Electrostrictive and Structural Properties of Poly(Vinylidene Fluoride-Hexafluoropropylene) Composite Nanofibers Filled with Polyaniline (Emeraldine Base)

Previous studies have reported that poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymers can exhibit large electrostrictive strains depending on the filler. This work examines the electrostrictive and structural properties of P(VDF-HFP) nanofibers modified with conductive polymer polyaniline (PANI). The P(VDF-HFP)/PANI composite nanofibers were prepared by an electrospinning method with different PANI concentrations (0, 0.5, 1, 1.5, 3 and 5 wt.%). The average diameter, water contact angle and element were analyzed by SEM, WCA and EDX, respectively. The crystalline, phase structure and mechanical properties were investigated by XRD, FTIR and DMA, respectively. The dielectric properties and electrostrictive behavior were also studied. The results demonstrated that the composite nanofibers exhibited uniform fibers without any bead formation, and the WCA decreased with increasing amount of PANI. However, a high dielectric constant and electromechanical response were obtained. The electrostrictive coefficient, crystalline, phase structure, dielectric properties and interfacial charge distributions increased in relation to the PANI content. Moreover, this study indicates that P(VDF-HFP)/PANI composite nanofibers may represent a promising route for obtaining electrostrictive composite nanofibers for actuation applications, microelectromechanical systems and sensors based on electrostrictive phenomena.


Introduction
Electroactive polymers (EAPs) are materials that respond to electrical stimulation by exhibiting significantly large strains. EAPs have gained tremendous attention from researchers worldwide because of their attractive merits of large bending deformation, quick response, thinness, flexibility, relatively low cost and light weight [1,2]. Among the known EAPs, poly(vinylidene fluoride) (PVDF) and its copolymers, such as polyvinylidenefuoride hexafuoropropylene (P(VDF-HFP)), have been widely studied due to their relatively large and fast electromechanical response, high breakdown strength, high mechanical and chemical stability, flexibility, low acoustic impedance, low cost and also processing advantages. Therefore, they have been extensively used for a wide range of applications, such as sensors, actuators, energy harvesting and biomedicine [3,4]. Previously, it has been reported that the piezoelectric coefficient (d 31 ), electromechanical coupling factor (k 31 ) and dielectric constant in P(VDF-HFP) are higher than those of a PVDF homopolymer [5]. P(VDF-HFP) is a semi-crystalline polymer that consists of a repeating unit of the CH 2 CF 2 monomer and commonly exists in at least four crystal forms, namely α-, β-, γand δ-phases. Among these polymorphs, the β-phase (all-trans (TTTT) conformation) is the Poly(vinylidene fluoridene-hexafluoropropylene) (P(VDF-HFP); Solef 11010/1001, Solvay Solexis, Inc., Albright, WV, USA) powder and N, N-dimethylformamide (DMF; D158550, Sigma-Aldrich, St. Louis, MO, USA) were used as a polymer matrix and solvent, respectively. The P(VDF-HFP) powder was dried at 60 • C for 6 h prior to being used. Polyaniline (PANI; emeraldine base; 576379, Sigma-Aldrich) was used as a filler doped into the neat polymer solution.
The P(VDF-HFP)/PANI composite nanofibers were fabricated by a simple electrospinning method. Firstly, 5 g dried P(VDF-HFP) powder was dissolved in 20 mL DMF solvent and kept continuously under a magnetic stirrer at 40 • C, stirring until a transparent solution was achieved. Thereafter, the PANI filler was mixed into the P(VDF-HFP) solution by adjusting the weight percent to 0.0, 0.5, 1.0, 1.5, 3.0 and 5.0 wt.%. The whole mixture was magnetically stirred for 2 h to achieve a homogeneous mixture. Next, each viscous solution was loaded into a 20 mL plastic syringe connected to a stainless-steel needle, as shown in Figure 1a. Figure 1b shows the electrospinning setup to prepare pure P(VDF-HFP) and Polymers 2021, 13, 3250 3 of 12 P(VDF-HFP)/PANI composite nanofibers. The solution was electrospun in the horizontal direction at a flow rate of 0.5 mL/h, controlled by a syringe pump (Nz1000 NEWERA Pump Systems Inc., New York, NY, USA). The distance from the tip (positive pole) to the collector (negative pole) was fixed at 20 cm and the applied voltage was 17 kV using a high-voltage supply (PHYWE Item No. 13671-93, Göttingen, Germany). Finally, the pure P(VDF-HFP) and P(VDF-HFP)/PANI composite nanofibers were obtained on an aluminum foil substrate pasted onto the collector. solvent and kept continuously under a magnetic stirrer at 40 °C, stirring until a transp ent solution was achieved. Thereafter, the PANI filler was mixed into the P(VDF-H solution by adjusting the weight percent to 0.0, 0.5, 1.0, 1.5, 3.0 and 5.0 wt%. The wh mixture was magnetically stirred for 2 h to achieve a homogeneous mixture. Next, e viscous solution was loaded into a 20 mL plastic syringe connected to a stainless-s needle, as shown in Figure 1a. Figure 1b shows the electrospinning setup to prepare p P(VDF-HFP) and P(VDF-HFP)/PANI composite nanofibers. The solution was electrosp in the horizontal direction at a flow rate of 0.5 mL/h, controlled by a syringe pu (Nz1000 NEWERA Pump Systems Inc., New York, USA). The distance from the tip (p itive pole) to the collector (negative pole) was fixed at 20 cm and the applied voltage w 17 kV using a high-voltage supply (PHYWE Item no. 13671-93, Germany). Finally, pure P(VDF-HFP) and P(VDF-HFP)/PANI composite nanofibers were obtained on an a minum foil substrate pasted onto the collector.

Surface Morphology, Water Contact Angle and Elemental Analyses
The surface morphology of all the nanofibers was investigated using a scanning e tron microscope (SEM; FEI Quanta 400, The Netherlands). Each sample was processe a magnification of 1000× and 10,000×. To explore the wettability of the nanofibers' s faces, the fiber diameters of all samples were determined, which also resulted in a cha in their water contact angle. The average diameter was determined from the SEM im using the ImageJ processing software (ImageJ; National Institutes of Health, 1.46, Ma land, USA). The water contact angle of the surface was also characterized using Dataphysics Contact Angle System (OCA-15EC, Germany) in static sessile drop mo Energy-dispersive X-ray (EDX; X-stream-2, Oxford, England) was used for the analysi the elemental components of the samples. The elemental mapping signal of each sam was used to determine the chemical components on the surface, such as fluorine (F) carbon (C).

Crystalline and Phase Structure Investigations
The crystallinity of pure and composite nanofibers was investigated using an Xdiffractometer (XRD; X'Pert MPD, Philips, The Netherlands) in the 2θ range of 5° to at a scanning rate of 0.05°/1 s. The percentage crystalline degree (Xc) of the sample co be calculated according to Equation (1) [14]:

Surface Morphology, Water Contact Angle and Elemental Analyses
The surface morphology of all the nanofibers was investigated using a scanning electron microscope (SEM; FEI Quanta 400, The Netherlands). Each sample was processed at a magnification of 1000× and 10,000×. To explore the wettability of the nanofibers' surfaces, the fiber diameters of all samples were determined, which also resulted in a change in their water contact angle. The average diameter was determined from the SEM image using the ImageJ processing software (ImageJ; National Institutes of Health, 1.46, Bethesda, MD, USA). The water contact angle of the surface was also characterized using the Dataphysics Contact Angle System (OCA-15EC, Filderstadt, Germany) in static sessile drop mode. Energy-dispersive X-ray (EDX; X-stream-2, Oxford, UK) was used for the analysis of the elemental components of the samples. The elemental mapping signal of each sample was used to determine the chemical components on the surface, such as fluorine (F) and carbon (C).

Crystalline and Phase Structure Investigations
The crystallinity of pure and composite nanofibers was investigated using an X-ray diffractometer (XRD; X'Pert MPD, Philips, Amsterdam, The Netherlands) in the 2θ range of 5 • to 90 • at a scanning rate of 0.05 • /1 s. The percentage crystalline degree (X c ) of the sample could be calculated according to Equation (1) [14]: where ΣA Cr and ΣA amr are the total value of the integral area of crystalline diffraction peaks and amorphous, respectively. Moreover, the phase structure of all samples was considered using a Fourier transform infrared spectrometer (FTIR; Vertex70, Bruker, Germany) in the range of 400-2000 cm −1 . The fraction of β-phase [F(β)] was given by Equation (2) [15]: where A β and A α are the absorption peaks of the crystalline phase at 840 and 763 cm −1 , respectively. The number 1.26 is the ratio of the absorption coefficients at 840 and 763cm −1 . The absolute β fraction (%β) was calculated by Equation (3) [16].

Dynamic Mechanical and Young's Modulus Analyses
Dynamic mechanical analysis (DMA; Perkin Elmer, Groningen, The Netherlands) can be used to identify the storage modulus (E ), loss modulus (E) and tan delta (tanδ). The DMA testing was performed from −100 to 140 • C with a heating rate of 5 • C/min at a constant frequency of 1.0 Hz. Furthermore, Young's modulus (Y) was investigated to determine the longitudinal strain (S 33 ) based on the sizeable mechanical deformation in the electrostriction application. The modulus of all samples was measured by a strain gauge setup with a force gauge (BFG50N, Mecmesin, West Sussex, UK). Rectangular samples were cut from the original sheets with a size of 5 × 30 mm 2 . The modulus of the samples was calculated from the slope of the stress (σ)-strain (ε) curve on the elastic region (ε < 5%).

Dielectric Properties and Electrostriction Behavior
The dielectric constant (ε r ) and loss tangent (tanδ) of P(VDF-HFP) nanofibers with different PANI concentrations were evaluated as a function of applied frequency in the range of 10 2 -10 5 Hz using an LCR meter (IM 3533, HIOKI, Nagano, Japan) at room temperature. For electrostrictive measurement, each P(VDF-HFP) nanofiber film was examined by measuring the electric field (E)-induced strain (S) with a photonic displacement sensor (MTI-2100 Fotonic sensor, Albany, NY, USA), as demonstrated in Figure 2. All samples were sandwiched between two brass electrodes, and the weight of the top brass disc was 5 g, which was advisable for small stress, to avoid clamping of the sample. The electric field-induced strain was applied along the thickness direction. The thickness deformation of all samples was determined using a photonic sensor and a lock-in amplifier (610E, Trek, Waterloo, WI, USA).  Figure 3 presents images of the specimens of pure P(VDF-HFP) and P(VD HFP)/PANI composite nanofibers at 0.0, 0.5,1.0, 1.5, 3.0 and 5.0 wt%. As the concentrati of PANI increases, the color of the composite nanofibers changes from white to navy blu which is the color of the PANI powder, indicating the presence of the additives in t composite. SEM images of pure P(VDF-HFP) and P(VDF-HFP)/PANI composite nan fibers are also shown in Figure 4. As can be observed in the SEM image of the pure P(VD  increases, the color of the composite nanofibers changes from white to navy blue, which is the color of the PANI powder, indicating the presence of the additives in the composite. SEM images of pure P(VDF-HFP) and P(VDF-HFP)/PANI composite nanofibers are also shown in Figure 4. As can be observed in the SEM image of the pure P(VDF-HFP) nanofiber mat with 1000× magnification, a number of bead-on-string fibers are randomly produced. For composite fibers, when PANI concentrations are doped from 0.5 to 3.0 wt.%, the bead-on-string structures evidently disappear, which can be attributed to the higher conductivity in the composite solution during the electrospinning process, as previously considered by Uyar T., et al. [17]. Remarkably, the structure of 5.0 wt.% P(VDF-HFP)/PANI composite is composed of bead-free, continuous and aligned fibers. This observation can be explained by the fact that the addition of the conductive PANI polymer results in stronger electrostatic repulsion, which reduces the surface tension forces, favoring the stretching of fibers to form the aligned fibers [18].  Figure 3 presents images of the specimens of pure P(VDF-HFP) and P(VDF-HFP)/PANI composite nanofibers at 0.0, 0.5,1.0, 1.5, 3.0 and 5.0 wt%. As the concentration of PANI increases, the color of the composite nanofibers changes from white to navy blue, which is the color of the PANI powder, indicating the presence of the additives in the composite. SEM images of pure P(VDF-HFP) and P(VDF-HFP)/PANI composite nanofibers are also shown in Figure 4. As can be observed in the SEM image of the pure P(VDF-HFP) nanofiber mat with 1000× magnification, a number of bead-on-string fibers are randomly produced. For composite fibers, when PANI concentrations are doped from 0.5 to 3.0 wt%, the bead-on-string structures evidently disappear, which can be attributed to the higher conductivity in the composite solution during the electrospinning process, as previously considered by Uyar T., et al. [17]. Remarkably, the structure of 5.0 wt% P(VDF-HFP)/PANI composite is composed of bead-free, continuous and aligned fibers. This observation can be explained by the fact that the addition of the conductive PANI polymer results in stronger electrostatic repulsion, which reduces the surface tension forces, favoring the stretching of fibers to form the aligned fibers [18].   The SEM images were also used to analyze the average fiber diameter using the Im-ageJ program. As demonstrated in Table 1, the diameters of all fiber samples are observed in the nanoscale range. The incorporation of PANI content into P(VDF-HFP)/PANI composite nanofibers results in an increase in the diameter of the nanofiber. The diameter of the pristine P(VDF-HFP) nanofiber is 120.3 ± 20.6 nm, which then increases from 265.5 ± 53.1 to and 326.5 ± 99.5 nm after adding PANI from 0.5 to 5.0 wt%, respectively. The enlarged diameter of the composite nanofibers may be caused by the PANI absorbed by the P(VDF-HFP) nanofibers mats, which is similar to the result previously reported by Dognani G., et al. [19]. The average diameter of the composite nanofibers increases with increasing PANI concentration due to the higher viscosity of the solutions after adding PANI. The water contact angle in pristine P(VDF-HFP) nanofibers is approximately 139.2 ± 3.2°, which then slightly decreases with the increasing amount of PANI in the composite, as shown in Table 1. This result is due to the inherent hydrophilic property of PANI, which contains abundant amine groups and has a water contact angle of 51.5° [20]. However, PANI filling is not sufficient to change the water contact angle of composite nanofibers, which still presented a hydrophobic surface (90° ≤ θ ≤ 150°). These composite nanofibers are suitable for some applications that are poor wettability. Moreover, the EDX spectra were recorded in order to study the chemical composition of the nanofibers. In Table 1, we show the weight percentage and atomic percentage of F and C elements existing in the nanofiber samples. The result confirms the presence of C and F elements originating from The SEM images were also used to analyze the average fiber diameter using the ImageJ program. As demonstrated in Table 1, the diameters of all fiber samples are observed in the nanoscale range. The incorporation of PANI content into P(VDF-HFP)/PANI composite nanofibers results in an increase in the diameter of the nanofiber. The diameter of the pristine P(VDF-HFP) nanofiber is 120.3 ± 20.6 nm, which then increases from 265.5 ± 53.1 to and 326.5 ± 99.5 nm after adding PANI from 0.5 to 5.0 wt.%, respectively. The enlarged diameter of the composite nanofibers may be caused by the PANI absorbed by the P(VDF-HFP) nanofibers mats, which is similar to the result previously reported by Dognani G., et al. [19]. The average diameter of the composite nanofibers increases with increasing PANI concentration due to the higher viscosity of the solutions after adding PANI. The water contact angle in pristine P(VDF-HFP) nanofibers is approximately 139.2 ± 3.2 • , which then slightly decreases with the increasing amount of PANI in the composite, as shown in Table 1. This result is due to the inherent hydrophilic property of PANI, which contains abundant amine groups and has a water contact angle of 51.5 • [20]. However, PANI filling is not sufficient to change the water contact angle of composite nanofibers, which still presented a hydrophobic surface (90 • ≤ θ ≤ 150 • ). These composite nanofibers are suitable for some applications that are poor wettability. Moreover, the EDX spectra were recorded in order to study the chemical composition of the nanofibers. In Table 1, we show the weight percentage and atomic percentage of F and C elements existing in the nanofiber samples. The result confirms the presence of C and F elements originating from the pure P(VDF-HFP) copolymer. After loading PANI into the P(VDF-HFP) composite, the F element is reduced and the C element is increased, corresponding to the chemical formula of the PANI [([C 6 H 4 NH] 2 [C 6 H 4 N] 2 ) n ] homopolymer added to the composite.   [21]. The peaks of the α-phase at 18.8 • and 26.6 • are obviously presented in pure P(VDF-HFP) nanofibers. After the addition of PANI in P(VDF-HFP), the peaks of the α-phase are completely diminished and two peaks of the β-phase at 20.3 • and 35.5 • appear, indicating the presence of the β-phase in the composite nanofibers. The high voltage applied during the electrospinning process contributed to aligning the electric dipoles in the P(VDF-HFP) solution, leading to the formation of the polar β-phase in the composite nanofibers [22]. As seen in Table 2, the degree of crystallinity (X c ) of pure P(VDF-HFP) nanofibers is 48.40%, and it increases upon the addition of PANI filler. The composite fibers are molecularly oriented during the electrospinning. nanofibers. After the addition of PANI in P(VDF-HFP), the peaks of the α-phase are completely diminished and two peaks of the β-phase at 20.3° and 35.5° appear, indicating the presence of the β-phase in the composite nanofibers. The high voltage applied during the electrospinning process contributed to aligning the electric dipoles in the P(VDF-HFP) solution, leading to the formation of the polar β-phase in the composite nanofibers [22]. As seen in Table 2, the degree of crystallinity (Xc) of pure P(VDF-HFP) nanofibers is 48.40%, and it increases upon the addition of PANI filler. The composite fibers are molecularly oriented during the electrospinning.  Table 2. Evaluated crystallinity and β-phase fraction in the samples.   Table 2. Evaluated crystallinity and β-phase fraction in the samples. The electroactive β-phase in the nanofiber samples was further examined via FTIR spectra. Figure 6 presents the FTIR spectra of pure and P(VDF-HFP)/PANI composite nanofibers with different PANI concentrations in the region of 400 to 2000 cm −1 . The vibrational bands at 534, 617, 765 and 976 cm −1 are determined to the characteristic peaks of the α-phase observed in the pure P(VDF-HFP) nanofiber web [23], indicating that it contains α-phase formation. With the accretion of PANI filler, the characteristic peaks of the α-phase are entirely diminished and the peaks of the β-phase as marked bands at 838, 1234 and 1276 cm −1 are increased. It can be inferred that with PANI loaded in P(VDF-HFP), not only is the α-phase reduced but also the β-phase is also induced in the composite nanofibers. In Table 2, the relative fraction of β-phase content (F(β)) with various concentrations of PANI filler is shown. It is found that the F(β) values increase from 73.53% to 85.79% when the minimum amount of PANI (0.5 wt.%) is added to P(VDF-HFP). The F(β) values slightly increase with the increasing amount of PANI filler. Furthermore, the absolute β fraction (%β), evaluated by Equation (3) using the XRD and FTIR results, gives information about the overall β-phase within the as-prepared nanofibers. It is evident that the %β values increase rapidly upon adding only 0.5 wt.% of PANI and then increase gradually with the PANI content. These observations denote the effect of varying PANI filler loaded into P(VDF-HFP) nanofibers prepared by electrospinning techniques, which can affect the orientation of the dipoles to promote β-phase formation [24]. Figure 6 presents the (a) storage modulus (E ), (b) loss modulus and (c) tan delta (tanδ) of pure P(VDF-HFP) and P(VDF-HFP)/PANI composite nanofibers using the DMA measurement. Moreover, the thermomechanical properties and glass transition temperature (T g ) can be observed. It is found that the storage modulus of all samples decreases with increasing temperature. The storage modulus of P(VDF-HFP)/PANI composite nanofibers is greater than that of pure P(VDF-HFP) nanofibers, which confirms the reinforcement effect at the molecule interfaces [25]. A slow reduction in E is observed from −40 • C to 0 • C, which is ascribed to the glass transition of P(VDF-HFP) [26]. This effect demonstrates that semiconductor nanofillers influence the glass transition temperature of the P(VDF-HFP) nanofibers. Figure 6c offers tanδ as a function of temperature for pure and composite nanofibers at varying concentrations of PANI filler. The first peak damping of the relaxation process provided the α-relaxation, which was related to the motion in a crystalline fraction, while the second peak relaxation process depicted the melting temperature [22]. Normally, the glass transition temperature of a polymeric material is determined from the peak of the tanδ curve [27]. The β-relaxation is associated with a dynamic glass transition in amorphous and other semicrystalline polymers. The damping combined with the β process appears to be comparatively high for the nanofiber with broad β-transition from −80 The elastic modulus increased after PANI loading due to the rule of mixing properties. The addition of a conductive filler is normally associated with an increase in the interfacial surface area in the mixed composite solution between the filler particles and the host matrix [28]. The Young's modulus of the composite nanofibers tends to be stable for higher PANI concentrations (with low Young's modulus for lower PANI concentration).

Mechanical Properties
Polymers 2021, 13, x FOR PEER REVIEW 8 of 13 loaded into P(VDF-HFP) nanofibers prepared by electrospinning techniques, which can affect the orientation of the dipoles to promote β-phase formation [24].  ) of pure P(VDF-HFP) and P(VDF-HFP)/PANI composite nanofibers using the DMA measurement. Moreover, the thermomechanical properties and glass transition temperature ( ) can be observed. It is found that the storage modulus of all samples decreases with increasing temperature. The storage modulus of P(VDF-HFP)/PANI composite nanofibers is greater than that of pure P(VDF-HFP) nanofibers, which confirms the  Figure 7a shows the dielectric constant (ε r ) as a function of frequency with various amounts of PANI in P(VDF-HFP) nanofibers. At a low frequency (10 2 Hz), a high dielectric constant is observed and it decreases with increasing frequency. The dielectric constant is Polymers 2021, 13, 3250 9 of 12 associated with free dipole vibration in an alternating field at low frequency. The dielectric constant increases with the PANI concentration, as listed in Table 3. The ε r value at 10 2 Hz increases from 1.48 to 2.58 for the 5 wt.% P(VDF-HFP)/PANI composite nanofibers, which is around 1.74 times that of the pure P(VDF-HFP) nanofibers. Clearly, the PANI reduces air gaps and induces surface charges, causing strong Maxwell-Wagner interfacial polarization. Furthermore, the dielectric loss tangent increases with the PANI concentration from 0.00071 to 0.02403 at 10 2 Hz. It is recommended for use in ferroelectric and dielectric applications.   Figure 7b presents the longitudinal strain ( ) as a function of an external electric field ( ) for pure and composite nanofibers at a frequency of 1 Hz. The electric fieldinduced strain is quadratically related to the applied electric field, which is demonstrated by ( is the apparent electrostrictive coefficient) and Maxwell stress [29]. The Maxwell stress effect involves electrostatic attractions and interactions with the charges on electrodes, expressed by / . Here, is the permittivity of free   Figure 7b presents the longitudinal strain (S 33 ) as a function of an external electric field (E 3 ) for pure and composite nanofibers at a frequency of 1 Hz. The electric fieldinduced strain is quadratically related to the applied electric field, which is demonstrated