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Abstract

This research presents an engineering approach to fabricate multilayered electrospun nanofiber mats with high conversion performance of mechanical to electrical energy as well as improved physical stability. Electrospun polyvinylidene fluoride nanofiber webs were prepared with predefined nanofiber alignments. Fiber alignments and layer-by-layer deposition angles are considered as a tool to adjust the piezoelectric responses of multi-layered fibrous mats. Samples with optimized drum speed and maximum aligned nanofibers were utilized to fabricate multi-layered mats in different layering angles from the fiber direction of base layer (0°, 30°, 60°, 90°, 120°, 150°, and 180 $°$ ). The effect of layering angle of multi-layered nanogenerators on their piezoelectric responses was investigated using an image analysis approach based on the fast Fourier transform. Multivariate analyses (ANOVA) performed to reveal the relationship between increasing drum speed and nanofibers alignment and the degree of crystallization as well as the formation of β-phase in the fiber crystalline structure. Results showed that increase in drum speeds had a relative improvement in the crystal structure and the formation of β-phase in the electrospun polyvinylidene fluoride nanofiber webs. Furthermore, electrical response of samples with well-aligned polyvinylidene fluoride nanofibers collected at 1800 r/min led to 94.49% improvement when they were exposed by a periodic mechanical impact compared to non-aligned polyvinylidene fluoride nanofiber webs. Piezoelectric response of multilayered samples with layering angles of 120 $°$ showed 41% improvement in their electrical output compared to those with 0 $°$ . These results teach us to establish engineering design rules for textile-based energy conversion devices with different piezoelectric coefficients.

Keywords

Nanofiber alignment power generator electrospinning piezoelectric PVDF

Introduction

Energy harvesting technology has attracted significant attention, because such devices are becoming smaller and lower-powered. Many research groups have been studying piezoelectric harvesting technology for converting mechanical vibrations into electric energy because of its outstanding power density. Many of these research studies have made performance improvements on the piezoelectric constant of active layers and/or the efficiency of charge transport [1].

It is believed that generator devices made of aligned fibers perform with improved electrical response compared to those with randomly orientated fibers. There are various methods for producing nano-web piezoelectric layer made of aligned nanofibers, one of the most common of which is the use of rotary drum for aligned nanofibers production [2]. The rotary drum method use to produce fibers with different alignment by changes in the speed of the drum [2,3].

Poly(vinylidene fluoride) (PVDF) is a representative piezoelectric material having greater flexibility, lower cost, less damage to the environment, and the highest piezoelectric constant among other piezoelectric polymers [4 –8]. The piezoelectric characteristics of PVDF are the result of the β-phase crystals in the polymer. β-phase is responsible for most piezoelectric responses and is obtained through the electrospinning process [9 –11]. The electrospinning process by applying the electric field and uniaxial drawing the polymer solution leads to molecular chain arrangement along the fiber axis, polarization of bipolar moments, dipoles orientation, and consequently, the formation of β-phase crystals [12].

In this study, first, the electrospun PVDF webs with various alignments were collected using a rotating drum at four different harvesting speeds using the electrospinning process to improve the electrical output of nanogenerator devices. We further investigated the effects of various take-up speeds on the degree of nanofiber alignment, the formation of β-phase in the crystalline structure of fibers as well as improvement of the electrical output.

In studies of piezoelectric response testing in nanogenerator devices, the layout angle of the active layers is usually 0° in the direction of the rotational drum rotation. In this study, we have tried to find the optimum piezoelectric response in a generator containing aligned nanofibers using layout of active layers at different angles. In the second stage of this research, we fabricated specific nanogenerator device. PVDF layers with high fiber alignment were selected from the first stage, and the layers were placed on each other at 0°, 30°, 60°, 90°, 120°, 150° and 180° angles from the fiber direction of the base layer. Output voltage of these samples is then compared to each other to find out the best layering strategies for multilayer devices in terms of their piezoelectric output.

Experimental details

Materials

PVDF formamide pellets with molecular weight viscosity of 275,000 (g/mol) were purchased from Sigma-Aldrich. The solvents used in this work were N,N-dimethyl formamide (DMF, Merck Chem. Co.) and acetone (Merk Chem Co.); both were used without any further purification.

Electrospinning PVDF fibers

For use as a piezoelectric active layer, the PVDF web was prepared by electrospinning. PVDF solution was prepared by the addition of PVDF pellets to DMF/acetone solvent mixture (DMF/acetone = 7/3 wt/wt), and then the resulting heterogeneous solution was stirred by a magnetic stirrer at a 60°C temperature for almost 4 h. PVDF solution was prepared on 27% (wt/wt). After stirring, a homogeneous solution was obtained and used for electrospinning. The electrospun PVDF fibers were prepared in a horizontal electrospinning setup (Fanavaran Nano-Meghyas Company, Iran). Electrospinning was performed by a 1.0 ml plastic syringe tipped with a 22-gauge stainless steel needle. The positive lead from a high voltage supply (Gamma High Voltage Research) is connected to the metal needle, with the applied voltage of 20 V. The solutions were injected into the needle at a constant rate of 0.5 ml/h with a syringe pump (KD Scientific, USA). A cylindrical drum collector (outer diameter 26 cm) wrapped with AL foil, with some modification to be able to put the conductive electrodes on it during the electrospinning, was used as the collector. The distance between the needle tip and the collector was 15 cm, and rotational speed of collector was from 500 to 1800 r/min.

Fabrication of power harvesting devices

In the first step to investigate the effect of different take-up speeds on output voltage and degree of fiber alignment for multilayer samples, nanofibers webs at 500, 1000, 1300, and 1800 r/min were produced on AL foils (3 $\times$ 7 ${cm}^{2}$ and thickness 70 $μ m$ ). By keeping the rotational direction on the foil, two rectangles (A) and (B) were cut with dimension 1 × 1.5 by 0° angle. Next, two rectangles (A) and (B) were put together to fabricate the generator device (Figure 1).

Figure 1.

Schematic diagram of fabricated power generator.

In the second step, to investigate the effect of different layering angle of multilayer samples, nanofibrous mats consisted of aligned nanofibers (electrospun web at 1800 r/min) were produced on a piece of AL foils (3×7 cm² and thickness 120 $μ m$ ). By keeping the rotational direction on the foil, a rectangle (A) was cut with dimension 1 × 1.5 by 0° and another rectangle (B) with the same dimension at 30° from the horizon axis. Next, two rectangles (A) and (B) were put together to make the generator device. In addition, in order to fabricate other generator devices at angles of 60°–180°, also individually, the (c) to (g) rectangles were must put on the (A) rectangle. This method led to the formation of a same certain angle at the cutting stage between the two nanofiber layers (active layer). Two pieces aluminum electrodes were used to transmit the charges produced by the generator device. A crystal adhesive tape was set just outside the nanofiber mat enabling as substrates and the bent nanofiber web to effectively recover to its original structure. The whole device was then sealed using a commercial crystal adhesive tape as substrates.

Characterization

Scanning electron microscopy (SEM) images was used for characterizing the microstructure and the morphology of the nanofibers (SEM, model: XL30, PHILIPS Co.). Each sample was sputter-coated with gold before the examination. The fiber alignment was measured from SEM images using Matlab software. The fiber alignment was obtained by using the two-dimensional fast Fourier transform (FFT) function to transform SEM images into FFT images. The nanofiber diameter was measured using image processing software (Image J, National Institutes of Health, USA). FTIR spectra of PVDF nanofibers were documented by spectrometer (model: NEXUS 670, Nicolet Co.) over a range of 400–4000 ${cm}^{- 1}$ . Nanofiber crystalline structure was studied by X-ray diffraction (XRD) (Equinox 3000 model, INEL France Co.) using $-$ Cu Kα radiation (wavelength 0.154 nm), and the samples were analyzed at room temperature. Differential scanning calorimeter (DSC) (model: DSC 2010, TA Instruments. co) measurements were performed at a heating rate of 20°C/min. To measure the electrical resistance of samples, a Keithley instrument (Keithley 2601a) with input voltage between –40 and 40 V was employed. To compare the properties of piezoelectric and electrical response of samples, a measurement system was developed (Figure 2) based on the previous studies (PiezoTester, Functional Fibrous Materials LAB, Iran, http://ffm.aut.ac.ir) [13,14]. The device for applying force was an eccentric connected to the main shaft derived by a servomotor. Beating frequency has a direct effect to the amount of applied force. On the other hand, by changing eccentric radius without changing the motor speed, applied force to the samples can be changed. The intensity of the applied force was measured with a load cell which is in contact with the sample. The signals generated by load cell and samples were magnified, and their noises were filtered, so that they could be characterized by an oscilloscope.

Figure 2.

Piezoelectric property measurement device [13].

To measure the open circuit voltage, we have used a standard 10:1 passive probe and connect it to the oscilloscope. Data sets were statistically analyzed by one-way ANOVA (P-value < 0.05) to compare the effect of drum speeds on fibers alignment and piezoelectric responses. The normality and equal variance tests were subjected to pairwise multiple comparisons (Duncan test, $α = 0.05$ ) before the ANOVA analysis.

Result and discussions

Fiber morphology

The influence of different take-up speeds (500, 1000, 1300, and 1800 r/min) on the resulting fibers morphology was studied by SEM micrographes. Figure 3 shows the SEM images of PVDF nanofibers prepared by electrospinning. Acording to the literature [15], nano-fibers collected at a low drum speed (500 r/min) are not aligned in the direction of rotation. The use of higher drum speeds (1000, 1300, and 1800 r/min) produces aligned fibers in the direction of rotation. In fact, at higher speeds, the centrifugal force around the rotary drum, with the help of shear force during the electrospinning process, helps to align the fiber in one direction [15]. However, there were still a few irregular fibers in Figure 3(e) and (g) due to the repulsive interaction of residual charges on the electrospun fibers [16]. The fiber-diameter distribution histogram drawn using the software SPSS is shown in Figure 3. The average diameters of 529.18, 448.94, 303.59, and 292.20 nm are evaluated for drum speeds of 500, 1000, 1300, and 1800 r/min, respectively.

Figure 3.

SEM images and fiber diameter distribution histograms of electrospun PVDF nanofibrous mats collected at different drum speeds (500 (a), 1000 (c), 1300 (e) and 1800 r/min (g)).

Image processing procedure to study the fiber alignment

Improving pizoelectric response in nanofiber mats is heavily dependent on well-aligned PVDF nanofibers. The degree of fiber alignment was evaluated using the derivatives of Fourier transform. The FFT function converts information present in an original data image from real space into mathematically defined frequency space [17]. The graphical depiction of the FFT frequency distribution generated by placing acircular projection on the FFT output image and conducting a radial summation of the pixel intensities for each degree between 0 $°$ and 360 $°$ , in 1 $°$ increments [2,18]. The results of the FFT analysis from SEM images (Figure 3(a), (c), (e), and (g)) as well as the intensity graph and FFT pattern are shown in Figure 4. In our analysis, the pixel intensities were summed along each degree and then plotted between 0 $°$ and 180 $°$ . In all images to this type of analysis, the FFT data set has been rotated 90 $°$ to correct for the mathematical transformation inherent, so the principal axis of orientation is directly determined from the position of the peak in the intensity plot. A height and overall shape peak is present in this plot (intensity plots of the samples collected at 500, 1000, 1300, and 1800 r/min). A high and narrow peak indicates a more uniform degree of fiber alignment, a broad peak, or a shoulder on the peak indicates that more than one axis of intensity graphs [19]. Peaks with uniform height are related to random fibers and contain spatial information of low frequency [2]. In the case of nonaligned PVD nanofibers (500 r/min), the intensity graph shows more than six peaks; on another side, it has almost low intensity at 90 $°$ . When the drum speed is increased (1000 r/min) related to low-alignment nanofiber, the graph reveals a broad peak around 90 $°$ . At FFT pattern with speeds 500 and 1000 r/min, a circular distribution of low frequency pixels is formed with high frequency pixels distributed dispersed in the center of the circle.

Figure 4.

FFT patterns and pixel intensity graphs as a function of the angle of acquisition for electrospun PVDF nanofiber processed with different collector speed.

At 1300 and 1800 r/min high speeds, FFT pattern displays a circular distribution that includes high frequency pixels focused on a 90 $°$ axis and represent the alignment fibers. But, the density graph generated by PVDF nanofibers collected at the velocity of 1300 and 1800 r/min showed a symmetrical and outstanding peak around the 90 $°$ angle. The density graph generated by PVDF nanofibers processed at 1800 r/min reveals just two peak with high intense compared with 1300 r/min. These results clearly suggest that PVDF nanofiber collected at 1800 r/min contain highly aligned nanofibers.

Crystalline structure of PVDF nanofiber

Figure 5 shows the FTIR spectra from PVDF nanofiber mats collected at different take-up speeds. The presence of absorption bands at 614, 766, 795, and 976 ${cm}^{- 1}$ was related to the α-phase crystalline structure, and bands at 510, 840, and 1279 ${cm}^{- 1}$ were associated with the β-phase [20 –23]. The intensity of the absorption band at 840 ${cm}^{- 1}$ increases with increasing fiber orientation.

Figure 5.

FTIR absorbance from PVDF nanofiber mats collected at different drum speeds.

To determine the percentage of $β$ -crystalline phase in each sample, the relative fraction of $β$ -phase could be calculated by the following equation (1), where $A_{α}$ and $A_{β}$ are the absorbencies of the $α$ and $β$ -phases at the 766 and 840 ${cm}^{- 1}$ characteristic peaks and $X_{α}$ and $X_{β}$ are crystalline rate of the $α$ and $β$ -phases, respectively [5,24,25]. As shown in Table 1, with increasing drum speed, the percentage of $β$ -phase content increases [13,26]. This suggests that nonpolar $α$ -phase is converted to polar $β$ -phase, and the crystalline phase and dipole alignment in the nanofbers were induced directly by the drum speed in the electrospinning process [27]. The results are shown in Table 1 and Figure 5 in summary. In addition, high ratio of stretching during the electrospinning process was in some way similar to the uniaxial mechanical stretching which could case $α$ to $β$ -phase transition [2,16,26]. The 1800 r/min drum speed thus provided a higher stretching ratio which resulted in the higher relative fraction of the $β$ -phase in the electrospun PVDF nanofiber

f (β) = \frac{X_{β}}{X_{β} + X_{α}} \times 100 = \frac{A_{β}}{1.26 A_{β} + A_{α}} \times 100

(1)

Table 1.

FTIR result of electrospun PVDF nanofiber mats (500–1800 r/min).

Drum speed (r/min)	$A_{α}$	$A_{β}$	$f_{β}$ (%)	Improvement (%)
500	0.120	0.355	70.13	–
1000	0.091	0.381	76.86	9.6
1300	0.080	0.386	79.29	13.06
1800	0.067	0.349	80.52	14.81

The DSC curve and data for the PVDF nanofibers collected at different drum speeds are shown in Figure 6 and Table 2. Equation (2) is also used to calculate sample crystallites, where $X_{c}$ indicates the degree of crystallinity of sample and ${Δ H}_{lit}$ is melting enthalpy for 100% crystalline PVDF sample [25,28]

X_{c} = \frac{{Δ H}_{m} (sample)}{{Δ H}_{Lit}} \times 100, ({Δ H}_{Lit} = 103.4 \frac{J}{g})

(2)

Figure 6.

DSC thermograms for the electrospun PVDF nanofibers collected at different drum speeds.

Table 2.

DSC result of electrospun PVDF nanofiber mats (500–1800 r/min).

Drum speed (r/min)	$T_{m} (°C)$	${Δ H}_{m} (\frac{J}{{\cdot g}^{- 1}})$	$X_{c}^{m}$ (%)	Improvement (%)
500	166.97	61.83	59.79	–
1000	167.11	62.24	60.19	0.67
1300	167.07	62.48	60.42	1.05
1800	166.49	58.14	56.22	–5.97

Results showed that the crystallinity percentage of nanofibers did not obviously enhance with increasing the drum speed. This could be explained by the fact that there are two contrary factors affecting the crystallization of PVDF nanofibers. Columbic force in the high electric field and mechanical drawing force caused by the rotating drum would enhance the orientation and crystallization of PVDF molecular chains in the direction of the fiber axis [26,29]. At the same time, the higher drum speeds also decreased the diameter of the jet fluids and increased their specific surface area. The increasing solvent evaporation rate and jet fluids curing rate then led to decreased crystallization time. To confirm formation of $β$ -phase in crystaline structure, XRD analysis was performed. Figure 7 shows XRD patterns of the aligned PVDF nanofibers collected at the drum speed of 500, 1000, 1300, and 1800 r/min. In the case of PVDF nanofiber mats, distinctive diffraction peaks were close to 18.5 $°$ reflections of the $α$ -phase while diffraction peaks at 20.6 $°$ represents the formation of the crystalline β-phase of PVDF.

Figure 7.

XRD patterns of electrospun PVDF nanofiber mats collected at different drum speeds.

These results indicate the co-existence of the $α$ - and $β$ -phase [16]. The results of FTIR data and XRD spectra indicate the presence of $β$ -phase in electrospun PVDF nanofibers.

The appearance of the peak diffraction at 20.6 $°$ represents the formation of an all-trans crystal conformation by the electrospinning process [16]. With increasing the orientation of the fiber in the electrospinning process, the diffraction peak decreases at 18.5 $°$ , which indicates the presence of the $α$ -phase [30]. These results are in agreement with the FTIR spectrum of the fibers (Figure 5). The fact proves that the formation of the β-phase in electrospun PVDF nanofibers is closely related to the orientation and stretching of the molecular chains along the fiber axis induced by two forces of Coulombic and stretch exerted in electrospinning [29].

Moreover, the presence of shear and elongation forces during electrospinning creates the lamellae to form fibrils aligned along the fiber axis. In other words, increasing the speed of the collector directly caused the increase of beta crystal [15]. Due to FTIR result (Figure 5), DSC (Figure 6), and XRD spectra (Figure 7), the contribution of the $β$ -phase increased in the crystalline structure. In fact, under the same conditions and in the time of crystallization, more force is applied to the liquid jet. Eventually, these results indicate that the electrospinning process with the orientation of polymeric chains and the formation of beta crystals by Coulombic forces exerted during the process caused improve the piezoelectric properties.

Pizoelectric properties

Piezoelectric properties of different power harvesting devices made from electrospun PVDF nanofiber webs collected at different drum speeds were examined by applying a periodic mechanical impact with a force of 2.6 N. When the impact is applied to the PVDF nanogenerator, a pizoelectric potential difference is produced between two electrodes, which leads to a free electron flow occuring around the external circuit and is detected as an output signal, and recovery, the pizoelectric potential disapears and an opposite potential. The free electrons in the external circuit flow back and forth, resulting in a alternating output [31].

For ensuring the reliability of these results, each sample was evaluated five times at the same testing condition ( $S_{1} (500 r / \min), S_{2} (1000 r / \min), S_{3} (1300 r / \min), and S_{4} (1800 r / \min)$ ). An image of the set-up used to measure the piezoelectric response is shown in Figure 2. Results are shown in Table 3 and depicted in Figure 8. As it was observed, increase in speed of collector caused an increase in output voltage, so the voltage of $S_{4}$ compared to $S_{1}$ shows almost 94.49% improvment.

Table 3.

Electrical response for measured samples.

Sample no.	Drum speed (r/min)	Alignment degree		Electrical response (mV)	Improvement (%)
$S_{1}$	500	Non		43.60	_
$S_{2}$	1000	Low		66.60	52.75
$S_{3}$	1300	Medium		72.40	66.05
$S_{4}$	1800	High		84.80	94.49
ANOVA
	Sumof squares	df	Mean square		F	Sig
Between groups	4468.50	3	1489.383		87.482	.000
Within groups	272.400	16	17.025
Total	4740.550	19

Figure 8.

Electrical response for measured samples.

$S_{1}$ – $S_{3}$ output enhancement was mainly due to their crystallinity percentage and $β$ -phase content improvement. $S_{4}$ improvement in addition to the previous reasons can be attributed to more alignment caused by increasing the drum speed to 1800 r/min. In other words, since the degree of fiber alignment in the $S_{4}$ sample is very high compared with other samples ( $S_{1}, S_{2}, and S_{3})$ , the uniformity overcomes the crystal structures’ weaknesses and finally caused higher output voltage [13,26]. There was a significant difference between the groups as a result of ANOVA testing and post-hoc analyses of pizoelectric response produced by the different drum speeds.

In the second step, piezoelectric properties of different generator devices made from electrospun PVDF nanofiber webs collected at drum speed of 1800 r/min with different layout angles (0°, 30°, 60°, 90°, 120°, 150°, and 180 $°$ ) were examined by applying a periodic mechanical impact with a force of 2.6 N.Each sample was evaluated twenty times at the same testing condition ( $S_{4 (0 °)}$ , $S_{4 (30 °)}$ , $S_{4 (60 °)}$ , $S_{4 (90 °)}$ , $S_{4 (120 °)}$ , $S_{4 (150 °)},$ and $S_{4 (180 °)}$ ). According to the results shown in Table 4, the output voltage is increased in 120° sample. Duncan’s post-treatment analyses divided all groups (angles) into three different categories. This test showed that three angles of 30°, 180° and 120° (each from one different categories) can signifcantly improve the piezoelectric response. Piezoelectric materials have inherent polarization and react intelligently to any factor that changed their polarization. As a result, when a force is applied to the fibers, whether mechanical or electrical, it will change the current polarization state of the polymer. As a result, this change of polarization creates instability in the polymer chain in the fibers. Based on the achieved results, it was found that an increase in drawing forces in the electrospinning caused by increasing the drum velocity will result in more nanofiber alignment and finally the oriented bipolar moments in the polymer chain. The application of high electric field and high stretching due to high velocity resulted the creation of an instability polarization state in the polymer molecular chain. This is also confirmed by the results of FTIR and XRD analyses, increasing β phase, and, consequently, increasing the piezoelectric properties of electrospun nanofibers at a speed of 1800 r/min. The reason for the increase of the piezoelectric response in the sample with the 1800 r/min drum speed (with 30°, 180, and 120° alignment angles) is shown in Figure 1.

Table 4.

Electrical response for measured samples with different layering angles.

Sample no.		Layout degree ( $°$ )	Electrical response (mV)	Improvement (%)
$S_{4 (0 °)}$		0	96	_
$S_{4 (30 °)}$		30	129.7	35.1
$S_{4 (60 °)}$		60	124.2	29.4
$S_{4 (90 °)}$		90	123.2	28.3
$S_{4 (120 °)}$		120	136	41
$S_{4 (150 °)}$		150	114.2	19
$S_{4 (180 °)}$		180	132.3	37.8
ANOVA
	Sum of squares	df	Mean square	F	Sig
Between groups	22,067.500	6	3677.91	7.8	.000
Within groups	62,648.750	133	471.04
Total	84,716.250	139

Conclusion

In this work, PVDF nanofibers with different fiber orientations were prepared from 27% (wt./wt.) solutions by electrospinning. The degree of fibers alignment in electrospun PVDF membrances increased with increasing drum speeds. XRD analysis, FTIR, and DSC characerizations showed that increase in drum speeds had a relative improvement in the crystal structure of electrospun PVDF nanofibers. In the first step, piezoelectric properties of different power harvesting devices made from electrospun PVDF nanofiber webs collected at different drum speeds were examined by applying a periodic mechanical impact with a fixed force of 2.6 N and led to 94.49% improvement in electrical output of the well-aligned PVDF nanofiber web collected at 1800 r/min compared to non-aligned PVDF nanofiber web collected at 500 r/min. In the second step, piezoelectric properties of different generator devices made from electrospun PVDF nanofiber webs collected at drum speed of 1800 r/min with different layout angles (0°, 30°, 60°, 90°, 120°, 150°, and 180 $°$ ) were examined by applying a periodic mechanical impact with a fixed force of 2.6 N. The results showed that the angle of 120° improved 41% of the output voltage than angle of 0°. The result indicated that use of generator devices made from electrospun PVDF nanofiber webs collected at drum speed of 1800 r/min with 120 $°$ layout angles can be considered as an effective method to increasing piezoelectric response.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Funding is received from Amirkabir University of Technology and INSF (Grant No. 95828199).

ORCID iDs

Masoud Latifi

Roohollah Bagherzadeh

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Tuning energy harvesting devices with different layout angles to robust the mechanical-to-electrical energy conversion performance

Abstract

Keywords

Introduction

Experimental details

Materials

Electrospinning PVDF fibers

Fabrication of power harvesting devices

Characterization

Result and discussions

Fiber morphology

Image processing procedure to study the fiber alignment

Crystalline structure of PVDF nanofiber

Pizoelectric properties

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

References