2.4.6. Electrostrictive Properties

The electrostrictive property of the sample was evaluated by measuring the deformation of strain-induced at low frequency and low electric field strength (f = 1 Hz, *E* ≤ 3 MV/m) using the photonic displacement apparatus (MTI-2100 Fotonic sensor, New York, USA, sensitivity 5.8 μm/V) setup demonstrated in Figure 2. The dimension of the sample was 3 × 3 cm<sup>2</sup> and the same thickness of 300 μm. The sample was sandwiched among brass electrodes (diameter 2 cm). The electric field (E3) of a high-voltage power supply (Trek model 610E, New York, USA) was applied along the thickness direction of the sample, which was the so-called "3" direction. The electric field-induced strain in polarizable materials was measured in the same direction and, hence, denoted as S3, then the electrostrictive coefficients (M33) were given, the relationship can be expressed according to the equation:

$$\mathbf{S}\_3 = \mathbf{M}\_{33} \mathbf{E}\_3^2 \tag{7}$$

The electrostrictive coefficient can be calculated from the slope of the strain (S3) versus the square of the electric field (E23). As a consequence, it can be expressed according to Equation (7).

**Figure 2.** The electrostriction setup.

#### **3. Results and Discussion**

#### *3.1. Structure and Morphology*

In Figure 3, SEM micrographs display the morphology of P(VDF-HFP) film and fibers. Figure 3a presents the surface of the P(VDF-HFP) film with a 50 μm scale bar showing a clearly smooth, homogeneous, and non-porous surface. The electrospun membranes show a random orientation distribution, high porosity, and smooth and bead-free fibers with an average diameter of 600 ± 50 nm in Figure 3b. Figure 3c–e shows the electrospun P(VDF-HFP) nanofibers after compressing at 30, 60, and 80 ◦C, respectively. The compression at an elevated temperature reduced porosity and flattened fibers, with an apparent increase in diameter. In Figure 3e, the surface of the fiber mat was almost similar to the film, but with some porosity remaining. The fibers had large contact surfaces which is good for exchanging electric charges. Nanofibers have potential use in the production of sensing devices, because they have a large specific surface that enhances their sensitivity as a sensor [7]. In a previous paper, Kang et al. [30] studied the effect of load compression parameters with electrospun nanofibers

at different types of polymers. They compressed poly(caprolactone); PCL and poly(vinyl alcohol); PVA and polyurethane; and PU and nylon nanofibers using a KES-G5s compression tester at different loads (0.5, 1, and 2 N, respectively) under room temperature. They explained that the movement of the fibers was related to the inter-fiber frictions when obtained fibers were passed. The changed density and loss of space between the layers occurred under an applied force. However, the morphology and structure of fibers under compression are not only magnitude force and direction force but also density materials, frictions of fibers, and operating temperature. In our case, the temperature conditions of 30, 60, and 80 ◦C for P(VDF-HFP) fiber mats were studied under a certain compression force. The morphology of P(VDF-HFP) film and fiber mats depends on the temperature effect, shown in Figure 3, and the changing temperature conditions related to the modification of the interfacial effect within P(VDF-HFP) film and fiber mats. Under the testing conditions, the P(VDF-HFP) fiber mats were strongly melted when the operating temperature increased due to the reduction of the glass temperature.

**Figure 3.** SEM images of the P(VDF-HFP) (**a**) film, (**b**) fiber, (**c**) 30 ◦C compressed fibers, (**d**) 60 ◦C compressed fibers, and (**e**) 80 ◦C compressed fibers.

#### *3.2. X-ray Di*ff*raction (XRD) Analysis*

To confirm the presence of β-phase crystals in P(VDF-HFP) fibers, XRD analysis was performed. Figure 4 displays the XRD patterns generated for the P(VDF-HFP) film, fiber, and fiber mat compressed at 30, 60 or 80 ◦C. The characteristic reflections of the crystalline phases were seen in the XRD spectrum at 2θ = 17.9◦ (020) and 26.8◦ (021) which indicate the large spherulites of the non-polar α-phase crystals while 2θ = 18.5◦ (110) and 20.1◦ (110) correspond to the smaller spherulites of γ-phase crystals that co-exist with the α-phase. The specific peaks at 2θ = 20.3◦ (110) and (200) and 36.7◦ (020) and (100) correspond to β-phase diffraction [31]. The P(VDF-HFP) film shows the largest non-polar α peak at 26.8◦ and the smallest β peaks at 20.3◦ and 36.7◦, because it lacks the electrical poling and mechanical stretching treatments. Thus, it shows the α-phase that is commonly the dominant phase in P(VDF-HFP) [32]. After electrospinning, the P(VDF-HFP) fibers and compassed fibers showed a strong β peak (110) at 2θ = 20.3◦ for the β-phase having an all-trans (TTTT) conformation, while the β peak (020) at 2θ = 36.7◦ was not clearly observed. Normally, the magnitude of this β peak (020) was quite small when compared with the β peak (110) which may be attributed to the formation of crystalline region and order of polymerization. Therefore, in our work, it was necessary to continue studying by Fourier transform infrared (FT-IR) analysis. It was apparent that the electrospinning successfully formed β-phase crystallites; this should enhance the electrostrictive properties of the nanofibers. The high voltage used during the electrospinning aligned the electric dipoles in the P(VDF-HFP) solution, and the degree of alignment was determined by the applied electric field [23]. In addition, the fibers after pressure and annealing treatments increased the crystalline and β-phases. This demonstrates that the formation of β-phase was induced by electrospinning, pressing, and annealing.

Furthermore, the crystallinity, Xc, of the film and fibers are presented in Table 1. The Xc increased from 49.47% to 55.03% with the compression temperature as shown by the strong peaks for β. The increased crystallinity could be due to the active interactions of the surfaces with polymer chains, inducing the formation of polar β-polymorphs from non-polar α spherulites. In other words, the fraction of crystalline material gradually increased. This appeared to be inferior to the α- to β-phase transformation, since the β-phase strongly depends on the overall crystallinity. However, using only XRD analysis is not enough for comparing the degrees of β-phase transformation, because the α and β peaks are close to each other and the changes are not clear. Therefore, Fourier transform infrared (FT-IR) analysis can provide additional data on the phase structure.

**Figure 4.** X-ray di ffractograms for P(VDF-HFP) film, fiber, and fiber mats compressed at 30◦, 60◦, and80 ◦C.


**Table 1.** Analysis of the β-phase fraction in the crystalline region of the samples.

#### *3.3. Fourier Transform Infrared Spectroscopy*

The FTIR spectra are displayed in Figure 5. These were employed to assess the α- and β-phases, the F(β) of the crystalline region and %β in the samples. According to the literature [32], the non-polar α-phase in P(VDF-HFP) is detected in absorbance bands around 490 cm<sup>−</sup><sup>1</sup> (−CF2 wagging), 530 cm<sup>−</sup><sup>1</sup> (−CF2 bending), 615 cm<sup>−</sup><sup>1</sup> (skeletal bending), 764 cm<sup>−</sup><sup>1</sup> (−CF2 bending), 795 cm<sup>−</sup><sup>1</sup> (−CH2 rocking), and 975 cm<sup>−</sup><sup>1</sup> (twisting) in the IR spectra. In contrast, the large absorbance peaks of the β-phase, attributed to the electroactive polar β-polymorph with a parallel dipole moment, were found at cm<sup>−</sup><sup>1</sup> (−CF2 stretching) and cm<sup>−</sup><sup>1</sup> (−CH2 rocking, −CF2 stretching, and skeletal C−C stretching) in the spectrum.

In the FTIR spectrum, the P(VDF-HFP) film presented the most α-phase at 490 and 764 cm<sup>−</sup>1. If comparing the IR spectra of the film and nanofiber in Figure 5, it showed a shift of the IR peak of the P(VDF-HFP) film which had a lower wavenumber. The normal α-phase of the P(VDF-HFP) film presented the spectrum peak of −CF2 wagging or out of plane bending and positioned at 490 cm<sup>−</sup><sup>1</sup> [32]. These results are due to the stress and variation in the morphology [33]. Moreover, it may be attributed to a reduction in mass of the molecule polymer chains which depend on the vibration frequency under absorption bands. In the P(VDF-HFP) fiber, all absorbance bands for the α-phase were missing while the absorbance peaks at 509 and 840 cm<sup>−</sup><sup>1</sup> were prominent, signifying a strong emergence of the electroactive β-phase. Therefore, the FTIR results demonstrate that electrospinning promotes the transition to β-phase crystals within the P(VDF-HFP) fibers. In addition, the β-phase of the fiber increased with the compression temperature.

An assessment of the relative fraction of the β-phase content, *F*(β), was executed from the IR spectra using the Lambert–Beer law stated in Equation (2). The *F*(β) for all samples is exhibited in Table 1. The P(VDF-HFP) fiber had F(β) ~85.90% exceeding the film by 11.79%. Electrospinning relies on high electric fields and allows the production of sub-micro to nano-scale fibers, with a β-phase fraction up to 86%without any post-treatment. Furthermore, *F*(β) in the fiber increased from 85.90% to 89.65% when compressed at an elevated temperature. This confirms the positive influence of high pressure on β-phase formation as previously reported. Scheinbeim et al. [14] verified that increasing the quenching pressure from 200 to 700 MPa increased the β-phase content in samples from 0% to 85%.

The absolute β fraction (%β) was estimated from the data of both the Xc and F(β) with Equation (3) as shown in Table 1. About 36.67 %β was obtained in the film, while the largest 49.33% was obtained with a compression at 80 ◦C of the fiber mat. The emergence of electroactive β-phase was clearly improved by electrospinning and compression at an elevated temperature which was corroborated by FTIR spectra and XRD patterns.

**Figure 5.** IR spectra of film and fiber P(VDF-HFP) for wavenumbers from 400 to 1000 cm<sup>−</sup>1.
