## 3 Results and Discussion
### Physical Properties of Nanocomposite Films  
The film thickness is affected by the film-forming production and the drying method. No significant differences between the films thicknesses were detected, which ranged between 66 to 70 \(\upmu\)m (Table 2).  
Figure 4: (**A**) Tensile strength and (**B**) elongation at break of neat and CNP-loaded starch-PVA films. Starch-PVA 40:60 (control film sample), starch-PVA loaded with 1% CNPs (control + 1% CNPs), starch-PVA film loaded with 3% CNPs (control + 3% CNPs), and starch-PVA film loaded with 5% CNPs (control + 5% CNPs). Different letters in each diagram indicate statistically significant differences at \(p<0.05\).  
The solubility of the films is one of the main characteristics of edible/biodegradable films, as some of the food products have a high water activity, and the packaging film will be in contact with them [36]. The water solubility (WS) of films represents the polymer's affinity for water molecules [37]. As shown in Table 2, the WS of neat starch-PVA films in distilled water was 79% and was significantly reduced by the addition of CNPs into the films at 3 and 5% \(w/v\). The decrease in WS of nanocomposite films can be attributed to the formation of hydrogen bonds between polymers and CNPs, which cannot be disrupted by the water molecules [29]. [PERSON] et al. [28] reported a similar observation, highlighting a remarkable WS decrease of methylcellulose films as a consequence of increasing CNPs.  
The WVP values of the starch-PVA nanocomposite films significantly decreased by adding CNPs into the film matrix (Table 2). Indeed, the WVP of neat starch-PVA films was 0.41 g-mm/kPa\(\cdot\)h\(\cdot\)m\({}^{2}\) and dropped to 0.28 g-mm/kPa\(\cdot\)h\(\cdot\)m\({}^{2}\) due to the addition of CNPs up to 5% \(w/v\). This experimental result can be ascribed to the CNPs' ability to increase the polymer structure compactness by placing themselves between voids in the polymer matrix, as evidenced by SEM micrographs (Figure 2), and, thus, to reduce the permeation of water molecules from the polymer matrix [33]. Indeed, it is well known that the WVP of the films is generally affected by the polymer nature, the manufacturing process, the ingredients/additives used for the film formation, the presence of voids, cracks and the polymer matrix homogeneity [38].  
The color characterization of films to be potentially used for the packaging of food materials is pivotal since it significantly impacts consumer acceptance of the packaging. The color change in natural polymers is generally influenced by the biopolymer nature, the film-forming process, the pH of the film-forming solution, the presence of plasticizers, nanoparticles, and thermal processing. The results presented in Table 2 show that by the addition of CNPs, the color parameters of the films significantly increased (\(p\leq 0.05\)), decreasing their lightness (L* value). Films containing 3 and 5% CNPs presented the lowest L* value. Similarly, [PERSON] et al. [27] reported that the yellow index of the tarq gum films containing CNPs increased mainly due to the yellow color of the CNPs extracted from crap's skin.  
Another important parameter for the selection of edible and biodegradable films for different applications is the opacity, since it can also affect the acceptance of products by consumers [39]. From the data in Table 2, it is evident that the opacity significantly increased with the amount of CNPs in the films. This experimental evidence can be ascribed to NPs agglomeration, which reduced the light transmittance from the polymer structure. Similar observations were also reported for CNPs-loaded fish gelatin films [29] and CNPs-loaded cellulose films [40].