*3.8. Mechanical Properties*

The tensile strength (TS), elongation at break (EAB), and elastic modulus (EM) of starch films with and without the addition of 5% NC are shown in Table 3. TS is the measurement of maximum strength of a film against applied tensile stress, EAB represents the ability of a film to stretch, and EM indicates the rigidity of a film. The TS of the pure starch film was 6.32 MPa. The TS of the starch/NC films was considerably greater than that of the pure starch film. The TSs of the starch nanocomposite films prepared from AH-NC, HH-NC, and US-NC were 9.35 MPa, 11.74 MPa, and 10.75 MPa, respectively, which correspond to 1.47, 1.85, and 1.70 times the TS of pure starch film. Li et al. [22] reported that NC can improve the tensile strength of starch films when NC is uniformly distributed in the starch matrix. The strong adhesion at the starch/NC interface and the creation of a rigid NC percolating network within the starch matrix facilitate efficient stress transfer from the soft starch matrix to the rigid NC and thus improve the strength of the starch nanocomposite [53]. Moreover, cationic modification is a hydrophilic modification that introduces hydrophilic quaternary ammonium salt groups; therefore, the cationic NC has excellent compatibility and strong interactions with starch because hydrogen bonds can form between them, which greatly increases the TS of the nanocomposite films.


**Table 3.** Mechanical properties and water vapor permeability of starch/NC composite films.

\* Different lowercase letters in the same column indicate a statistically significant difference (*p* < 0.05).

The reinforcing effect of NC on the starch films is also different for the different preparation methods. This is potentially due to the different aspect ratios of the three types of NC, which could affect the dispersion of NC in the starch matrix [6]. Since the AH-NC has the highest aspect ratio, hydrogen bonding might prevent it from being uniformly dispersed throughout the starch matrix [28]. Conversely, HH-NC has a low aspect ratio and thus is expected to be well-dispersed in the starch film. This conjecture was verified using the SEM results of the films, which are discussed later in this section.

The effects of preparation method of NC on the EAB of starch/NC composite films were shown in Table 3. As expected, the incorporation of NC into the starch matrix substantially decreased the EAB and the results were in agreement with previous studies [54,55]. The decrease in EAB is attributed to the geometry and rigid nature of NC as well as the formation of a stiff NC network linked by hydrogen bonds and entanglements. Moreover, the addition of NC could hinder the plasticizing efficacy of glycerol and decrease the mobility of starch chains, which would result in brittle nanocomposite films [56,57]. Effects similar to those observed for NC have been reported for other biopolymers, such as whey protein isolate and agar [42,58]. Finally, EM, which indicates the rigidity of a film, increased significantly (*p* < 0.05) when 5% NC was added to the starch film.
