1.

POLYMERS
1.1 GENERAL INTRODUCTION AND ITS SCOPE
Polymers form a very important class of materials without which the life
seems very difficult. They are all around us in everyday use; in rubber, in plastic, in
resins, and in adhesives and adhesives tapes. The word polymer is derived from greek
words, poly= many and mers= parts or units of high molecular mass each molecule of
which consist of a very large number of single structural units joined together in a
regular manner. In other words polymers are giant molecules of high molecular
weight, called macromolecules, which are build up by linking together of a large
number of small molecules, called monomers. The reaction by which the monomers
combine to form polymer is known as polymerization [1]. The polymerization is a
chemical reaction in which two or more substances combine together with or without
evolution of anything like water, heat or any other solvents to form a molecule of high
molecular weight. The product is called polymer and the starting material is called
monomer.
1.2 HISTORICAL DEVELOPMENT OF POLYMERS
Polymers have existed in natural form since life began and those such as
DNA, RNA, proteins and polysaccharides play crucial roles in plant and animal life.
From the earliest times, man has exploited naturally-occurring polymers as materials
for providing clothing, decoration, shelter, tools, weapons, writing materials and other
requirements. However, the origin of todays polymer industry is commonly accepted
as being the nineteenth century when important discoveries were made concerning the
modification of certain natural polymers. In eighteenth century, Thomas Hancock
gave an idea of modification of natural rubber through blending with ceatrain
additives. Later on, Charles Goodyear improved the properties of natural rubber
through vulcanization process with sulfur. The Bakelite was the first synthetic
polymer produced in 1909 and was soon followed by the synthetic fiber, rayon, which
was developed in 1911. The systematic study of polymer science started only about a
century back with the pioneering work of Herman Staudinger. Staudinger has given a

1|Page

new definition of polymer. He in1919 first published this concept that high molecular
mass compounds were composed of long covalently bonded molecules.
1.3 CLASSIFICATION OF POLYMERS
Polymer is a generic name given to a vast number of materials of high
molecular weight. These materials exist in countless form and numbers because of
very large number and type of atoms present in their molecule. Polymer can have
different chemical structure, physical properties, mechanical behavior, thermal
characteristics, etc., and on the basis of these properties polymer can be classified in
different ways, which are summarized in Table 1.1, whereas, important and broad
classification of polymers are described in the next section.
Table1.1: Classification of Polymers
Basis of Classification

Polymer Type

Origin

- Natural, Semi synthetic, Synthetic

Thermal Response

- Thermoplastic, Thermosetting

Mode of formation

- Addition, Condensation

Line structure

- Linear, Branched, Cross-linked

Application

and

Properties

Physical

- Rubber, Plastic, Fibers

Tacticity

- Isotactic, Syndiotactic, Atactic

Crystallinity

Polarity

- Polar, Non polar

Chain

- Hetro, Homo-chain

1.3.1

Non

crystalline(amorphous),

Semi-crystalline,

Crystalline

Origin
On the basis of their occurrence in nature, polymers have been classified in

three types [2]:A. Natural polymer:- The polymers, which occur in nature are called natural
polymer also known as biopolymers. Examples of such polymers are natural
rubber, natural silk, cellulose, starch, proteins, etc..
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B. Semi synthetic polymer:- They are the chemically modified natural polymers
such as hydrogenated, natural rubber, cellulosic, cellulose nitrate, methyl cellulose,
etc.
C. Synthetic polymer:- The polymer which has been synthesized in the laboratory is
known as synthetic polymer. These are also known as manmade polymers.
Examples of such polymers are polyvinyl alcohol, polyethylene, polystyrene,
polysulfone, etc..
1.3.2

Thermal Response
On the basis of thermal response, polymers can be classified into two groups

[3]:A. Thermoplastic polymers:- They can be softened or plasticized repeatedly on

application of thermal energy, without much change in properties if treated with
certain precautions. Example of such polymers are Polyolefins, nylons, linear
polyesters and polyethers, PVC, sealing wax etc..
B. Thermosetting polymers:- Some polymers undergo certain chemical changes on
heating and convert themselves into an infusible mass. The curing or setting
process involves chemical reaction leading to further growth and cross linking of
the polymer chain molecules and producing giant molecules. For example,
Phenolic, resins, urea, epoxy resins, diene rubbers, etc.
1.3.3 Mode of Formation
On the basis of mode of formation, polymers can be classified as [2]:A. Addition polymers:- They are formed from olefinic, diolefnic, vinyl and related
monomers. They are formed from simple addition of monomer molecules to each
other in a quick succession by a chain mechanism. This process is called addition
polymerization.

Examples of such polymers are polyethylene, polypropylene,

polystyrene.

3|Page

B. Conden
nsation poly
ymer:- Theey are formeed from inteermolecularr reactions between
b
bifunctioonal or po
olyfunctionaal monomeer moleculees having rreactive fu
unctional
groups such
s
as OH
H, -COOH, -NH2, -NCO
O, etc..
1.3.4 Lin
ne Structuree
On the basiis of structu
ure, polymerrs are of thrree types.
A. Linear polymer:p
If the monoomer units are joined in
i a linear ffashion, polymer is
said to be
b linear pollymer.

Linear Hom
mopolymer

Linear Cop
polymer

B. Brancheed polymerr:- When m

monomer un
nits are join
ned in brannched mann
ner, it is
called brranched pollymer.

Braanched Hom
mopolymer

Branched
B
Coopolymer

C. Cross liinked polymer:- A poolymer is said

s
to be a cross linkked polymeer, if the
monomeer units are joined
j
togetther in a chain fashion.

Cross linkeed Homopollymer

Cross linkeed Copolym

mer

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plication an
nd Physicall Propertiees
1.3.5 App
Deppending on its ultimate form and use
u a polymer can be cllassified as [2]:A. Rubberr (Elastomeers):- Rubbber is high
h moleculaar weight ppolymer wiith long
flexible chains and
d weak interrmolecular forces. Th
hey exhibitss tensile strength in
000 psi andd elongation
n at break ranging
r
bettween 300-1000% .
the rangge of 300-30
Examplees are natural and synthhetic rubberr.
B. Plastics:- Plastics are
a relativelly tough substances wiith high moolecular weiight that
m
with
h (or withoout) the app
plication of heat. Thesee are usuallly much
can be molded
strongerr than rubbeers. They exxhibit tensiile strength ranging beetween 4000-15000
psi and elongation at break rannging usuallly from 20
0 to 200% oor even high
her. The
examplees of plastics are, polyeethylene, po
olypropylenee, PVC, pollystyrene, ettc.
C. Fibers:-- Fibers arre long- chhain polym
mers characterized by highly cry
ystalline
regions resulting
r
mainly
m
from secondary forces.
f
They
y have a muuch lower elasticity
e
than plastics and elastomers. T
They also haave high ten
nsile strenggth ranging between
b
20,000- 150,000 psi., are light weight and
d possess mo
oisture absoorption prop
perties.
1.3.6 Taccticity:It may
m
be defined
d
as the geom
metric arran
ngement (oorientation) of the
characteristtic group off monomer uunit with reespect to thee main chainn (backbone) of the
polymers. On
O the basiss of structurre, polymer may be claassified into three group
ps:A. Isotacticc polymer:- It is the ttype of poly
ymer in whiich the charracteristic group are
arrangedd on the sam
me side of thhe main chaain.

Issotactic Pollypropene

5|Page

B. Syndiottactic polym
mer:- A poolymer is said
s
to be syndiotactic
s
c if the sid
de group
(characteristic group) are arrannged in an alternate
a
fash
hion.

Syyndiotatic Po
olypropene
C. Atactic polymer:- A polymer is said to be
b atactic, iff the charactteristic grou
ups (side
group) are
a arranged
d in irregulaar fashion (rrandomness) around thee main chaiin. It has
proper strength and
d more elastiicity.

A
Atactic Poly
ypropene
1.4 BIOCO
OMPOSIT
TES
Com
mposite are attractive m
materials beecause they combine ma
material prop
perties in
ways not foound in natu
ure. Such m
materials oftten result in
n lightweighht structuress having
high stiffneess and tailo
ored properrties for speecific appliccations, ther
ereby saving
g weight
and reducing energy needs [4-66]. Fiber-reeinforced pllastic compposites beg
gan with
cellulose fiber
f
in pheenolics in 1908, laterr extending
g to urea aand melamiine, and
reaching coommodity status
s
in thhe 1940s with
w glass fiber in unsaaturated polyesters.
From guitaars, tennis raacquets, andd cars to miicrolight airrcrafts, elecctronic comp
ponents,
and artificiaal joints, co
omposites arre finding use
u in diversse fields.
Com
mposite maaterials deriived from biopolymer
b
and synthhetic fibers such as
glass and carbon
c
also come undeer biocompo
osites. Biocomposites dderived from
m plantderived fibeer (natural/b
biofiber) annd crop/biod
derived plasstic (biopolyymer/bioplaastic) are
6|Page

likely to be more ecofriendly, and such biocomposites are sometimes termed green
composites [4].
1.4.1

Natural/Biofibers as Reinforcements in Biocomposites

The worlds supply of natural resources is decreasing and the demand for

sustainable and renewable raw materials continues to rise. Biofiber-reinforced

composites represent a potential nontraditional, value-added source of income to the
agricultural community.
Jute is from India and Bangladesh; coir is produced in the tropical countries of
the world, with India accounting for 20% of the total world production; sisal is also
widely grown in tropical countries of Africa, the West Indies, and the Far East, with
Tanzania and Brazil being the two main producing countries; kenaf is grown
commercially in the United States; flax is a commodity crop grown in the European
Union as well as in many diverse agricultural systems and environment throughout
the world, including Canada, Argentina, India, and Russia. Flax fiber accounts for less
than 2% of world consumption of apparel and industrial textiles, despite the fact that
it has a number of unique and beneficial properties. Hemp originated in Central Asia,
from which it spread to China, and is now cultivated in many countries in the
temperate zone. Ramie fibers are the longest and one of the strongest fine textile
fibers mostly available and used in China, Japan, and Malaysia.
Most of the polymers by themselves are not suitable for load-bearing
applications due to their lack of sufficient strength, stiffness, and dimensional
stability. However, fibers possess high strength and stiffness. Unfortunately, they are
not suitable for use in load-bearing applications by themselves because of their
fibrous structure. In fiber-reinforced composites, the fibers serve as reinforcement by
giving strength and stiffness to the structure while the plastic matrix serves as the
adhesive to hold the fibers in place so that suitable structural components can be
made. A broad classification (nonwood and wood fibers) of natural fibers is
represented schematically in Fig. 1.1.
Currently several nonwood fibers (e.g., hemp, kenaf, flax, and sisal) are being
utilized commercially in biocomposites in combination with polypropylene for
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automotivee application
ns. Now froom need off society an
nd research point of viiew it is
much impoortant to worrk on leaf bbased nonwo
ood fibers [4
4].

Fig 1.1: Scchematic reepresentatioon of reinfo

orcing natu
ural/biofibeers classificcation [4]
1.5 MECH
HANISM OF POLYM
MERIZATIO
ON
Thee linking together off a large number
n
of small moolecules terrmed as
monomers with each
h other to foorm a macrromolecule or polymerr molecule through
r
iss termed ass polymerization.
chemical reactions

d as the
It can also be defined

fundamentaal process by
b which loow molecullar weight compounds
c
are converrted into
high moleccular weigh
ht compounnds. In addiition to thee structural and compo
ositional
differences between polymers
p
F lory stresseed the very
y significannt difference in the
mechanism
m by which polymer m
molecules arre build up.. Although Flory contiinued to
use the teerms "addiition polym
merization" (polymerization by repeated addition
processes)

and

"co
ondensation

polymeriization"

(p
polymerizattion

by repeated
r

condensatioon processes, i. e., w

with the elimination
e
of small molecules)) in his
discussion of polym
merization mechanism
m. The cu
urrent term
minology classifies
c
polymerizaation into steep growth ppolymerizatiion and chaain growth ppolymerizattion [7].
Thee degree off polymerizzation is related to thee molecular
ar mass (M) of the
polymer annd is given by
b the equattion,
8|Page

1.1
where, m is the mass of the monomeric unit.

1.5.1

Condensation Polymerization or Step-growth Polymerization

Condensation Polymerization is a chemical reaction in which polymer is

formed and a small molecule of by-product with a lower molecular weight is released.
The by-product eliminated is called as condensate. The reaction can take place
between two similar or different monomers. It is also called as step-growth
polymerization [8-9].
1.5.2 Addition Polymerization or Chain Polymerization
In addition polymerization, two or more molecules of monomers attach
together to form a polymer. In this polymerization, there is no elimination of any
molecule. It is a chain reaction and no by product is released. It is obtained by linking
together the monomer molecules by a chain reaction to give a polymer whose
molecular weight is exactly an integral multiple of that of the monomer as in the case
of polyethylene obtained by polymerization of ethylene. Only a single monomer is
involved in addition polymerization and hence the polymer is homopolymer and
contains the same monomer units. Addition polymerization reaction is usually
induced by light, heat or a catalyst for opening the double bond of the monomer and
creating the reactive sites [9].

1.6 CHARACTERIZATION OF POLYMERS

It is comparable to the synthesis of organic compound, composites,

biocomposites without a subsequent characterization of its various properties
Synthesized material characterization is therefore of very great importance. Some of
the important aspects related to characterizations have been described in this section.

9|Page

1.6.1 Molecular Weight and its Distribution

The molecular weight of a polymer is of prime importance in the polymers

synthesis and application. It is important because it determines many physical
properties. The term molecular weight is a ratio of the average mass per formula unit
of substance to 1/12 th of an atom of 12C. Molecular weight (MW) and its distribution
(MWD) has a considerable effect on macroscopic properties of polymer such as
toughness, tensile strength, adherence and environmental resistance, etc. [10]. A
simple chemical has fixed molecular weight but when we discuss about the molecular
weight of polymer, we mean something different from that which applies to small
sized compounds. Since polymers are mixture of molecules of different molecular
weight, the molecular weight is expressed in the term of average value. This
average molecular weight is basically based on either average number of repeating
units, known as number average molecular weight, or average weight, known as
weight average molecular weight. The number average molecular weight is obtained
from the number ni of macromolecules for each degree of polymerization Pi by taking
for each degree of polymerization the product of the number of polymer molecules
and their degree of polymerization ni pi and dividing the sum of these product by the
total number of monomers No:

Mn

n p
i 1

No

1.2

The weight average molecular weight M w is obtained in a similar manner

from mass mi of each degree of polymerization Pi according to the following
equation:

Mw

m p
i 1

Mo

1.3

When the molecular weight distribution is very narrow, the number average
and weight average molecular weights are essentially equal. When the distribution is
broad, the weight average molecular weight is considerably greater than the number
10 | P a g e

average moolecular weeight and bbroader thee distributio

on, the greeater the diifference
between thhem as show
wn in Figurre 1.2. From
m these mollecular averrages the molecular
m
weight distrribution is represented
r
by their rattio

Mw
Mn

Fig1.2: Scchematic reepresentatiion of the weight av

verage andd number average
mollecular weight
w
disttribution as a fun
nction of the deg
gree of
polyymerization [11].
Tennsile and im
mpact strenngths increease with molecular weight. Th
he melt
viscosity of the polym
mer, howeveer, shows a different trend.
t
At veery high molecular
m
weights, thhe melt viscosity risees more stteeply than
n at low m
molecular weights.
w
Molecular weight distribution allso affects properties of polymerrs. To kno
ow of a
nowledge of
o both the average molecular
m
polymer prroperly, we must havee a good kn
weight as well
w as its diispersion paatterns.
1.6.2 Cryystallization
n
Cryystallization in polymerr has alway
ys been the subject of great scienttific and
academic interest,
i
since polymeers are kno
own to exh
hibit a varieety of strucctures at
various lenngth scales, such as unnit cell, lam
mella, and spherulites.
s
It is an intteresting
property reelated to ph
hase transittion which determines the final pproperties of
o many
technologiccally relevaant and sccientifically
y exciting systems [[12, 13]. Polymer
P
11 | P a g e

crystallization controls the structural formation process of polymeric materials and

thereby dominates the properties of final polymer product. Polymer crystallization is
usually divided into two separate processes : primary nucleation and crystal
growth [14].
In general, a crystalline phase must primarily nucleate. The fundamental kinetics of
nucleation are often complicated to determine because the rate of heterogeneous
nucleation at defects, impurities, and surfaces is much faster than the homogeneous
rate of nucleation within the pure bulk liquid. After a nucleus forms, the kinetics of
crystal growth determines the overall rate of the phase transformation. For growth
from solid or liquid solutions, the rate limiting step in the kinetics is often mass
diffusion. For crystal growth from a melt, heat transfer plays an important role for
metals, ceramics, and semiconductors, but for large molecules and polymers, the
microscopic kinetics associated with attaching a large molecule or polymer segment
to the growing crystal are typically the most important consideration [14].
The crystalline and amorphous components influence polymer properties. Actually,
crystallinity of a polymer sample is expressed in terms of that fraction of the sample
which is crystalline. The definition of degree of crystallinity is, of course, based on
the premise that crystalline and non-crystalline components of a polymer can coexist''. The highest crystallinity is generally associated with polymers which have a
simple unit cell structure and a relatively high degree of molecular order. The overall
property (Q) of a partially crystalline polymer can be expressed as a sum of its two
components as:
1.4
where Qc and Qa are contributions of the crystalline and amorphous components of
the sample, respectively. A number of methods can be used to detect crystallinity and
estimate its degree: X-ray diffraction, infrared-absorption spectroscopy, polarized
light microscopy, density, differential thermal analysis and nuclear magnetic
resonance spectroscopy, etc.. Accurate and undisputed measurement of the volume
fraction of crystallinity in a polymer is not easily accomplished because each of the
method of measurement mentioned above is concerned with a different physical
aspect of material. Nevertheless, the great practical consequences of crystallinity and
12 | P a g e

orientation on mechanical properties dictate that at least relative changes in these

factors be observed by whatever means are applicable, and that these be correlated
with changes in processing and fabrication methods and with end-use behaviour. It
was found that the preparation method of samples influence the morphology and
crystallization behaviour of blends.
1.6.3 Glass Transition Temperature
In the study of polymers and their applications, it is important to understand
the concept of the glass transition temperature Tg. The glass transition is a
phenomenon observed in linear amorphous polymer. It occurs at fairly well defined
temperature when the bulk material ceases to be brittle and glassy in character and
become less rigid and more rubbery. The knowledge of Tg is essential in the selection
of materials for various applications.
Many Physical properties change profoundly at the glass transition
temperature, including mechanical properties and electrical properties. All of these are
dependent on the relative degree of freedom for molecular motion within a given
polymeric material and each can be used to monitor the point at which the glass
transition occurs [15-17].
1.7 MECHANICAL PROPERTIES OF POLYMERS
To study the necessary set of valuable properties, polymers differing from one
another by their chemical structure and properties are usually mixed together, either
homogeneously or heterogeneously. The mechanical properties of inhomogeneous
mixture are worse than those of individual polymers, while the mechanical properties
of homogeneous mixture are good.
Polymers can exhibit the features of glassy, brittle solid or an elastic rubber, or
that of a viscous liquid, depending on the temperature and time scale of measurement.
The studies on mechanical properties of polymer can, therefore, be carried out by
subjecting them to some form of mechanical stress either continuous or in a periodic
manner at different rate. Some of the important properties of polymers with regard to
their use as engineering material are tensile strength, compressive and flexural
strength, hardness, creep, fatigue resistance and impact resistance.
13 | P a g e

Toughness of a polymer is the ability to absorb mechanical energy without fracturing.

The property such as tensile strength is the maximum amount of tensile load per unit
area a material can withstand, while the tensile elongation gives the measure of
increase in length in response to a tensile load expressed as a percent of the original
length. Elongation at break is the maximum elongation the plastic can undergo.
Engineering applications of polymers are governed to a great extent by strain
hardening considerations. The designer using polymeric materials must, therefore,
understand their mechanical behaviour with respect to the maximum permissible
strains to avoid failure. As for most materials, a simple tensile stress-strain curve
provides a good start towards understanding the mechanical behaviour of a particular
polymer. This curve is usually established by continuously measuring the force
developed as the sample is elongated at constant rate of extension until it breaks.
Portions of the curve in Fig.1.3 represent the stress-strain behaviour of any polymer
and are used to define several useful quantities. The initial slope provides a value for
Young's modulus (or the modulus of elasticity) which is a measure of stiffness. The
curve also gives yield stress, strength and elongation at break. The area under the
curve or work to break is a rough indication of the toughness of the polymeric
material. The stress at the knee in the curve (known as the yield point) is a measure of
the strength of the material and resistance to permanent deformation. The stress at the
breaking point, commonly known as ultimate strength, is a measure of the force
required to fracture the material completely.
A hard, brittle material such as an amorphous polymer far below its Tg,
usually has an initial slope indicative of very high modulus, moderate strength, a low
elongation at break, and a low area under the stress-strain curve (Fig. 1.4). Polymeric
materials showing hard brittle behaviour at room temperature or below are
polystyrene, poly (methyl methacrylate) and many phenol-formaldehyde resins.
Hard and strong polymers have high modulus of elasticity, high strength, and
elongation at break of approximately 5 percent. The shape of the curve often suggests
that the material has broken where a yield point might be expected. This type of curve
is characteristic of some rigid poly(vinyl chloride) formulations and polystyrene
polyblends. Hard, tough behaviour is shown by polymers such as cellulose acetate,
14 | P a g e

cellulose nitrate
n
and nylons; theey have hig
gh yield po
oints and hhigh modulu
us, high
strengths annd large elo
ongations. M
Most polym
mers of thiss group shoow cold-draawing or
"necking" during
d
the stretching
s
pprocess. Colld-drawing is importannt in syntheetic fiber
technologyy, and is ussed to deveelop strength. Polymerric materialls that are soft and
tough show
w low modu
ulus and yieeld values, moderate
m
strength at brreak, and very high
elongation ranging frrom 20 to 100 per cent.
c
This type
t
of strress-strain curve
c
is
characteristtic of plasticcized PVC and rubberss (elastomerrs).
Thee two mech
hanical perfformances creep
c
and stress
s
relaxaation are reelated to
each other. In creep, elongation
e
ttakes placee under the applicationn of constan
nt stress,
o
when a specimenn is held at constant
while in strress relaxatiion, decreasse in stress occur
and essenttially instan
ntaneously induced strain.
s
The varying sstress or strain
s
is
measured as
a a function
n of time. R
Repeated flexing of a saample throuugh a given distance
often causees a sample to fail at a lower stress than it could for a ssingle flexu
ure. This
feature is reeferred to ass fatigue.

Fig 1.3: Generalized

G
d tensile strress-strain curve for some polym
meric materrials

15 | P a g e

Fig 1.4: Teensile stresss-strain currves for fou

ur types of polymeric material.

g may be ccarried out by subjectting the sam

mple to altternating
Fatiigue testing
tensile and compressiv
ve stress. Thhe fatigue resistance
r
usually
u
decrreases with increase
in rigidity.
Imppact strength
h is a measuure of tough
hness or resistance to bbreakage under high
velocity im
mpact cond
ditions. Froom this poiint of view
w, polymeri
ric materialls under
normal connditions of use are thhought to be
b either brittle or tou
ough. For example,
e
polystyrenee, poly(metthyl methaccrylate) and
d unmodifiied, non pllasticized PVC
P
are
usually ratted as britttle, breakinng with a sharp fraccture; plastiicized

PV
VC's are

considered to be toug
gh. In generral, polymeeric materiaals are eithe
her brittle or tough,
depending on the tem
mperature annd rate of impact, i.e. rate of deeformation.. Impact
strength off polymers and derivved plasticss depends on the possition of th
he glass
transition temperature (Tg) wiith respectt to room
m temperatture and ease
e
of
crystallizatiion. Far beelow the gglass-transition, amorp
phous polym
mers break
k with a
brittle fractture, but they become tougher ass the service temperatuure approacches Tg..
Above Tg, amorphous polymers aare in a rub
bbery state so
s that the term impacct ceases
to have anyy significancce
1.7.1

Mecchanical prroperties off a thin film

m Polymer
Thee mechanicaal propertiees of any po
olymer blen
nd and com
mposite also
o depend

on its dimeensional size weather itt is thinner or thicker in appearannce. The sizze of the
blend makees it usable in various ffields. Todaay polymerss are used eextensively in many
applicationns in thin film
m form, likke film coatiing, adheren
nt epoxy unnder fills to localize
16 | P a g e

the stress concentration during thermal expansion, in electronic packaging etc.. The
mechanical state within the polymer necessarily varies from point to point within the
thin film, as can the strength of the adhesion between the polymer and the substrate.
In most practical cases, even in thin film applications such as found in electronic
packaging applications, the typical dimensions of the polymer component are much
larger than the dimensions of individual molecules. At scales much larger than this,
the material can be considered as continuum and the usual mechanics can be invoked
to model the deformation behavior, and bulk tensile properties (which will vary with
resin and processing) can accurately describe the time dependent properties of the thin
film.
1.7.2 Microhardness of a Polymer
If we discuss the mechanical properties of polymer and biocomposites,
hardness of the material is one of the most important aspects in its mechanical
characterization; it is generally defined as resistance of a material to plastic
deformation, usually by indentation. It is the property of a material, which gives it
the ability to resist permanent deformation (bend, break, or have its shape changed),
when a load is applied. The greater the hardness of the material, the greater is the
resistance it has towards deformation. The one of the best available definition is given
by Ashby [18], which states hardness is a measure of the resistance to permanent
deformation or damage.
Microhardness is the hardness of a material measured at low loads as
determined by forcing an indenter such as Vickers or Knoop into the surface of the
material under 5 g to 160 g load. Usually the indentations are so small that they must
be measured with a microscope. Micro indenters work by pressing a tip into a sample
and continuously measuring applied load, penetration, depth and cycle time. Nanoindentation tests measure hardness by indenting with very small (of the order of 1
nano-Newton) indentation forces and measuring the depth of the indention that is
made.
The hardness test measures the mean contact pressure, when an indenter is
pressed onto the surface of a flat specimen.

It provides the simple and non-

destructive means of assessing the resistance of material to plastic deformation. In

this present work Vickerss microhardness indentation testing has been utilized to
17 | P a g e

study various properties of polymer and polymer biocomposite blends, which has also
been utilized by various workers. This typical hardness test involves applying a fixed
load to the indenter, and measuring the resultant size of indentation. This has been
related empirically to the yield stress of the material. This work presents the results of
micro-indentation testing on electrically stressed polymer thin films [19-26].
1.8 ELECTRICAL PROPERTIES OF POLYMER
Materials such as glass, ceramics, polymers and biocomposites are non
conducting materials. They prevent flow of current through them. When these types
of non-conducting materials are placed in an electric field, they modify the electric
field and they themselves undergo appreciable changes as a result of which they act as
stores of electrical charges. When charge storage is the main function, the materials
are called dielectrics. For a material to be a good dielectric, it must be an insulator. As
good insulators, polymers posses excellent dielectric properties. Many authors have
reported theoretical and experimental work related to these properties [29-52]. The
common electrical properties of interest are discussed below:
1.8.1

Dielectric polarization
Polarization with the application of an electric field, due to the displacement

of charge particles inside the material forming dipoles, is known as internal

polarization. Some of the internal polarizations are electronic, orientational (dipolar),
space charge and barrier polarization.
Dielectric can also be charged by direct injection of charge carriers, when high
electric field (105 V/cm) is applied between the electrodes in intimate contact with it.
Charges get sprayed or deposited due to corona discharge or dielectric breakdown in
the thin air gap between the electrode and the dielectric. If the injected charge in the
surface is of same sign as that of electrode in contact, it is called homo-charge; if it is
opposite in sign then it is called hetero-charge.
The total polarization is the resultant of both internal and external polarization.
P = Pint + Pext

1.5

18 | P a g e

Polymers as dielectrics are known to store charge permanently when subjected

to field - temperature treatment; such quasi permanently charged dielectrics are
known as Electret.
1.8.2 Dielectric strength
It measures the highest current that can be applied to a plastic before it allows
current to pass. It is expressed as the voltage just before this happens divided by the
thickness of the sample (in volts/m). It is affected by temperature, thickness, how the
sample was conditioned, rate of voltage increase, test duration and contamination
etc.
1.8.3

Electrical conduction
The electrical conductivity of polymer, x, measures the presence of free ions

not connected chemically with the macromolecules. It also depends on presence of

low molecular weight impurities that can serve as source of ions. The chemical
constitution has only an indirect effect on the mobility of the ions.
In glassy state the conductivity of the polymer is approximately 1013 to 1019 ohm1.
With increasing temperature, the conductivity of polymer increases according to the
exponential law as,
x = A eURT

1.6

where, A is a Coefficient mainly dependent on temperature, R is Universal gas

constant, and U is the Activation energy
1.8.4 Dielectric constant (or permittivity)
Dielectric constant, , indicates how easily a polymer/plastic can be polarized
relative to vacuum. It is defined as the ratio of the capacity of an electric capacitor
filled with the substance to that of the same capacitor in vacuum, at a definite external
field frequency. This dimensionless number which is important in high frequency
applications varies with temperature, moisture, frequency and thickness.
1.8.5 Dissipation factor
This measures the energy dissipated during rapid polarization reversals, as
with an alternating current. It can be seen as the ratio of energy lost as heat to current
19 | P a g e

transmitted. It is usually measured at 1 MHz. This factor should be low when

polymers are used as insulators in high-frequency applications such as radar and
microwave equipments.
1.8.6 Dielectric loss
Dielectric loss, , measures the part of the energy of an electric field that is
dissipated irrecoverably as heat in the dielectric. Dielectric loss in polymeric materials
is due to the independent movement of chain sections consisting of large number of
monomer units.
1.8.7 Volume resistance
A standard measure of conductivity when a direct current potential is applied
across a material is volume resistivity (measured as ohm area of the smaller
electrode/ specimen thickness). Materials measuring volume resistance above 108
ohm-cm are insulators.
1.8.8 Surface resistance
This expresses how well current flows over the surface of a material between
electrodes placed on the same side of a specimen. While volume resistance is a
property of the material, surface resistance measures how susceptible a plastic is to
surface contamination, especially moisture. It is useful when surface leakage may be a
problem but since it is not measurable exactly it should be used with wide margin of
safety.
1.9 STATEMENT OF THE PROBLEM
Composite materials are attractive because they combine material properties in
ways not found in nature. Such materials often result in lightweight structures having
high stiffness and tailored properties for specific applications, thereby saving weight
and reducing energy needs [4,54-55]. Typically, a manmade composite would consist
of a reinforcement phase of stiff, strong material, frequently fibrous in nature,
embedded in a continuous matrix phase. The advantage of such a coupling is that the
high strength and stiffness of the fiber may be exploited.
Biocomposites are composite materials comprising one or more phase(s)
derived from a biological origin. In terms of the reinforcement, this could include
20 | P a g e

plant fibers such as cotton, flax, hemp and fibers from recycled wood or waste paper,
or even by-products from food crops. Biocomposites offer a significant non-food
market for crop-derived fibers and resins.
Considerable growth has been seen in the use of biocomposites in the
automotive and decking markets over the past decade or so, but application in other
sectors has hitherto been limited. Nevertheless, with suitable development, the
potential exists for biocomposites to enter new markets and thus stimulate an increase
in demand for non-food crops. Fibers provide strength and stiffness and act as
reinforcement in fiber-reinforced composite materials; ultimately the properties of a
composite are governed by the inherent properties of these fibers. Natural fibers can
be subdivided into vegetable, animal and mineral fibers [55].
Biodegradation is the process by which organic substances are broken. The
term is often used in relation to ecology, waste management and environmental
remediation (bioremediation).

Biodegradable composite which can be easily

degraded in natural environment are gaining public interest. Biodegradable plastics

and biocomposites are being developed in number of research laboratories concerned
about the persistence and ecological effect of synthetic plastic in the environment
worldwide including

applications such as waste management, recycling,

sustainability, packing, automotive, construction, etc [54-56].

Wide range of bio-based biodegradable polymers which are utilized to prepare
biodegradable polymer and biocomposite such as Polyhydroxyalkanoates (PHAs),
Polylactides (PLA), Cellulose esters, Starch plastics which are Renewable resourcebased and Aliphatic polyesters, Aliphatic-aromatic polyesters, Poly ester amide, Poly
vinyl alcohol are Petroleum/fossil fuel-based. Poly vinyl alcohol (PVA) is one of the
best biodegradable polymers and it is widely utilized to prepare the biocomposites.
The characterization of biocomposite can be made on the basis of structural,
mechanical, electrical, thermal and biodegradation. The study of the surface structure
of biocomposite can be made with the help of various characterization techniques like
FTIR, XRD, SEM, TEM, AFM, STM, etc [56-62]. The surface structure is also
responsible for the biodegradation of the biocomposites. The mechanical behavior of
various types of biodegradable materials depends, mainly, on their chemical
composition and the application conditions. Various additives are added into the bio21 | P a g e

blends to improve their properties, which sometimes even reach the levels of the
conventional plastics. It is well known that the environmental conditions during
production, storage, and usage of these materials influence their mechanical
properties. The mechanical characterization can be made with the help of
Microhardness and Tensile Stress, tensile strength [56,58,59,63].

The study of

thermal properties like thermal stability, thermal expansion of biocomposites can be

made with the DSC and TGA [64-66]. Biodegradation of polymeric materials affect
a wide range of industries; information on degradability can provide fundamental
information facilitating design and life-time analysis of materials. The common tests
to check the Biodegradability of biocomposites are Weight Loss Test, FT-IR
Technique and 1H NMR Spectroscopy [67]. The TSDC technique is a basic tool to
identify and evaluate the polarization and charge storage and decay phenomena, so it
can be utilized to check the electrical behavior of biocomposite films [29-52].
With the view to have a sufficiently good knowledge of structural, mechanical,
electrical, and thermal behaviour and to characterize the material for such applications
a detailed investigation has been under taken on biocomposite of PVA and palm leaf.
The measurement techniques employed include:
1.

IR/XRD/DSC/AFM and SEM studies on samples to investigate the

structural, morphological, thermal aspects of the film specimens.

2.

Microhardness measurement to study the effect of composition and load

on the pure PVA film and PVA incorporated palm leaf biocomposite film
specimens.

3.

Tensile strength and elongation studies on samples to investigate the

mechanical properties of the specimens.

4.

Short circuit thermally stimulated depolarization current measurement on

samples polarized under varying field and temperature conditions.

1.10 SELECTION OF MATERIAL

1.10.1 Polyvinyl Alcohol (PVA)
Many authors have studied biocomposites; some of the studies are based on the
composition of PVA with agro-industrial waste, corn fiber starch, glycerol, cellulose.
22 | P a g e

The sample has been prepared with different techniques. The structural, thermal and
mechanical characterization has been carried out. The effect of various types of
thermal pretreatment on the properties of biocomposites has been investigated.
Chiellini et al have studied composite films based on biorelated agro-industrial
waste with PVA and reported that the mechanical properties of prepared agroindustrial based biocomposite films were found to be dependent upon the nature and
environmental conditions [56]. Cinelli et al have studied the properties of injection
molded composition containing corn fiber and PVA and reported that the addition of
starch moderately reduces tensile properties of composite, lowering the elongation
and increasing Youngs modulus while tensile strength remains constant. Yun et al
have studied the mechanical properties with the functional group of additives for
starch/ PVA films and reported increase in flexibility and strength of films and degree
of swelling (DS) of films[68].
Mao et al have studied the mechanical properties, morphology and
biodegradability of Extruded Cornstarch- Glycerol PVA blends and reported that
addition of PVOH to blends slows down the biodegradation process and improves
physical and mechanical properties[59]. Jayshekara et al have studied biodegradation
by composting of surface modified starch and PVA blended films and reported that
PVA can be facilitate as biodegradation of composite films[69].
Imam et al have widely studied the composites based on PVA and reported
that the FTIR based biodegradation and hexamethoxymethyl-melamine (HMMM)
based composite for improving water resistance of PVA based biocomposite[64].
Rudnik has studied the thermal properties of biocomposite and reported increase in
thermal stability of modified starch[70]. The thermal stimulated discharge current in
polymer composite films has been reported by many authors on polymeric films
[29-52,71,72].
PVA is one of the most important polymeric materials to prepare composites
of biomaterials. The chemical structure of Polyvinyl alcohol is shown in Fig 1.5. The
general properties of PVA are appearance as white granule or powder, specific gravity
of 1.25~ 1.32, bulk density of above about 0.4 ~ 0.7, ph value is about to 5 ~ 7 with
glass transition temperature (Tg) = 850C, melting point of 210 ~ 2300C, thermally
stable, gradual discoloration at about 1000C, darkens rapidly above 1600C, gradual
23 | P a g e

decomposittion above 1800C as m

melting poin
nt is similarr to decomp
mposing tem
mperature
and is soluuble in wateer and resistt to oil, aniimal, plant and greasee oils. The different
d
type of bioocompositess of PVA hhave been studied
s
with corn-starcch, lingo-ceellulosic
fiber, agroo-industrial waste, staarch, glycerrol with diifferent asppect of stu
udy like
mechanicall, surface strructure andd thermal pro
operties.

Chemical Structure off PVA.

Fig 1.5: C
Thee different types
t
of bi odegradable biocompo
osites have been prepared for
various bioological app
plications iincluding packing,
p
agriculture, w
waste manaagement,
recycling, sustainabili
s
ty, automottive, constrruction, con
ntact lenses,, bio-mediccal field,
etc[55-60]. Applicatio
on of hexxamethoxym
methyl-melaamine (HM
MMM) witth PVA
reduces its water sensiitivity[56].
1.10.2 Alexxander Palm
m Leaf
Thee other nam
mes of thiis palm arre; Alexand
dra palm, Kind palm
m, King
Alexander palm,
p
Aalex
xandrae pallm. It has a medium sized, gray trrunk (1 1.5 feet in
diameter). Crownshafft is a brighht green colo
our (a wood
dy shaft at tthe top of th
he trunk
composed of
o tightly wrapped
w
leaaf base). Th
he type of the
t leaf is ppinnate. It is a fast
growing soolitary singlee trunk palm
m of height of near abo
out 40 feet. Palm leaf contains
fibers insidde the leaf an
nd it has maany potentiaal applicatio
ons in agricuulture and villages.
v
1.10.3 Bio Compositees of PVA aand Palm Leaf
L
Thee worlds su
upply of nnatural resou
urces is deecreasing annd the dem
mand for
sustainable and renew
wable raw m
materials co
ontinues to
o increase. B
Biofiber-reinforced
compositess represent a potential nontradition
nal, value-aadded sourcce of incom
me to the
agriculturall communitty. Due to thhe above mentioned
m
prroperties off Polyvinyl Alcohol
and Palm leeaf powder the biocom
mposite of PVA
P
and Paalm Leaf caan be developed. In
present studdy the bioccomposite oof PVA avd
d Palm leaf powder hass been succcessfully
prepared and
a
this biiocompositee can be the potenttial materiaal with co
onducive
24 | P a g e

structural, mechanical and electrical properties for various applications like

automobile, agriculture and industries sectors.
1.11

Possible Outcome
The

developed

biocomposite

may

definitely

exhibit

the

properties

incorporating the individual qualities of PVA and Palm Leaf powder. It may lead to
develop a green material with higher mechanical properties, improved microhardness,
improved structural morphology, improved thermal properties and improved charge
storage properties, providing high strength to weight ratio, for various applications as
biodegradable and environmental friendly material.

25 | P a g e

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