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https://doi.org/10.1177/15589250221125437
Journal of Engineered Fibers and Fabrics
Volume 17: 1 –8
© The Author(s) 2022
DOI: 10.1177/15589250221125437
journals.sagepub.com/home/jef
Introduction
In recent years, energy harvesters have been attracting the
attention of researchers and scientists as noticeable by
expanding the number of publications and the develop-
ment of product prototypes.1–8
Running portable electronic devices with batteries such
as sensors, watches, light-emitting diodes, and so on has
obstacles because of batteries disadvantages such as the
need to recharge or replace them, occupying a significant
percentage and weight of portable products, and dangerous
environmental impact.8,9 Consequently, the best choice for
exceeding these difficulties is using energy harvesters
which are defined as devices that collect energy from the
surrounding environment and convert it into electric power
for later use.10,11
Piezoelectric materials can generate electrical energy
when subjected to mechanical deformation.12–14 Piezoelectric
polymers have many advantages due to their lightweight,
flexibility, and simple manufacturing process compared
with other piezoelectric materials.15,16 Therefore, they are
used for various applications such as wearable sensors,
actuators, artificial skin, energy harvesting, drug delivery,
wound healing, and so on.8,17–23
Enhanced piezoelectric performance
of electrospun PVDF nanofibers by
regulating the solvent systems
Bilal Zaarour1 and Wanjun Liu2,3
Abstract
In recreant years, the attention of researchers has been focused on enhancing the electrical outputs of energy harvesting
devices. This study reports the generation and characterization of electrospun polyvinylidene fluoride (PVDF) nanofiber
webs obtained from different solvents (Acetone (ACE), ACE: N, N-dimethylformamide (DMF) /3:1, ACE: DMF/1:1,
ACE: DMF/1:3, and DMF). These electrospun webs will be used as active layers for piezoelectric nanogenerator (PENG).
We found that fibers electrospun using DMF have the highest phase content (F(β)), while fibers electrospun using ACE
have the lowest one. Furthermore, the results show that PENG based on fiber web electrospun using DMF has the
highest electrical outputs, whereas, the lowest electrical outputs were for PENG based on fiber web electrospun using
ACE. We believe this work can serve as a good reference for investigating the effect of solvent systems on diameters of
fibers, crystalline phases, and piezoelectric properties.
Keywords
Solvent systems, PVDF nanofibers, electrospinning, nanogenerator, β-phase
Date received: 7 June 2022; accepted: 25 August 2022
1
Textile Industries Mechanical Engineering and Techniques Department,
Faculty of Mechanical and Electrical Engineering, Damascus University,
Damascus, Syria
2 Key Laboratory of Textile Science & Technology, Ministry of
Education, College of Textiles, Donghua University, Shanghai, China
3 Engineering Research Center of Technical Textiles, Ministry of
Education, College of Textiles, Donghua University, Shanghai, China
Corresponding author:
Bilal Zaarour, Textile Industries Mechanical Engineering and Techniques
Department, Faculty of Mechanical and Electrical Engineering,
Damascus University, Damascus, 20872, Syria.
Email: Bilalzaarour121@hotmail.com
1125437JEF0010.1177/15589250221125437Journal of Engineered Fibers and FabricsZaarour and Liu
research-article2022
Original Article
2 Journal of Engineered Fibers and Fabrics
Polyvinylidene fluoride (PVDF) which is a semi-
crystalline polymer with a molecular structure [C2H2F2]n
is considered the most important piezoelectric polymer
thanks to its excellent piezo-, pyro-, and ferroelectric
properties, outstanding mechanical properties, low cost,
low density, high flexibility, good chemical stability, abil-
ity to be generated in different surface morphologies, and
so on.24,25
PVDF can be found in different polymorphs (α, β, γ,
δ, and ε). The β, γ, and δ are the polar phases while the α
and ε are non-polar phases.26 Generally, there is a positive
relationship between the piezoelectric response of the
PVDF and β phase content [F(β)] because the β-phase has
all-trans planar zigzag chain conformation (TTTT).27,28
However, improving the F (β) of PVDF fiber webs is
still a big challenge for researchers. Different methods
were used to improve the F(β) of PVDF such as plasticizer
treatment,29 mechanical drawing,30,31 thermal treatment,32
electrospinning,33 hydrated salt,34 the inclusion of nano-
fillers,35,36 electric poling,37 and so on.38
Electrospinning is an effective method for fabricating
nonwoven fiber webs with fiber diameters ranging to a few
hundred nanometers, as well as enhancing the F (β) of
PVDF webs at the same time because poling and stretch-
ing the polymer jet at high applied voltage during the
electrospinning process orientate the dipoles of PVDF
molecular chains, leads to a further transformation of α
to β crystalline phase.39 Electrospun fibers have outstand-
ing properties such as low density,40,41 high specific sur-
face area,42,43 good pore structures,44,45 tiny diameters,46,47
excellent mechanical properties,48,49 and so on. Therefore,
they can be used in multiple applications.50–52
Previously, our group studied the influence of surface
morphology on the piezoelectric properties of PVDF fiber
that can be used directly as active layers to form a piezoe-
lectric nanogenerator (PENG). We found that the PENG
based on the aligned wrinkled fiber web has the highest
electrical outputs owing to their high F(β), pillar wrinkled
surfaces, fewer air gaps between the fibers, and interior
pores.2
In addition, we demonstrated the effect of plasticizer
treatment on the electrical outputs of PENG based on
PVDF fiber webs. The results showed that the F(β) was
enhanced after plasticizer treatment resulting in improving
the electrical outputs of PENG owing to the interaction
between a plasticizer and PVDF.29
Furthermore, we explored the effect of the molecular
weight of electrospun PVDF nanofibers on the electrical
outputs of the PENG-based. We noticed that increasing the
molecular weight leads to enhancing the F (β) due to their
high F (β) and high roughness.53
The main objective of this study is to explore the rela-
tionship between the solvent system and the electrical out-
put of PENG based on the electrospun PVDF nanofibers
web.
To the best of our knowledge, up to now, no study has
comprehensively studied the effect of the solvent systems
(single solvent system and binary solvent system) on the
piezoelectric properties of PVDF fiber webs. We report the
formation of PVDF fiber webs using both high boiling
point solvent (HBPS), low boiling point solvent (LBPS),
and HBPS/ LBPS and studied their piezoelectric proper-
ties (Table 1). In addition, we designed a PENG based on
PVDF nanofiber webs electrospun using different solvent
systems and measured its electrical outputs.
Experimental
Materials
PVDF pellets (Mw = 530,000 gmol−1) were purchased
from Sigma-Aldrich, USA. Acetone (ACE) and N,
N-dimethylformamide (DMF) were bought from Shanghai
Chemical Reagents Co., Ltd, China. All chemicals were
used as received.
Electrospinning
18% (w/v) PVDF pellets were dissolved in ACE, DMF,
and ACE/ DMF at different solvent ratios (3:1, 1:1, and
1:3). Then, the solution was loaded into a plastic syringe.
A syringe needle (21 gauge) was used as the spinneret,
which was fixed on a syringe pump (single-syringe infu-
sion pump KDS 100, KDmScientific Inc., Holliston,
USA). A high-voltage supplier (high-voltage direct-cur-
rent power supply, DW-P503-2ACDE, Tianjin Dongwen
Co., Ltd., Tianjin, China) was connected to the syringe
needle (Figure 1). The solution concentration was pre-
sented as weight/volume ratio (w/v%). To keep the electro-
spinning process under control, electrospinning parameters
were adjusted at the needle to collector distance, flow rate,
applied voltage, temperature, and relative humidity (RH),
and were adjusted at 18 cm, 1.5 ml/h, 18 kV, 22°C, and
60%, respectively.
Fabrication PENG. It consists of an active layer of PVDF
nanofiber web with a thickness and working area of
Table 1. The basic properties of solvents.54
Solvent Boiling point (°C) Vapor pressure (kPa, 20°C) Surface tension (mN/m) Viscosity (mPa s, 25°C)
ACE 56 24 23.3 0.33
DMF 153 0.36 35 0.82
Zaarour and Liu 3
100 μm and 15 cm2, respectively. Fabric electrodes were
selected to increase the friction between them and the
PVDF nanofiber web. The electrodes were brushed with a
silver paste to ensure the electrical connection. For the
electric contacts, copper wires were adhered using a dou-
ble-sided carbon adhesive tape. To prove the protection,
the whole sensor was completely packaged with the polyu-
rethane (PU).
Characterization. The surface morphology of the electro-
spun PVDF fibers was detected under field emission scan-
ning electron microscopy (FE-SEM, S–4800 Hitachi,
Japan). Fiber diameter was measured using image analysis
software (Adobe Acrobat X Pro 10.1.2.45). X-ray diffrac-
tion (XRD) was carried out on a diffractometer (Panalyti-
cal XRD, Netherland) using Cu radiation 1.54 Å. All
samples were scanned in the 2θ range of 5° to 30°. Fourier
transform infrared (FTIR, USA) spectra were recorded on
a Bruker Optics spectroscopy in ATR mode. Differential
scanning calorimetry (DSC, USA) was measured by heat-
ing the samples from 40°C to 190°C at the heating rate of
10 °C/min in nitrogen atmosphere. The thickness of the
webs was checked using a micrometer (Anytime, USA).
The open-circuit voltage and the short-circuit current of
the PENGs with the working area of 15 cm2 were meas-
ured via an oscilloscope (LeCroy, Wavesurfer 104MXs-B,
USA) and current preamplifiers (Stanford Research
SR570, USA), respectively, under impacts frequency of
5 Hz, peak force of 10 N. Periodic force of 10 N was
applied to the samples with a Mark-10 ESM 303 force
tester fitted with an M5-500 force gauge and flat compres-
sion plates.
Results and discussion
Solvent systems
To discover the relationship between solvent systems and
piezoelectric properties of electrospun PVDF nanofibers,
18% of PVDF pellets were dissolved in both single solvent
system and binary solvent system. DMF which is classi-
fied as a good solvent for PVDF was used as a single sol-
vent system, while ACE which is categorized as a poor
solvent for PVDF was used also as a single solvent system,
whereas ACE/DMF at different solvent ratios (3:1, 1:1,
1:3) were used as binary solvent system.55
The results showed that the diameter of fibers obtained
using ACE was 1639 ± 143 nm, while the diameter of fib-
ers formed using ACE/DMF at the solvent ratios of 3:1,
1:1, 1:3 was 1533 ± 132 nm, 1221 ± 113 nm,773 ± 51 nm,
respectively, whereas, the diameter of fibers generated
using DMF was 592 ± 45 nm (Figure 2). It can be noticed
the diameter of fibers increased by increasing the ratio of
LBPS (ACE) owing to its fast evaporation rate during the
traveling from the tip to the collector.55
Crystalline phase characterization
To detect the effect of the solvent systems on the crystal-
line phases of the electrospun PVDF nanofibers webs, the
crystal structure of samples electrospun using different
solvents was checked. The XRD patterns of electrospun
PVDF nanofibers formed using different solvents are
shown in Figure 3(a). The results showed peak at 2θ = 18.4°
which refers to α phase corresponding to the (020) crystal
plane, and peak at 2θ = 20.6° which refers to β phase
Figure 1. Schematic diagrams demonstrating the electrospinning technique.
4 Journal of Engineered Fibers and Fabrics
corresponding to the (110) and (200) plane. The sample
electrospun using DMF showed the highest intensity of β
crystal phase, while the sample electrospun using ACE
exhibited the lowest one. FTIR spectrophotometry was
used to confirm the crystal phase structure of studied
samples.
Figure 3(b) exhibited that the characteristic bands of the
α phase crystals had been observed at bands of 762 and
976 cm−1, while β phase crystals had been identified at
840 cm−1 (CH2 rocking) and 1274 cm−1 (trans band).
PVDF can be existed in different polymorphs: α and δ
phases trans-gauche–trans-gauche (TGTG′), β phase all
trans (TTTT), and (T3GT3G′) for γ and ε phases. It is
worth mentioning that there is a positive relationship
between the F (β) of the PVDF fibers and the piezoelectric
response. F(β)of the studied samples can be calculated this
using equation56:
F=X/X+X= A / K/KA+A
=A / 1.26A +A
ββαββ βα
αβ
βαβ
()
()()
Where Xα and Xβ are the crystalline rate of α and β phases,
respectively. Aα and Aβ represent the height of absorption
bands at 762 and 840 cm−1, respectively. Kα = 6.1 × 104 cm2/
mol and Kβ = 7.7 × 104 cm2/mol are the absorption coeffi-
cients at the respective wavenumber.
F (β) was 70.13% for sample electrospun using ACE,
81.79%, 83.28%, and 88.14% for samples formed using
ACE/DMF at the solvent ratios 3:1, 1:1, and 1:3, respec-
tively, and 93.88% for sample generated using DMF. To
determine the crystallinity of samples (ΔXc), DSC analy-
sis was used (Figure 3(c)). ΔXc content can be calculated
using this equation56:
Figure 2. SEM images of PVDF nanofibers electrospun using different solvents: (a) ACE, (b) ACE:DMF/3:1, (c) ACE:DMF/ 1:1,
(d) ACE:DMF/1:3, and (e) DMF.
Zaarour and Liu 5
∆∆
∆∆
X= X /XX+YX
cm
αβ
()
Where, ∆Xm is the melting enthalpy of the sample; ∆Xα =
93.07 Jg−1 and ∆Xβ = 103.4 Jg−1 are the melting enthalpy of
a 100% crystalline sample in α and β phases, respectively,
while X and Y are the amount of α and β phases in the
sample, respectively.
ΔXc content was 48.27% for samples formed using ACE,
50.28%, 52.89%, and 54.78% for samples electrospun
using ACE/DMF at the solvent ratios 3:1, 1:1, and 1:3,
respectively, and 59.19% for the sample obtained using
DMF.
It should be noted that the ΔXc as well as the F (β)
increase when the ratio of HBPS increases thanks to the
enhanced degree of molecular orientation during the elec-
trospinning of the PVDF fibers (Figure 3(d)). In other
words, ΔXc and F (β) improve by increasing the time of
evaporation solvents.53 The F (β) and ΔXc content of all of
the fiber webs formed are listed in Table 2.
To discover the influence of solvent systems on piezo-
electric properties of the PENG, five PENGs based on
PVDF fiber web electrospun using ACE, ACE/DMF:3/1,
ACE/DMF:1/1, ACE/DMF:1/3, and DMF were fabricated.
For comparison, each PENG consists of a small piece of
PVDF fiber web with a thickness of 100 μm and a working
area of 15 cm2 and was positioned between conductive
fabric electrodes. For the electric contacts, copper wires
were used. In addition, the PENG was covered by the PU
to protect it, improve its mechanical properties, and protect
Figure 3. (a) XRD patterns for the PVDF nanofiber webs electrospun at different solvents, (b) FTIR spectra for PVDF nanofiber
webs electrospun at different solvents, (c) DSC for PVDF fiber webs electrospun at different solvents, and (d) ∆Xc and F(β) for
PVDF nanofiber webs electrospun at different solvents.
Table 2. β Phase content and crystallinity of electrospun PVDF nanofiber webs electrospun using different solvents.
F(β) and ∆Xc
contents
Solvents
ACE ACE/DMF:3/1 ACE/DMF:1/1 ACE/DMF:1/3 DMF
F(β) (%) 70.13 81.79 83.28 88.14 93.88
∆Xc (%) 48.27 50.28 52.89 54.78 59.19
Piezoelectric effect of PVDF nanofiber webs electrospun at different solvents
6 Journal of Engineered Fibers and Fabrics
us from electrical noises (Figure 4(a) and (b)). For a per-
fect comparison, all PENGs were tested at the same
repeated compressive impacts (peak force 10 N and fre-
quency 5 Hz). The results indicated that the voltage and
current outputs of the PENGs were 0.98 V and 1.23 μA
using ACE, 1.43 V and 1.41 μA using ACE/DMF:3:1,
1.61 V and 2.21 μA using ACE/DMF:1:1, 2.11 V and
2.87 μA using ACE/DMF:1/3, 2.72 V and 3.35 μA using
DMF (Figure 4(c) and (d)). Herein, it is obvious that the
electrical outputs of PENG increased by increasing the
ratio of HBPS (Table 3). These results should be attributed
to the high F(β) of samples.34 The highest electrical
outputs of the PENG based on the PVDF nanofibers
electrospun using DMF solvent should be attributed to its
high F(β), and tiny diameter of fibers.
Conclusions
In summary, the effect of solvent systems (ACE, ACE:
DMF/3:1, ACE: DMF/1:1, ACE: DMF/1:3, and DMF) on
the electrical outputs of the PENG-based on electrospun
PVDF nanofiber webs were demonstrated. The results
exhibited that the F (β) and ΔXc of electrospun PVDF
fiber webs can be enhanced using HBPS owing to enhanc-
ing the degree of molecular orientation during the electro-
spinning of the PVDF fibers. Furthermore, we found that
Figure 4. (a) Schematic diagram of the PENG, (b) digital photo of the actual PENG, and (c and d) voltage and current outputs
generated by the piezoelectric device based on the PVDF nanofiber webs electrospun at different solvents.
Table 3. The electrical outputs of electrospun PVDF nanofiber webs electrospun using different solvents.
Electrical
outputs
Solvents
ACE ACE/DMF:3/1 ACE/DMF:1/1 ACE/DMF:1/3 DMF
Voltage (V) 0.98 1.43 1.61 2.11 2.72
Current (μA) 1.23 1.41 2.21 2.87 3.35
Zaarour and Liu 7
the PENG based on PVDF fiber webs electrospun using
DMF had the highest voltage and current outputs thanks to
its high F (β) and small diameter of fibers. We believe our
work may serve as an important reference for improving
the voltage and current outputs of PENG.
Declaration of conflicting interests
The author declared no potential conflicts of interest with respect
to the research, authorship, and/or publication of this article.
Funding
The author received no financial support for the research, author-
ship, and/or publication of this article.
ORCID iD
Bilal Zaarour https://orcid.org/0000-0001-6572-872X
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