*3.4. Thermal Analysis*

The study of the thermal behavior was done using the DSC technique. The data are summarized in Table 2. On comparing the P(VDF-HFP) fiber and film, the onset of the melting ( Ton m ) and peak melting ( T<sup>p</sup> m) temperatures of the fiber increased with the electrospinning. This indicates that the high electric field (and possibly the dimensions of the sample) influenced crystallization in the sample. For compressed the fiber mats, both Ton m and T<sup>p</sup> m decreased with compression temperature. In addition, the final melting temperature ( Tf m) in all cases was in the range from 140 to 180 ◦C, which corresponds to the melting temperatures of the crystalline phases.

In this analysis, the melting enthalpy (ΔHm) of compressed P(VDF-HFP) fibers increased with compression temperature, because the particle size increased significantly as seen in the SEM images (Figure 3). Moreover, Madan [34] reported that the specific heat increases as particle size decreases, while the melting entropy and enthalpy decrease. Increased crystallinity can contribute to the mechanical properties of materials. The increased crystallinity may be due to the fact of good interactions and interfacial adhesion between the polymer matrix and the dispersed phase domain surfaces which would also restrict molecular mobility.

The onset crystallization temperature (Tonc ), peak crystallization temperature (Tpc ), and final crystallization temperature (Tfc) decreased with the compression temperature. Elevating the compression temperature reduced the P(VDF-HFP) crystallization temperature progressively, indicating a reduced crystallization rate of the P(VDF-HFP) crystals. Besides, the difference, ΔTc, increased as the compression temperature decreased. This means that the crystallization rate of the P(VDF-HFP) fibers from the melt was elevated.


**Table 2.** Thermal properties of the samples.

Tonm : onset melting temperature; Tpm: peak melting temperature; Tfm: final melting temperature; ΔTm = Tfm − Tonm ; ΔHm: melting enthalpy; Tonc : onset crystallization temperature;.Tpc : peak crystallization temperature; Tfc: final crystallization temperature; ΔTc = Tonc − Tfc; ΔHc: crystallization enthalpy.

−26.2

## *3.5. Mechanical Properties*

Fiber 80 ◦C 135.5 158.2 170.9 35.4 36.3 138.6 134.2 129.6 9.0

Dynamic mechanical analysis helps assess the thermomechanical properties and the glass transition temperatures of polymers. The storage modulus (*E*) and the tan delta as functions of temperature are displayed in Figure 6. The storage modulus decreases with temperature in Figure 6a although not linearly. The storage modulus displays three distinct regions: (1) a glassy high modulus region at low temperatures where the segmental motions are restricted; (2) a transition region with a substantial decrease in *E*; (3) and a rubbery area with severe decay in the modulus above the glass transition temperature. The storage modulus in both the glassy and rubbery regions increased due to the thermal compression at 30◦ to 80 ◦C and was comparatively high in the glassy region relative to the P(VDF-HFP) fiber. The high storage modulus of 80 ◦C for the compressed P(VDF-HFP) fiber at low temperatures confirms the reinforcement effect at the molecule interfaces. It can be attributed to the restricted molecular mobility in the P(VDF-HFP) fibers by the strengthened interactions with the polymer matrix [35]. A gradual decrease of E is observed from −40 to 0 ◦C which is ascribed to the glass transition of P(VDF-HFP( [36]. It can be seen that the compression temperature influenced the glass transition temperature of the P(VDF-HFP) fibers.

Figure 6b presents the loss tangent (tanδ) as a function of temperature for the P(VDF-HFP) fiber and compressed fiber mats. The dielectric relaxation process can be used to explain the ability motions and cross-linking structure in the amorphous phases and crystalline fraction which are related to the dynamic glass transition. Under the relaxation process at lower than room temperature, the β-relaxation for P(VDF-HFP) fiber was −55 ◦C which can be obtained from the value of the maximum in the loss tangent. It was found that the board of the β-transition for the P(VDF-HFP) fiber presented from −80 to −20 ◦C. Moreover, it was clearly shown that the β-transition for the P(VDF-HFP) fibers was lower than the fiber mats. This effect occurred in the cooperative segment's mobility of the polymer chains in the amorphous regions. Above the room temperature, the damping relaxation process was observed as two peaks. The first peak damping of the relaxation process provided the α-relaxation which was related to the motions in a crystalline fraction [29], while the second peak relaxation process depicted the melting temperature.

Generally, the glass transition temperature (*T*g) of a polymeric material is determined from the peak of the tanδ curve [37]. The tanδ has a peak at approximately −40 ◦C assigned to the glass transition of pure PVDF. In Figure 6b, the glass transition temperatures (*T*g), are approximately −56.17, −44.83, −50.83, and −48.97 for the fiber and the 30, 60 and 80 ◦C compressed fiber mats, respectively. In fact, the *T*g of the polymers was related to the polymer chains flexibility. When the rigid regions within the polymer increased, it led to an increase in the value of *<sup>T</sup>*g. In the case of fiber mats, the compression process enhanced the rigidity of the polymer based on the crystallinity fraction or hard segments. However, the *T*g also depended on the heating or cooling rate and the stress rate.

**Figure 6.** The dynamic mechanical analysis curves of P(VDF-HFP) fiber and compressed fiber mats. (**a**) Storage modulus and (**b**) tan delta.

## *3.6. Electrical Properties*

Figure 7a–c presents the dielectric constant (εr), loss tangent (tanδ), and conductivity (σ)as functions of frequency from 10<sup>0</sup> to 10<sup>5</sup> Hz for the P(VDF-HFP) film, fibers, and fiber mats compressed at 30◦, 60◦, and 80 ◦C. For all samples (Figure 7a), the dielectric constant decreased with frequency. This was because the dipoles of the dielectric materials cannot follow rapid changes in the field direction [38]. At a high frequency, the dielectric constant then only depends on the dipolar polarization, while the alignment of the dipoles lags behind the field in the polymer matrix. The dielectric constant strongly decreased in the low frequency range up to 20 Hz and then suffered a softer decrease at higher frequencies. This was due to the electric polarization inside the matrix which arises from the electrospinning and compression of the P(VDF-HFP) fibers.

All samples had the highest dielectric constant at low frequency, which can be explained by the Maxwell–Wagner polarization in a heterogeneous material [39]. Consequently, the organization of filler within the composites or multilayer dielectric, including electrospun fibers, can enhance the Maxwell–Wagner interfacial polarization with surface charge distribution. The maximum dielectric constant was 8.4 at 1 Hz for fiber mats compressed at 80 ◦C. Clearly, the thermal compression reduced air gaps and added surface charges causing strong Maxwell–Wagner interfacial polarization.

Interfacial polarization occurs whenever there is an accumulation of charges at interfaces among regions (phases) within a material. Grain boundaries frequently have interfacial polarization, as they can trap charges migrating in an applied field. Dipoles formed by the trapped charges increase the polarization. Interfaces also arise in heterogeneous dielectric materials, for example, when there is a dispersed phase in a continuous matrix. This principle is schematically illustrated in Figure 8. The schematic illustrates the anticipated electroactive β-phase interaction mechanism between the phase and chains of P(VDF-HFP) in the matrix. The P(VDF-HFP) film was prepared without stretching or poling, and the α-P(VDF-HFP) film is shown in Figure 8a. The fibers were then fabricated by electrospinning, thus they contained a β-phase and formed a highly porous fiber mat. Therefore, the interactions among the fiber surfaces were weak because of the air gaps (pores) among the fibers. During thermal compression, the interaction of negatively charged surfaces with C–F and positive –CH2 dipoles from (CH2–CF2) monomers in the P(VDF-HFP) alters the polarity and gives rise to nucleation of the β-phase in fibers [40]. Thus, the compressed fibers had cooperative interactions, apparently with synergistic e ffects giving an extremely high dielectric constant.

In recent work, the increase in the dielectric constant depended on the crystalline fraction in the polymer [3]. This fraction is normally accompanied by dipole polarization, which increases the melting enthalpy of crystalline domains and can greatly increase the dielectric constant. The observed melting enthalpy and crystallinity were highest for the fiber mats compressed at 80 ◦C, matching the highest dielectric constant for this case. Moreover, a high specific surface area and overlap without fusing the compressed fibers are the keys to achieving a high dielectric constant [41].

Figure 7b presents the dielectric loss versus frequency. Obviously, the dielectric loss increased with the applied frequency. Large dielectric losses are caused by the charges at high frequencies (10<sup>4</sup> Hz), which is typical owing to the polarization loss and DC conduction loss [42]. On the other hand, the decreased dielectric constant also relates to increased dielectric loss. The AC conductivity in all cases increased at high frequencies, as displayed in Figure 7c. The observed increases in electrical conductivity may be attributed to the polarization of the bound charges [43]. The electrical conductivity linearly increased with frequency, indicating that the number of charge carriers also increased. The electrical conductivity of the fiber mats compressed at 80 ◦C was the highest, and this might be attributed to the conductive networks formed by the surface contacts in the fibrous matrix and the free electrons within it.

**Figure 7.** *Cont*.

**Figure 7.** Variation of the dielectric constant (**a**), loss tangent (**b**) and AC conductivity (**c**) with frequencies from 10<sup>0</sup> to 10<sup>5</sup> Hz for the film, fibers, and compressed fiber mats.

**Figure 8.** Schematic of the proposed β-phase transformation mechanism.
