthalate
polybutadiene styrene
polybutylene terephthalate
polycarbonate
polycarbonate/acrylonitrile butadiene styrene blend
polycaprolactone
polycyclohexylene terephthallate
glycol modified polycyclohexyl terephthallate
polymonochlorotrifluoroethylene
poly(d,l-lactic acid)
polyethylene
polyether block amide or polyester block amide
polyetheretherketone
polyetherimide
polyetherketone
polyetherketone etherketone ketone
polyetherketoneketone
polyethylene napthalene
polyethylene oxide
polyethersulfone
polyethylene terephthalate
glycol modified polyethylene terephthalate
261
262
Appendix II
(cont.)
Abbreviation
Polymer
PFA
perfluoroalkoxy
PI
PI
PIB
PIR
PLA
PLLA
PMAN
PMMA
PMP
PMS
PO
POM
PP
PPA
PPC
PPC
PPE
PPI
PPO
PPOX
PPS
PPSU
PPT
PS
PS-b-PI
PSO, PSU
PTFE
PTMT
PU,PUR
PUU
PVA
PVAc
PVB
PVC
PVCA
PVDA
PVDC
PVDF
PVF
PVK
PVOH
PVP
SBSa
polyimide
polyisoprene
polyisobutylene
polyisocyanurate
polylactic acid
poly(l-lactic acid)
polymethactylonitrile
polymethylmethacrylate (acrylic)
polymethylpentene
paramethylstyrene
polyolefin
polyoxymethylene (acetal)
polypropylene
polyphthalamide
chlorinated polypropylene
polyphthalate carbonate
polyphenylene ether
polymeric polyisocyanate
polyphenylene oxide
polypropylene oxide
polyphenylene sulfide
polyphenylene sulfone
polypropylene terephthalate
polystyrene
polystyrene/polyisoprene block copolymer
polysulfone
polytetrafluoroethylene
polytetramethylene terephthalate
polyurethane
polyurethaneurea
polyvinyl alcohol (sometimes polyvinyl acetate)
polyvinyl acetate
polyvinyl butyryl
polyvinyl chloride
polyvinyl chloride acetate
polyvinylidene acetate
polyvinylidene chloride
polyvinylidene fluoride
polyvinyl fluoride
polyvinyl carbazole
polyvinyl alcohol
polyvinyl pyrrolidone
styrene–butadiene–styrene triblock copolymer
Appendix III Classification of fibers
Fiber
Regenerated
fiber
Natural fiber
Plant fiber
Seed fiber
Cotton, etc.
Bast fiber
Ramie, flax,
hemp, jute, etc.
Animal fiber
Hair fiber
Mineral fiber
Asbestos
Regenerated
synthetic fiber
Regenerated
starch fiber
Zein, PLA, etc.
Synthetic fiber
Regenerated
Cellulose fiber
Viscose, modal,
cuprammonium,
acetate, lyocell, ect.
Regenerated
protein fiber
PET, PP, PA, PVC,
etc.
Inorganic fiber
Carbon fiber
Ceramic fiber
Metal fiber
Glass fiber
Silk fibroin,
collagen, casein,
soybean, etc.
Wool, alpaca,
camel, rabbit,
yak, etc.
Silk fiber
Silkworm, spider
silk, etc.
Leaf fiber
Sisal, pineapple
leaf, etc.
Fruit fiber
Kapok, coconut,
etc.
Stem fiber
Bamboo, wood,
etc.
Root fiber
Lotus root, etc.
263
Appendix IV Polymers and solvents for
electrospinning
1
Natural polymers
Cellulose
Cellulose acetate
Ethyl cellulose
Chitin
Chitosan
Collagen
Dextran
DNA
Alpha-elastin
Gelatin
Hyaluronic acid
(HA)
Tropoelastin
Silk fibroin
Spider silk fibroin
Zein
264
lithium chloride (LiCl, 8%)/N,N-dimethyl acetamide (DMAc)
N-methylmorpholine oxide (NMMO)/water ¼ 85/15
Acetic acid/ acetone ¼ 3/1
Acetic acid/DMAc ¼ 2/1,3/1,5/1,10/1
Tetrahydrofuran (THF)/dimethylacetamide (DMAc) ¼ 100/0, 60/40,
50/50
1,1,1,2,2,2-hexafluoro-2-propanol (HFIP)
90% acetic acid
Trifluoroacetic acid (TFA)
TFA /dichloromethane (MC) ¼ 90/10, 80/20, 70/30
1,1,1,2,2,2-hexafluoro-2-propanol (HFIP)
water
Dimethyl sulfoxide (DMSO)/ N,N-dimethyl formamide (DMF) ¼ 60:40
Water:ethanol 7:3
1,1,1,3,3,3-hexafluoro-2-propanol (HFP)
2,2,2-trifluorothanol
acidic aqueous solution (pH ¼ 1.5)
1,1,1,3,3,3-hexafluoro-2-propanol (HFP)
98% formic acid
hexafluoroacetone trihydrate/ HFA3H2O
formic acid
ethanol/water ¼ 70/30, 80/20, 90/10
Polymers and solvents for electrospinning
2
Synthetic polymers
Polymer
Nylon 6
PAA
PAN
PBI
PC
PCL
PDLA
PE
PEO
PET
PLA
PLLA
P(LLA-CL)
PLGA
PMMA
PMMA-rTAN
PPy
MEH-PPV
PS
PSU
PTT
PU
PUU
PVA
PVK
PVP
SBS
Solvent
1,1,1,3,3,3-hexa-fluoro-2-propanol (HFIP)
1,1,1,3,3,3-hexa-fluoro-2-propanol (HFIP)/N,N-dimethyl formamide
(DMF) ¼ 95/5
ethanol/water ¼ 40/60
dimethylformamide (DMF)
N,N-dimethylacetamide, lithium chloride (4%)
dichloromethane
THF/DMF ¼ 100:0, 70/30, 60/40
chloroform
acetone
methylene chloride (MC)/ dimethylformamide (DMF) ¼ 40/60
chloroform/methanol
dimethyl formamide
dichloromethane/trifluoroacetic acid ¼ 1/1
p-xylene
water
water/ethanol ¼ 60/40
trifluoroacetic acid
chloroform: acetone 2:1
N,N-dimethyl formamide (DMF)
dichloromethane
acetone
N,N-dimethyl formamide (DMF)
dimethylformamide/toluene¼90/10
dimethylformamide/toluene¼90/10
chloroform with DBSA
1,2-dichloroethane
dimethylformamide
pyridine
Trifluoroacetic acid (TFA) /methylene chloride (MC) ¼ 50/50
N,N-Dimethyl formamide
tetrahydrofuran (THF)/ethanol ¼ 50/50
N,N-dimethylformamide: tetrahydrofuran
dimethyl formamide
water
acetone
methylene chloride(MC)/ dimethylformamide (DMF) ¼ 75/25, 40/60
dichloromethane
ethanol
2-propanol
tetrahydrofuran/dimethylformamide ¼ 3/1
265
Index
alignment 205
composite slicing 207
electric field 206
electrospinning 208
film rubbing 207
liquid crystal solution 206
magnetic field 205
mechanical stretching 207
amorphous 27
differential scanning calorimetry 131
applications 47
applied voltage 52
atomic force microscopy 108, 125
battery 242
bioactive nanofiber 148
boiling point 55
carbon nanofiber 223
carbon nanotube 192
carbon nanotube fiber 209–210
chemical vapor deposition 210
electrospinning 214
liquid crystal spinning 212
solid-state spinning 212
traditional spinning 214
wet spinning 213
challenge 244
characterization 102
bioactivity 161
degradation 163
electrical conductivity 136
magnetic properties 140
tissue compatibility 163
concentration 50
configuration 25
conformation 25
conservation laws 67
contact angle 134
covalent functionalization 201
cross-linking 26
crystal structure 27
differential scanning calorimetry 131
266
dielectric constant 57
differential scanning calorimetry 131
dispersion 197
drug delivery 61, 156–157, 244
dye-sensitized solar cell 243
electrical conductivity 53
electrical-active nanofiber 62
electroactive nanofiber 170
conductive 167
magnetic nanofiber 179
PAn.HCSA 171
photonic 182
Poly(3,4-ethylenedioxythiophene)(PEDOT) 172
polyaniline 171
superior electrical conductivity 169
supermagnetic nanofiber 180
supermagnetism 179
electrolyte 242
electrospinning 49
electrospinning mechanism 65, 244
electrospinning process 49
EMI shielding 180
energy 242
feeding rate 57
fiber formation 39
fiber properties 40
process 43
structure 40
theoretic modulus 82
fiber size 7
filtration 60, 115, 243
formation of nanofibers 45
chemical vapour deposition 45
conjugate spinning 45
drawing 47
electrospinning 49
meltblown technology 47
phase separation 46
self-assembly 47
template synthesis 47
four-element model 21
Index
Fourier transform infra-red spectroscopy 116
fuel cell 242
mechanical properties 84
nuclear magnetic resonance 121
graphite nanoplatelet 220
optical microscopy 102
optical nanofiber 62
orientation 28
orifice diameter 55
heterogeneity 14
historical development of nanotechnology 2
humidity 56
hydrogen bonding 24
intrinsic viscosity 36
linear kinetics 14
Maxwell model 17
measurement of order and disorder 29
mechanical dispersion 199
ball milling 200
high shear stress 200
ultrasonication 199
mechanical testing 125
microtensile testing 123
medical prostheses 61
melt electrospinning 58
melt spinning 39
mercury porosimetry 115
modeling
jet 68
one-dimensional (1-D) model 70
parametric analysis 74
Rutledge’s model 72
Spivak–Dzenis model 72
Taylor cone 67
three-dimensional (3-D) models 72
Wan–Guo–Pan model 73
molecular bondings 23
molecular weight 29, 36, 53
molecular weight distribution 29
nanoclay 215
nanocomposite fiber 192
carbon nanofiber 226
graphite 222
nanoclay 218
nanofiber 5
nanofiber technology 45
nanomaterials 5
nanosensor
biosensor 159
nanotechnology 1 240
Newton’s law 66
noncovalent functionalization 200
nonlinear behavior 15
nonlinearity 14
nonwoven
deformation 86
geometry 84
photonic nanofiber
fluorescent nanofiber 184
photo-catalytic nanofibers 186
porosimetry 115
power transmission line 243
protective clothing 59
purification 197
centrifugation 197, 199
electrophoresis 199
filtration 197, 199
gas-phase oxidation 197
vapor-phase oxidation 197
wet chemical oxidation 197–198
quality control 245
Raman spectroscopy 119
reinforcement 59
scale-up manufacturing 245
scanning electron microscopy 103
scanning tunnelling microscopy 110
sensor 62
simulation 75
size-effect 81
solubility parameter 33
solutions 33
spinning angle 55
spinning distance 54
structural characterization 102
structure of hierarchy of textile materials 81
supercapacitor 242
surface tension 52
temperature 52
thermal behavior 32 128
differential scanning calorimetry 131
thermogravimetric analysis 129
thermo analysis 128
tissue scaffold 61, 149, 244
transmission electron microscopy 106
van der Waals forces 24
viscoelastic 16
viscosity 36, 50
Voigt (Kelvin) model 20
wet spinning 40
wettability 134
267
268
Index
wound dressing 61, 244
woven fabric
geometry 97
mechanical properties 96
X-ray diffraction 112
small-angle X-ray scattering 114
wide-angle X-ray diffraction 114
yarn properties
geometry 88
linear fiber assemblies 90
mechanical properties 87
staple yarns 93