*3.3. Physicochemical Properties of the PVDF Composite Nanofibers*

The FTIR was used to analyze the chemical structure and phase composition of the PVDF composite nanofibers. Characteristic bands appeared at 1402 and 840 cm−<sup>1</sup> in the spectrum of the pure PVDF nanofiber corresponded to the stretching and rocking vibration of the CH2 groups, respectively (Figure 3a) [28,29,36]. The four bands observed at 1275, 1178, 760, and 482 cm−<sup>1</sup> corresponded to the deformation, stretching, bending, and wagging modes of the CF2 groups in PVDF. An absorption peak due to the skeletal vibration of C–C bonds in PVDF was also observed at 880 cm−1. After adding the POSS–EGCG conjugate or pure API-POSS, the PVDF composite nanofibers exhibited a new absorption peak at 1110 cm−<sup>1</sup> associated with the stretching mode of Si–O–Si bonds, which is the typical absorption peak of the POSS inorganic framework [37]. In particular, the absorption peaks at 1402, 1275, 880, 840, and 482 cm−<sup>1</sup> represent characteristic vibration modes of the piezoelectric β-phase of PVDF [28,29].

**Figure 3.** (**a**) FTIR spectra of the PVDF composite nanofibers and (**b**) changes in β-phase content in the nanofibers calculated from the FTIR spectra (*n* = 5). Significant difference from the pure PVDF nanofiber was denoted as *p*\* < 0.05.

The FTIR data was used to calculate the relative β-phase content (F(β)) in the PVDF composite nanofibers. As presented in Figure 3b, the F(β) values of all samples were higher than 75% because the crystalline phase change of the PVDF from α-phase to β-phase was occurred by the electrostatic forces applied to the polymer droplet during the electrospinning [30]. In addition, the piezoelectric β-phase increased from 77.4 ± 2.5% to 88.1 ± 3.2% after adding the POSS–EGCG conjugate. According to previous reports, converting α-phase to piezoelectric β-phase in PVDF/ceramic composites was ascribed to the interaction between the negatively charged ceramic nanoparticles and the positively charged CH2 groups of PVDF [27,28]. Moreover, hydroxylated ceramic nanoparticles interacted more strongly with PVDF through the hydrogen bonds formed by the fluorine atoms on the PVDF with the hydroxyl groups on the surface of the ceramic nanoparticles. Therefore, we may deduce that, in our experiments, PE06 exhibited a higher F(β) value than those of the other composite nanofibers because of strong dipole and hydrogen bonding interactions between the PVDF and POSS–EGCG conjugates.

The surface elemental composition of the PVDF composite nanofibers was assessed using XPS. The XPS survey scan spectrum of the pure PVDF nanofiber exhibited two separate peaks which corresponded to the F 1s orbital (688 eV) and the C 1s orbital (287 eV) (Figure 4a). After adding the POSS–EGCG conjugate or pure API-POSS, the XPS spectra exhibited six separate peaks assigned to PVDF, the POSS–EGCG conjugate, and API-POSS: F 1s (688 eV), O 1s (535 eV), N 1s (400 eV), C 1s (287 eV), Si 2s (155 eV), and Si 2p (105 eV). The intensities of the peaks assigned to the POSS–EGCG conjugate increased with increasing contents of the POSS–EGCG conjugate. In addition, the crystalline phases of the PVDF composite nanofibers were examined by means of XRD as shown in Figure 5. All samples exhibited two diffraction peaks mainly attributed to the piezoelectric β-phase of PVDF at 20.4◦ and 35.9◦, which were indexed to the β(110)/(200) and β(020)/(101) planes, respectively [27,28].

**Figure 4.** (**a**) X-ray photoelectron (XPS) spectra of the PVDF composite nanofibers and (**b**) the magnified XPS spectra of the areas in (a) marked by rectangle.

**Figure 5.** XRD patterns of (a) PVDF, (b) PE02, (c) PE04, (d) PE06, and (e) PO06 composite nanofibers.

The investigation on the surface hydrophilicity of biomedical membranes is very important to confirm the affinity between and the membranes and the cell. Suitable hydrophilicity can influence the attachment and proliferation of the cells on the surface of the membranes. Therefore, the performance of membranes in biomedical applications may depend on the hydrophilicity [38]. Here we measured the water contact angle to evaluate the hydrophilicity of the composite nanofibers. Figure 6 exhibited the water contact angle data measured for the composite nanofibers; the PVDF nanofiber revealed a high water contact angle (127.7 ± 9.5◦) due to its hydrophobic property. In addition, the water contact angles were hardly changed by the addition of the POSS–EGCG conjugate, i.e., 129.0 ± 8.7◦ for PE02, 128.9 ± 6.8◦ for PE04, and 130.4 ± 8.6◦ for PE06.

GBR membrane should be strong enough to withstand the force during new bone tissue regeneration. Thus, the effect of the addition of the POSS–EGCG conjugate on the mechanical properties of the composite nanofibers was assayed. As shown in Table 1, the resulting data obtained on the basis of stress–strain measurements exhibited that the tensile strength and Young's modulus increased with increasing contents of the POSS–EGCG conjugate. This means that the POSS–EGCG conjugate can cause temporary intermolecular cross-linking of polymer chains and thus can provide enhanced mechanical strength [39]. However, the addition of POSS–EGCG conjugate induced a slight decrease in elongation at break of the composite nanofibers.

**Figure 6.** Contact angle images for water droplet on (**a**) PVDF, (**b**) PE02, (**c**) PE04, (**d**) PE06, and (**e**) PO06 composite nanofibers. (**f**) Average contact angles on PVDF composite nanofibers determined by contact angle meter (*n* = 5). Insets are photographs of PVDF composite nanofibers.

**Table 1.** Mechanical properties of the PVDF composite nanofibers with different contents of the POSS–EGCG conjugate.


<sup>a</sup> Content of API-POSS (wt%).
