*2.5. Morphology by SEM*

The morphology of the top-surface of the films was observed to determine if there was any change in the texture with an increase in the content of nanocellulose. This was performed with a SM-510 scanning electron microscope (TOPCON, Tokyo, Japan) at 5 kV at a working distance of 18 mm. Composite films were gold-palladium coated for 90 s on a Denton Vacuum model DESK II sputter (Moorestown, NJ, USA).

### *2.6. Thermal Properties*

The effect of incorporating the CNC on the thermal resistance of the starch–chitosan films was determined using a DSC 2500 Discovery differential scanning calorimeter (TA Instruments, New Castle, DE, USA) in a temperature range of 0 to 250 ◦C, in a nitrogen atmosphere (50 cm<sup>3</sup> min−<sup>1</sup> ).

#### *2.7. X-ray Diffraction*

To determine if the presence and increase of the nanomaterial content modified the structural arrangement of the polymeric matrix, the latter was determined using a Siemens D500 powder diffractometer (Munich, Germany), according to the methodology suggested by Fonseca-García, Jiménez-Regalado [29].

#### *2.8. Water Vapor Adsorption Isotherms*

The water vapor absorption capacity of the materials was evaluated at room temperature (≈25 ◦C) according to the methodology proposed by Jiménez-Regalado, Caicedo [13], in a relative humidity (RH) range of 11 to 90% (*a<sup>w</sup>* = 0.1 to 0.9). The saline solutions were prepared according to the methodology of Aguirre-Loredo, Rodriguez-Hernandez [30]. The experimental data were fitted to the Guggenheim–Anderson–de Boer (GAB) mathematical model (Equation (1)). In Equation (1), *X* is the moisture content absorbed by the material, *Xm* is the content of water that adheres directly to the material, forming the first layer of molecules, also called monolayer; *C* is a constant related to the sorption in the monolayer; and *K* is a constant associated with the sorption of water molecules in the layers after the monolayer, which forms multilayers [30].

$$X = \frac{X\_m \text{CKa}\_w}{(1 - Ka\_w)(1 - Ka\_w + \text{CKa}\_w)} \tag{1}$$

#### *2.9. Mechanical Properties*

The mechanical properties of tensile strength and elongation at break of the materials were determined in a texture analyzer (TA.XT Express Enhanced, Stable Micro Systems, Godalming, UK) equipped with clamps. The films were cut into strips 1 cm wide by 6 cm long, with a gap of 3 cm between clamps. The equipment operated at a speed of 1 mm·s −1 [13,31]. Results are reported as the mean and standard deviation of at least ten replicates.
