*3.3. Surface Morphology*

Visually, there was no change in the appearance of the starch–chitosan films with incorporated nanomaterials, nor with the increase in their concentration (Figure 2a, only one photography of the biodegradable materials is presented because those obtained with the other formulations were visually the same). However, when observed with a scanning electron microscope (SEM), a change in their morphology was observed with the addition of CNC. The top surface morphology of the films is shown in Figure 2.

**Figure 2.** (**a**) Photograph and top surface morphology of corn starch–chitosan films with cellulose nanocrystals (CNC) at (**b**) 0%, (**c**) 0.5%, (**d**) 2.5%, and (**e**) 10% (*w*/*w*). **Figure 2.** (**a**) Photograph and top surface morphology of corn starch–chitosan films with cellulose nanocrystals (CNC) at (**b**) 0%, (**c**) 0.5%, (**d**) 2.5%, and (**e**) 10% (*w*/*w*).

The control film (Figure 2b), that is, the film containing no nanomaterials, presented a more homogeneous appearance. An increase in surface roughness or heterogeneity was observed in the presence of CNC. The surface appeared to be better structured and less rough with low concentrations of CNC (0.5 and 2.5% *w*/*w*), as seen in Figure 2c,d. In contrast, with higher CNC content, the films had a more irregular surface (Figure 2e), which may be a consequence of the reduction in the dispersibility and an aggregation of nanomaterials [6,19,20,41]. This change in the morphology of the composite films with the addition of CNC was similar to that reported by Chen, Shi [20] for cassava starch films with The control film (Figure 2b), that is, the film containing no nanomaterials, presented a more homogeneous appearance. An increase in surface roughness or heterogeneity was observed in the presence of CNC. The surface appeared to be better structured and less rough with low concentrations of CNC (0.5 and 2.5% *w*/*w*), as seen in Figure 2c,d. In contrast, with higher CNC content, the films had a more irregular surface (Figure 2e), which may be a consequence of the reduction in the dispersibility and an aggregation of nanomaterials [6,19,20,41]. This change in the morphology of the composite films with the addition of CNC was similar to that reported by Chen, Shi [20] for cassava starch films with cellulose crystals in micro- and nano-sizes.

Figure 3a,b show thermogravimetric analysis curves and their respective derivatives that

cellulose crystals in micro- and nano-sizes.

*3.4. Thermal Properties* 

#### *3.4. Thermal Properties*

Figure 3a,b show thermogravimetric analysis curves and their respective derivatives that detail the behavior of biopolymeric samples with varying contents, i.e., between 0 and 10%, of CNC. In general, the biopolymers had similar degradation temperatures (~300 ◦C); however, the plasticizer content and the addition of particles have been shown to significantly influence the thermal behavior at processing temperatures between 120 ◦C and 200 ◦C. *Polymers* **2022**, *14*, x FOR PEER REVIEW 9 of 17 the plasticizer content and the addition of particles have been shown to significantly influence the thermal behavior at processing temperatures between 120 °C and 200 °C.

**Figure 3.** (**a**) TGA, (**b**) DTG curves, (**c**) DSC thermogram of biopolymer (corn starch–chitosan) and nanocomposite films incorporated with different concentrations of CNC (0 to 10% *w*/*w*). **Figure 3.** (**a**) TGA, (**b**) DTG curves, (**c**) DSC thermogram of biopolymer (corn starch–chitosan) and nanocomposite films incorporated with different concentrations of CNC (0 to 10% *w*/*w*).

In this study, the plasticizer content was constant. As such, the addition of nanoparticles maintained stability and, in some cases, improved it. The first zone of mass loss corresponded to the evaporation of water. Later, in the extended zone between 150 °C and 250 °C, a more pronounced negative slope (abrupt fall) of the control sample was observed. This second area represents the outlet of the plasticizer. This behavior could be explained by the interaction between the CNC and the plasticizer [13,20], or by the barrier effect of CNC that caused a delay in the release of the plasticizer [16]. Likewise, the degradation temperatures in the range of 255 °C and 365 °C (Figure 3b) presented bifurcation due to the degradation of each polymer. Similar degradation processes and degradation temperature ranges have been reported for other biomaterials based on starch and chitosan with cellulose nanoparticles [20,39]. The effect on the CNC sample was 0.5% (containing a low percentage of CNC). This result is consistent with other studies that described an adequate interaction through the formation of hydrogen bonds between the hydroxyl groups of cellulose and starch [40] and between the hydroxyl groups and the amino groups of chitosan [34]. In this study, the plasticizer content was constant. As such, the addition of nanoparticles maintained stability and, in some cases, improved it. The first zone of mass loss corresponded to the evaporation of water. Later, in the extended zone between 150 ◦C and 250 ◦C, a more pronounced negative slope (abrupt fall) of the control sample was observed. This second area represents the outlet of the plasticizer. This behavior could be explained by the interaction between the CNC and the plasticizer [13,20], or by the barrier effect of CNC that caused a delay in the release of the plasticizer [16]. Likewise, the degradation temperatures in the range of 255 ◦C and 365 ◦C (Figure 3b) presented bifurcation due to the degradation of each polymer. Similar degradation processes and degradation temperature ranges have been reported for other biomaterials based on starch and chitosan with cellulose nanoparticles [20,39]. The effect on the CNC sample was 0.5% (containing a low percentage of CNC). This result is consistent with other studies that described an adequate interaction through the formation of hydrogen bonds between the hydroxyl groups of cellulose and starch [40] and between the hydroxyl groups and the amino groups of chitosan [34].

The DSC thermogram (Figure 3c) shows curves with an amplified band between ~50 °C and ~160 °C, characteristic of partially gelatinized and plasticized starch materials. The The DSC thermogram (Figure 3c) shows curves with an amplified band between ~50 ◦C and ~160 ◦C, characteristic of partially gelatinized and plasticized starch materi-

transitions found in pure polymers are reported in a previous work showing Tg for starch and chitosan of around 57 °C and 112 °C, respectively [13]. In the same way, a chitosan-

als. The transitions found in pure polymers are reported in a previous work showing Tg for starch and chitosan of around 57 ◦C and 112 ◦C, respectively [13]. In the same way, a chitosan-starch blend in the development of films by casting, which implies the prior mixing of film-forming solutions, revealed optimal interaction, resulting in a homogeneous morphology. Thus, a calorimetric analysis showed a decrease in the enthalpy of gelatinization (Table 2) when the destructuring of the starch granules was promoted with acid solutions (acetic acid was used in the preparation of the chitosan film-forming solution) [41]. The values of the glass transition temperature (Tg) are of low intensity (Table 2); however, the samples increased with the increase in the concentration of CNC due to the restriction in the mobility in the amorphous regions of the starch that generated intermolecular interactions between the nanocrystals and the matrix. On the other hand, the incorporation of CNC showed an irregular trend in the values of the melting temperatures due to possible variations in the crystalline domains that each concentration of CNC induced on the polymeric matrix. In the case of 0.5% and 5% (*w*/*w*) CNC, the fusion of the starch crystallites was observed around 155 ◦C; some authors have reported higher values (at 160 ◦C) [42]. The shift of this thermal transition slightly towards lower temperatures was probably due to the plasticizing effect of the CNCs on the matrix, which means that lower temperatures are needed to melt these types of nanocrystals.

**Table 2.** Temperature data (expressed in ◦C) related to thermal analysis of nanocomposite films incorporated with CNC (0 to 10% *w*/*w*).


(--) It was not possible to determine.
