## 3 Results
### Microstructure of the Films  
Figure 4 shows the FESEM micrographs of the cross-sections of the TPS-based films incorporating different CMF concentrations (1, 3, and 5 wt%) obtained by cryo-fracture and their corresponding samples using DH-modified starch, that is, TPSDH. The TPS control film samples revealed a smooth and homogeneous cross-section surface pattern, indicating a good starch thermoplasticization, as also reported by other authors for starch films obtained in similar conditions [50; 51]. In contrast, the TPSDH films exhibited remaining starch granules, which suggests changes in the starch thermoplasticization behavior after the DH treatment. In this context, [PERSON] et al. [31] reported that the treatment of native starch by DH provokes partial damages in the semi-crystalline and crystalline regions of the granules, which prevent the granule swelling and delays its gelatinization. Furthermore, the DH treatment can oxidize and convert the -OH groups present in the starch chains to carbonyl (C=O) and carboxyl (-COOH) groups, which alters the gelatinization profile [52]. A progressive increase in the number of non-gelatinized starch granules was also observed in the cross-sections of the films of non-modified starch when the CMF concentration increased. However, this number increased in the samples prepared with DH-modified starch. The starch thermoplasticization occurs during heating, in the extruder or melt-mixing equipment, in the presence of a plasticizer, such as glycerol, at high temperatures (130 \({}^{\circ}\)C) and pressures [21]. The microstructural features of the microcomposite films revealed that the CMF incorporation interfered the starch gelatinization with glycerol. This could be explained by the competitive interaction of the plasticizer with both CMFs and starch through -H intermolecular bonds with the cellulose chains, decreasing their availability and subsequently provoking gelatinization of the granular structure of starch. This effect was mainly observed in the TPSDH samples, where the thermal treatment involved changes in its granular structure that already limits its gelatinization. Figure 3: Cumulative distributions of cellulose microfiber (CMF) length (\(l\)) (**a**) and width (\(w\)) (**b**) obtained from image analyses of optical microscopy and field emission scanning electron microscopy (FESEM), respectively. A minimum of 60 measurements for each parameter was performed. Figure 4, which shows the fracture surfaces of film samples, revealed that the CMFs incorporated into the films exhibited good compatibility with both types of TPS matrices, that is, non-modified and DH-modified starch. This observation can be deduced from the good interfacial adhesion observed between the fibers and starch matrix due to the absence of the interface between the fibers. The results are shown in Figure 5, which shows the fracture surfaces of the film samples, revealed that the CMFs incorporated into the films exhibited good compatibility with both types of TPS matrices, that is, non-modified and DH-modified starch. This observation can be deduced from the good interfacial adhesion observed between the fibers and starch matrix due to the absence of the interface between the fibers. Figure 4: Field emission scanning electron microscope (FESEM) micrographs of the fracture surfaces of thermoplastic starch (TPS) and TPS modified by dry heating (TPSDH) films with contents of cellulose microfibers (CMFs) of 1 wt% (TPS1, TPSDH1), 3 wt% (TPS3, TPSDH3), and 5 wt% (TPS5, TPSDH5). Images were taken at 300\(\times\) with insets at 2000\(\times\). of gap between the fiber surface and the TPS matrix. Nevertheless, the film cryo-fracture does not seem to promote the breakage of the fibers, but their separation from the matrix, resulting in hollows that arise from the outgoing fiber during the fracture of the film samples. Similar findings were observed by [PERSON] et al. [40] and [PERSON] et al. [28] in TPS films reinforced with CMFs from rice husk.