*3.5. Mechanical Properties Analysis*

Figure 4 shows the mechanical properties of the GA, PA66 films and GA/PA66 films with various weight ratios. The elastic modulus, elongation at break, and tensile strength of the GA film were 0.52 MPa, 7.98%, and 0.03 MPa, respectively. This result implied that the GA film was a material with poor ductility and mechanical strength. After blending with PA66, the elongation at break of the GA film increased substantially from 7.98% to 30.36% at the weight ratio of 1:1 (GA/PA66), and the tensile strength of the GA film were considerably enhanced from 0.03 MPa up to 1.42 MPa, which was 48 times higher, but there was a certain range of differences compared with pure PA66 nanofiber film. This might be due to the fact that the reduction of the diameter of nanofibers led to an increase in the density of inter-fiber alignment, and the uniformity of the nanofiber film might be improved [41]. In a similar study of Zhang et al. [21], they found that the mechanical properties of the GA films was also improved with the addition of poly(ε-caprolactone). In addition, the elongation at break of GA/PA66 was significantly improved compared to the pure GA film. These results indicate that the composite films had higher mechanical strength due to the addition of PA66.

## *3.6. Water Contact Angles Analysis*

Figure 5 shows the WCAs of the films at the equilibration times of 0 s and 3 s. The WCA of the GA film was 111.73◦ at 0 s and 71.87◦ at 3 s, which implied the GA film had a comparatively hydrophilic surface. This was because GA was a hydrophilic material [42]. The WCA of the PA66 film was 136.83◦ at 0 s and 137.07◦ at 3 s, which meant the PA66 film had a comparatively hydrophobic surface. However, the hydrophilicity of GA/PA66 films was significantly increased since the composite films had a super hydrophilic surface of 0◦ at both 0 s and 3 s. Similar results were reported by Cao et al. [43], who found that compared to pure polyethersulfone film, the hydrophilicity of the polyethersulfone/PA66 filtration film surface was improved. This may be related to the fact that a considerable number of hydrophilic groups existed in PA66 [44], and the homogeneous dispersion of gelatin in the mixing system, thus exposing the hydrophilic groups more sufficiently to

the surface. The hydrophilic surfaces of materials have been considered as an important factor in increasing the antimicrobial agen<sup>t</sup> loading and antimicrobial activity. Karam et al. [45] found that hydrophilic surfaces could load more antimicrobial agents (nisin) and had higher antimicrobial activity than hydrophobic surfaces. Therefore, the GA/PA66 composite film presents promising potential in loading high doses of antimicrobial agents for food packaging.

**Figure 4.** Mechanical properties of the nanofibrous films: (**a**) stress-strain curves; (**b**) elastic modulus; (**c**) elongation at break; (**d**) tensile strength. Values denoted with different letters (a–e) are significantly different (*p* < 0.05), where a is the highest value.

**Figure 5.** Water contact angles of the films at 0 s and 3 s (equilibration time).

## *3.7. Water Vapor Permeability Analysis*

The rate of water vapor transmission was investigated to be proportional to the porosity of the nanofiber film due to the fiber diameter [46]. Figure 6 shows the WVP values of the GA, PA66 films and GA/PA66 films with various weight ratios. It is well acknowledged that WVP is an inevitable factor of packaging products, which is associated with the exchange of water with the food and the environment [34]. The WVP values of the pure GA and PA66 nanofibrous mat were 16.99 and 12.61 g mm/m<sup>2</sup> h kPa, respectively. When mixing with PA66, the WVP decreased significantly at weight ratios of 2:1 and 1:2 (GA/PA66), and the WVP for the GA/PA66 composite film declined from 16.99 to 9.93 g mm/m<sup>2</sup> h kPa at weight ratios of 2:1 (GA/PA66), indicating that the water vapor barrier performance was improved. This improved water vapor barrier performance could be attributed to a higher crystallization, which could function as a nucleating element, producing increased crystallinity and impermeable regions in the film [47]. In a similar study of the modification of gelatin by Mohammadi et al. [48], they also found that the WVP of GA/chitosan composite film significantly decreased after chitosan was added.

**Figure 6.** Water vapor permeability of the nanofibrous films. Values denoted with different letters (a–c) are significantly different (*p* < 0.05), where a is the highest value.
