*2.3. Mechanical Properties*

Table 1 shows the mechanical properties of pure PVA film and PVA with different CH weight ratios. The ultimate stress of PVA film is 25.08 ± 4.32 MPa, and the specific deformation is 72.97% ± 7.26%. When the concentration of CH in the PVA/CH films is increased, the ultimate stress of the films is also increased, while the elongation at break of the films is decreased. Compare to the pure PVA film, the stress of PVA/CH film increases from 29.76 ± 4.81 MPa to 35.39 ± 5.35 MPa, while the strain decreases from 77.13% ± 7.94% to 72.91% ± 8.17%. Bispo et al. proposed that films formed from polymer blends present intermediate values of maximum stress compared with those of films comprising the pure components [22]. Therefore, the mechanical properties of the PVA/CH films may suggest that the PVA polymer has greater influence on the tensile stress and maximum specific deformation. The increases in CH content cause change in stress and strain, due to the thermodynamic immiscibility and inherent incompatibility between thermoplastic polymers and CH [23]; the CH also disrupts the intramolecular hydrogen bonding of the PVA molecules [23]. Moreover, as the CH addition is increased, this changed the intermolecular hydrogen bonds formed between the amine and hydroxyl groups of CH [24]. However, PVA/CH-2.5 shows the best mechanical properties (Young's modulus 63.57 ± 6.98 MPa; those results proved that blend miscibility of the two polymers as a function of molecular interactions between PVA and CH [25].

**Table 1.** Mechanical properties of pure PVA film and PVA/CH with different CH weight ratios.


Notes: Values with the same letter are not statistically different, according to Duncan's Multiple Range Test at *p* < 0.05; a, b, means with the same letter in the same column are not significant different (*p* > 0.05).

#### *2.4. Oxygen Permeability and Water Vapor Permeability*

The OPs of packaging materials are of considerable importance in food preservation. Low OPs indicate excellent oxygen barrier properties. Table 2 shows the OP and WVP values for all films. The OP of the pure PVA film is 0.12 ± 0.04 cm<sup>2</sup> <sup>m</sup>−<sup>2</sup> atm−<sup>1</sup> day−<sup>1</sup> MPa. The addition of CH reduces the abilities of the blend films to act as oxygen barriers. The OPs of PVA/CH-2 and PVA/CH-2.5 increase slightly, from 0.15 ± 0.07 to 0.16 ± 0.08 cm<sup>2</sup> <sup>m</sup>−<sup>2</sup> atm−<sup>1</sup> day−<sup>1</sup> MPa. This can be attributed to the lower crystallinities of the blended films with low CH concentrations [26]. The OPs increase

rapidly with the CH content when the latter is over 30 wt %, resulting in blend films with poorer oxygen barrier properties.


**Table 2.** Oxygen permeability and water vapor permeation of different films.

Notes: Values with the same letter are not statistically different, according to Duncan's Multiple Range Test at *p* < 0.05; a, b, means with the same letter in the same column are not significant different (*p* > 0.05).

The WVPs of PVA/CH-2 and PVA/CH-2.5 films are 12.43 ± 3.72, and 14.93 ± 4.09 g cm−<sup>1</sup> <sup>s</sup>−<sup>1</sup> Pa<sup>−</sup>1, respectively. This demonstrates that the WVPs of the composite films are higher than that of the pure PVA film (10.11 ± 2.14 g cm−<sup>1</sup> <sup>s</sup>−<sup>1</sup> Pa<sup>−</sup>1); however, the differences are not significant. PVA/CH-3 film exhibits significantly higher WVPs (*p* < 0.05) than PVA. Those results indicated that the interaction between PVA and CH can increase the water vapor barrier properties. Furthermore, increasing the CH content clearly increases the WVPs of the films. The hydrophilic nature of CH favors the transport of water molecules through the film [27], and composite films with higher CH contents exhibit lower crystallinities [28].
