Humidity Controlled Mechanical Properties of Electrospun Polyvinylidene Fluoride (PVDF) Fibers
Humidity Controlled Mechanical Properties of Electrospun Polyvinylidene Fluoride (PVDF) Fibers
Humidity Controlled Mechanical Properties of Electrospun Polyvinylidene Fluoride (PVDF) Fibers
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1. Introduction
Many applications of electrospun polymer fibers depend greatly on their mechanical properties,
which are governed by processing conditions in electrospinning [1–3]. With the right set of parameters,
we are able to obtain a broad range of morphologies and physicochemical properties from the
same raw material [2,4–6]. Thus far, electrospun fibers have been researched in a wide variety of
applications such as cost-effective water harvesting systems [7,8], energy harvesters [9–11], tissue
engineering scaffolds [12,13], filters [14–16], composite strengtheners [17–19], sensors [20], drug
delivery systems [21,22] and many more.
Characterization of mechanical properties of single polymer fibers is difficult due to often micro-
and nanosized dimensions of the samples, making the handling and testing challenging [23]. There are
many reports on fiber meshes that were measured using macroscale experimental methods [23–26].
Such an approach gives important information on how electrospun membranes behave under load
together with a few proposed models of stretched electrospun samples [27]. Importantly, there are
many parameters in electrospinning and all of them affect the properties of produced fibers [28,29].
One of them is relative humidity (RH) [30]. Humidity influences fiber diameters [31], crystallinity [32],
internal structure [33], surface roughness [34], and mechanical properties of electrospun fibers [30,35,36].
Recent reports have shown that key properties such as stiffness, strain at break, or Young’s modulus
can be controlled using just RH [30,35,36]. The importance of RH is often correlated with the phase
separation processes that take place in a humid atmosphere. Vapor-induced phase separation (VIPS) is
frequently proposed as the main mechanism of phase separation in the electrospinning process. In this
phenomenon, humid air condenses on polymer jet mid-flight leading to changes in morphology and
porosity as well as affect the structure of obtained fibers [37,38].
In this study, we investigate how RH affects the mechanical properties of polyvinylidene fluoride
(PVDF), which has a broad range of applications due to its high mechanical strength, chemical
resilience, biocompatibility, and piezoelectric properties [39–41]. As a result of those properties,
PVDF can be found in applications from tissue engineering to water and energy harvesting [8,32,42],
where controlled mechanical properties of fibers are highly desirable [43]. Thus, we focus in this paper
on the mechanical properties of PVDF fibers electrospun at low (30%) and high (60%) RH. To fully
understand how processing parameters affect the properties of electrospun fibers, testing of individual
fibers was performed. Detailed investigation of individual fibers under stress was carried out utilizing
in-situ tensile testing in scanning electron microscope (SEM). Additionally, the mechanical properties
were discussed with the crystallinity values for similar samples. This work shows the strong influence
of RH on the morphology and mechanical properties of individual electrospun PVDF fibers.
2.1. Electrospinning
PVDF (Mw = 275,000 g mol−1 , Sigma Aldrich, London, UK) 24 wt.% was dissolved in acetone
(analytical standard, Avantor, Gliwice, Poland) and dimethylacetamide (DMAC, analytical standard,
Avantor, Gliwice, Poland) in a 1:1 ratio. The PVDF solution was stirred for 4 h at a constant speed of
700 rpm on a hot plate set to 55 ◦ C (IKA RCT basic, Staufen, Germany). The solution was electrospun
using EC–DIG (IME Technologies, Waalre, The Netherlands) apparatus with a climate upgrade system.
A constant voltage of 15 kV was applied to the stainless-steel needle with an inner diameter of 0.8 mm
situated at a 14 cm distance from the collector. The flow rate of the solution for all samples was set
to 4 mL·h−1 . Ambient temperature was kept constant at T = 25 ◦ C and the RH was set to 30 or 60%.
In this study, the samples are labeled as PVDF30 and PVDF60 for their respective RH. To obtain the
aligned fibers the rotating collector at 2500 rpm was used to deposit fibers for 5 s on custom-made
frames with a 3 × 3 mm window.
Figure
Figure Experimental
1. 1. Experimentalset-up
set-upforforthe
the tensile
tensile testing. (a) Macrophotography
testing. (a) Macrophotographyofoftensile
tensile stage
stage placed
placed
Figure
under
under 1.microscope,
thethe Experimental
(b)set-up
microscope, tensile
(b) for
tensile the
stage tensile
with
stage testing.
a mounted
with (a)
a mounted Macrophotography
sample, (c) sample
sample, ofimage
image
(c) sample tensile
ofstage
of PVDF placed
fiber
PVDF located
fiber
in under the
the frame.
located microscope,
in the frame. (b) tensile stage with a mounted sample, (c) sample image of PVDF fiber
located in the frame.
Figure 2. SEM images of initial morphology and cross-sections from PVDF electrospun at (a,c) 30 and
Figure
(b,d) 2.
Figure 60%2.SEM
RH,images
SEM imagesofofinitial
andmorphology
initial
respectively morphology andofcross-sections
and
(e) distribution cross-sections from
from
fiber diameters PVDFelectrospun
as PVDF electrospun
measured at at
from SEM (a,c)
(a,c) 30 30
images. andand
(b,d)
(b,d) 60%60% RH,
RH, respectivelyand
respectively and(e)
(e)distribution
distribution of fiber diameters
diametersasasmeasured
measuredfrom
fromSEM
SEM images.
images.
Fibers 2020, 8, 0065 4 of 9
Toughness was calculated as a function of area under the curve using OrginPro using integrate
function which is based on a given equation:
Z b
f (x)dx, (1)
0
where b = strain at failure, f (x) = stress and x = strain. Strain at the maximum stress was calculated
as strain value in the maximum height of the stress-strain curve and tensile strength was given as
the maximum value of stress. Both calculations were carried out using the integrated function in
OriginPro. The error was based on the standard deviation.
Table 1. Mechanical properties of single fibers as calculated from room condition measurements.
Fibers 2020, 8, 0065 5 of 9
Strain at Toughness W Tensile Strength Strain at Maximum Young’s
Sample
Failure Ɛf [%] [MPa] Rm [MPa] Stress max [%] Modulus [MPa]
PVDF30
the 74.19
individual ± 12.46 fibers
PVDF60 69.83
when± 13.94
compared 1.59 ± 0.41 samples.53.64
to PVDF30 ± 10.96 behavior was
A similar 13.3 ±reported
4.3
PVDF60
on 309.89
electrospun ± 39.74 312.11(PAN)
polyacrylonitrile ± 70.74and polymethyl
1.56 ± 0.36 methacrylate
225.08 ± 58.93 fibers [26,56].
(PMMA) 3.5 ± 1.2
Figure 3. (a)
(a) Stress-strain
Stress-strain curves
curves of single PVDF fibers electrospun at RH
RH of
of 30
30 and
and 60%.
60%. In-situ
investigation in SEM of samples (b) PVDF30 and (c) PVDF60, with the red arrows indicating necked
regions in tested fibers.
In our previous study, we reported that the overall crystallinity was 40.7 ± 0.7% and 34.1 ± 0.2%
Table 1. Mechanical properties of single fibers as calculated from room condition measurements.
for randomly aligned PVDF30 and PVDF60, respectively [32]. Notable, those results are probably
lowered as electrospun
Sample
fiber crystallinity
Strain at Toughness has been reported
Tensile Strength to be increased
Strain at Maximumwith the rotation
Young’s speed
Failure εf [%] W [MPa] Rm [MPa] Stress εmax [%] Modulus [MPa]
of the collecting drum [57]. However, increase in crystallinity for PVDF30 fibers is caused by lower
fiber PVDF30
diameter and 74.19a±lack 69.83 ±
12.46 of voids in13.94 1.59 ±
the material. 0.41
Notably, 53.64 ±
Aristein et10.96 13.3 ± 4.3
al. [51] proposed that the
PVDF60 309.89 ± 39.74 312.11 ± 70.74 1.56 ± 0.36 225.08 ± 58.93 3.5 ± 1.2
supramolecular structure of the amorphous phase was considered as a determining factor of fiber
stiffness, which was verified on electrospun nylon 6 fibers. Overall crystallinity has a modest effect
on theIntensile
our previous study,
properties of we reported thatelectrospun
semi-crystalline the overall crystallinity
fibers. Hence, wasour40.7 ± 0.7%
study has and
shown ± 0.2%
34.1that the
for randomly
higher aligned
stiffness PVDF30
of PVDF30 and isPVDF60,
fibers caused respectively [32]. Notable,
by a more compact those
structure results
that leadsare toprobably
ordered
lowered asorientation
molecular electrospun fiber
with ancrystallinity has been reported
additional contribution to be
of higher increased
overall with theConversely,
crystallinity. rotation speed less
of the collecting drum [57]. However, increase in crystallinity for PVDF30 fibers
crystalline structure and internal voids in PVDF60 can lead to increased strain at failure, following is caused by lower
fiber the
with diameter and rise,
toughness a lack
seeofTable
voids1.in the material. Notably, Aristein et al. [51] proposed that the
supramolecular structure of the amorphous phase was considered as a determining factor of fiber
stiffness, which was verified on electrospun nylon 6 fibers. Overall crystallinity has a modest effect
on the tensile properties of semi-crystalline electrospun fibers. Hence, our study has shown that the
higher stiffness of PVDF30 fibers is caused by a more compact structure that leads to ordered molecular
orientation with an additional contribution of higher overall crystallinity. Conversely, less crystalline
Fibers 2020, 8, 0065 6 of 9
structure and internal voids in PVDF60 can lead to increased strain at failure, following with the
toughness rise, see Table 1.
4. Conclusions
This study uncovered an extremely strong influence of water vapor on the resultant mechanical
properties of single electrospun PVDF fibers. The changes in RH resulted in completely different
morphologies of fibers. At RH of 30% smooth and solid fibers were produced, while 60% RH led to
wrinkled morphology with internal voids. Vast differences in appearance translated to drastic changes
in mechanical properties of obtained materials. Mechanical testing on single fibers uncovered that
PVDF30 fibers were ~400% stiffer than their PVDF60 counterparts. Such change was caused by ordered
supramolecular structure and lower diameters. Moreover, while PVDF60 fibers produced exhibited
strain at failure of almost 310% compared to 75% for PVDF30. An in-situ investigation has shown that,
in both cases, multiple necks appear during stretching. This difference was the result of the higher
crystallinity and non-porous structure of PVDF30 fibers. Our findings clearly show the influence of
relative humidity on a fundamental level, which affects the properties and morphology of electrospun
PVDF fibers. Those findings can be used to steer the mechanical properties of entire fiber networks at
no additional cost and minimal alterations to existing solutions.
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