*3.10. Surface Morphology*

The surface morphologies of pure starch film and starch nanocomposite films with 5% NC are shown in Figure 7 at 10,000× magnification. The surface of the pure starch film was smooth with homogeneous morphology and compact structure because of the plasticization effect of glycerol [60]. Compared to the pure starch film, the incorporation of AH-NC increased the surface roughness, which is attributed to the agglomeration of AH-NC in the starch matrix [22]; however, it was difficult to observe the individual fibers in the film because of the small size and bundle structure of AH-NC in the starch matrix. Well-dispersed NC particles were observed in the starch/HH-NC and starch/US-NC films; this suggests that there was strong interfacial adhesion or good compatibility between the HH-NC or US-NC and the starch matrix, which could be attributed to the size and zeta potential of the NC, the chemical similarities of starch and NC, and hydrogen bonding between the two components [22]. Similar results have been reported for starch nanocomposite films containing pineapple leaf cellulose nanofibers [61]. The homogeneous distribution of NC in starch films could greatly improve their tensile strength and water vapor barrier properties.

**Figure 7.** Surface morphology of (**a**) pure starch, (**b**) starch/AH-NC, (**c**) starch/HH-NC, and (**d**) starch/US-NC films.
