FTIR is a useful tool to determine not only the chemical structure but also the conformational structure of the protein-based materials. As seen in FTIR spectra in
Figure 5a, all the prepared electrospun PANI/silk mats showed similar results. The peaks found close to 3000 cm
−1 were due to the C-H stretching from silk fibroin and PANI. Furthermore, three major bands from silk fibroin, including amid I (C=O stretch, 1600–1700 cm
−1), amid II (N-H deformation, 1500–1600 cm
−1), and amid III (C-N stretch and N-H bending, 1200–1300 cm
−1), also appeared [
25,
26]. Among these three amide modes, the amide I band is commonly utilized to determine the secondary conformational structure of silk fibroin. In general, the β-sheet crystalline conformation has a strong absorption band at 1620–1640 cm
−1 and the random coil structure is normally found at 1640–1650 cm
−1 [
26]. Accordingly, wavenumbers of 1647 and 1627 cm
−1, which belong to random coil and β-sheet crystalline structure, respectively, were chosen to investigate the effect of adding PANIs on the silk structure.
Figure 5b shows that there was no appreciable peak appear at 1627 cm
−1 for silk fibroin without adding any PANI, indicating the prepared electrospun silk nanofibers with few well-defined crystalline structures. However, the peak of 1627 cm
−1 appeared when adding 5% of PANI in the silk fibroin. Furthermore, β-sheet crystalline conformation (1620–1640 cm
−1) dominated compared to random coil structures (1640–1650 cm
−1) when the added amount of PANI increased to 30%. These results suggest that adding PANI could induce the formation of β-sheet crystalline structure in the prepared electrospun silk nanofibers.
The mechanical properties of the resulting PANI/silk composite nanofibers’ mats are shown in
Figure 6. The pure silk electrospun mats had tensile strength of 3.91 MPa with elongation strain of 3.75%. The tensile strength decreased to 2.5 MPa and 1.25 MPa when adding 2.5 wt.% and 5 wt.% of PANI. These results contribute that the molecular weight of PANI is less than silk so that more PANI led to lower tensile strength. However, the tensile strength maintained close to 1.3 MPa as the added amount increased to 15%. Moreover, the tensile strength increased to 1.8 MPa, instead of continuing to decrease, for the composite mat with 20% of PANI. These results are because the beaded morphology in the electrospun composite fibers (
Figure 3e,f). Porosity measurement shows that more PANI led to low porosity, which indicated more compact composite mats. Therefore, composite mats with higher amount of PANI could have higher tensile strength. In contrast, the elongation strain of composite mats decreased when increasing the added amount of PANI. The reason is because PANI could be considered as a rigid polymer since its glass transition temperature is higher than 200 °C [
27]. Therefore, rigid composite mats were obtained when introducing more PANI, so that its elongation strain decreased.
The textiles’ surface showing hydrophobicity is desirable for preventing fabrics from wetting. Thus, the water contact angle test was utilized to reveal the water wettability of the prepared PANI/silk composite nanofibers’ mats. Both silk fibroin and PANI are considered as hydrophilic polymer since their surface energy is close to 50 mJ m
−2 [
28,
29]. However, the water contact angle of our electrospun silk mats was larger than 90° (
Figure 7a), indicating the hydrophobic surface. The improved hydrophobicity of the silk fibroin/PANI composites was due to the entrapped air within the porous structures, as illustrated in
Figure 7b. The water droplet partially wets the side surfaces of the composite fibers and partially sits on hydrophobic air pockets [
30]. The resulting water contact angle was maintained close to 120° for composite mats with up to 20% PANI. However, water contact angle decreased to 108° when adding 25% PANI, due to the beaded morphology. The water wetting properties of porous textiles’ structures largely depend on surface roughness, pore size, and interpore spacing [
31]. The mats with 25% PANI showed lowest porosity so that the water droplet can impregnate into mats more easily due to fewer air pockets on the surface, leading to smaller water contact angle compared to ones with less than 25% PANI.