*3.9. Water Vapor Barrier Properties*

The water vapor permeability (WVP) of a film is one of the most important properties for food packaging applications. Because of its hydrophilic nature, starch-based films usually have high WVP and thus poor water vapor barrier properties, which significantly restricts its application in food packaging. As shown in Table 3, the water vapor barrier properties of pure starch films could be improved by the incorporation of NC. The WVP of the pure starch film, 1.65 <sup>×</sup> <sup>10</sup>−<sup>12</sup> <sup>g</sup>·cm·cm−2·s−1·Pa<sup>−</sup>1, decreased by approximately 14%, 22%, and 18% (*p* < 0.05) when 5% of AH-NC, HH-NC, and US-NC was incorporated into the starch matrix, respectively. This decrease in WVP as the result of the incorporation of NC agrees with previous reports [22,59].

The NC in the starch nanocomposite films functions as an impermeable barrier against WVP because strong hydrogen bonding interactions reduce the diffusion coefficient of the films by increasing the diffusion path for water vapor through the film [42]. Unexpectedly, the lower the aspect ratio of the NC in the starch matrix, the lower the WVP of the resultant starch nanocomposite films. This result might be ascribed to the agglomeration of high aspect ratio NC in the starch matrix, which creates diffusion pathways for water vapor transport within the film and thus facilitates WVP. Therefore, the uniform distribution of NC in the starch matrix played a more significant role than high aspect ratio in improving the water vapor barrier properties of starch nanocomposite films.
