*3.4. FTIR*

*3.4. FTIR*  Figure 3 shows the spectra of the films stored for different durations. After 16 months of storage, the FTIR-ATR analysis showed modifications in the absorption in the region of 1300 cm−1 to 900 cm−1 for all materials (F127 0%, F127 1%, F127 3%, and F127 5%); the bands in this region are sensitive to the gelation of starch because of their association with C–O stretching of the ring, linkages (C–O–C), and COH groups. Notably, the band at 1017 cm−<sup>1</sup> is reported as sensitive to amorphous starch, which is constant in the spectra after 16 months of storage [21–23]. Casu and Reggiani [24] reported that the band at 3300 cm−1, attenuated after 16 months of storage, can be assigned to the O-H stretching of the groups in amorphous amylose. Moreover, the molecule of water showed that the absorption bands at 3300 and 1646 cm−1 are associated with OH stretching and deformation vibrations, respectively [24]. In addition, the absorption band at 2883 cm−1 was associated with Figure 3 shows the spectra of the films stored for different durations. After 16 months of storage, the FTIR-ATR analysis showed modifications in the absorption in the region of 1300 cm−<sup>1</sup> to 900 cm−<sup>1</sup> for all materials (F127 0%, F127 1%, F127 3%, and F127 5%); the bands in this region are sensitive to the gelation of starch because of their association with C–O stretching of the ring, linkages (C–O–C), and COH groups. Notably, the band at 1017 cm−<sup>1</sup> is reported as sensitive to amorphous starch, which is constant in the spectra after 16 months of storage [21–23]. Casu and Reggiani [24] reported that the band at 3300 cm−<sup>1</sup> , attenuated after 16 months of storage, can be assigned to the O-H stretching of the groups in amorphous amylose. Moreover, the molecule of water showed that the absorption bands at 3300 and 1646 cm−<sup>1</sup> are associated with OH stretching and deformation vibrations, respectively [24]. In addition, the absorption band at 2883 cm−<sup>1</sup> was associated with C-H bond stretching, which did not show a modification in the four materials after 16 months [25]. Finally, another important phenomenon identified in the spectra of all films after storage was the disappearing of the absorption band at 1150 cm−<sup>1</sup> , which was associated with C-O stretching of C–O–C in glycosidic linkage; this result suggests the depolymerization of the starch molecule [26].

C-H bond stretching, which did not show a modification in the four materials after 16 months [25]. Finally, another important phenomenon identified in the spectra of all films after storage was the disappearing of the absorption band at 1150 cm−1, which was associated with C-O stretching of C–O–C in glycosidic linkage; this result suggests the depoly-

**Figure 3.** FTIR spectra of biodegradable films of corn starch-chitosan with pluronic F127 at ratios of **Figure 3.** FTIR spectra of biodegradable films of corn starch-chitosan with pluronic F127 at ratios of 0%, 1%, 3%, and 5% stored for 0 and 16 months.

#### 0%, 1%, 3%, and 5% stored for 0 and 16 months. *3.5. X-ray Diffraction*

merization of the starch molecule [26].

*3.5. X-ray Diffraction*  To determine if there was retrogradation or modification in the conformation of the structural matrix of the biodegradable films, Figure 4 shows the XRD patterns of biodegradable films F127 0%, F127 1%, F127 3%, and F127 5%. The patterns show that there was a higher atomic ordering in the biodegradable film in F127 0% after 16 months of storage. Peaks at 16°, 19.65°, and 22° were defined after storage; these peaks were related to the crystalline starch, which indicates the retrogradation of starch in this film. However, in films F127 1%, F127 3%, and F127 5%, there was no increase in the atomic ordering after 16 months of storage, as peaks related to starch could not be identified. In F127 3% and F127 5% films, there were two peaks at 19° and 23°, which were less intense after 16 months; these peaks were associated with pluronic 127 [11]. The X-ray patterns indicate that pluronic F127 avoided retrogradation after the storage period of 16 months. Mina Hernandez [27] reported a significant increase in the rearrangement of the polymer chains of mixtures of thermoplastic starch and polycaprolactone during short storage periods (5 and 26 days). Furthermore, the author found that the retrogradation of starch-based polymers occurs more rapidly when the materials are stored in conditions of high RH, which To determine if there was retrogradation or modification in the conformation of the structural matrix of the biodegradable films, Figure 4 shows the XRD patterns of biodegradable films F127 0%, F127 1%, F127 3%, and F127 5%. The patterns show that there was a higher atomic ordering in the biodegradable film in F127 0% after 16 months of storage. Peaks at 16◦ , 19.65◦ , and 22◦ were defined after storage; these peaks were related to the crystalline starch, which indicates the retrogradation of starch in this film. However, in films F127 1%, F127 3%, and F127 5%, there was no increase in the atomic ordering after 16 months of storage, as peaks related to starch could not be identified. In F127 3% and F127 5% films, there were two peaks at 19◦ and 23◦ , which were less intense after 16 months; these peaks were associated with pluronic 127 [11]. The X-ray patterns indicate that pluronic F127 avoided retrogradation after the storage period of 16 months. Mina Hernandez [27] reported a significant increase in the rearrangement of the polymer chains of mixtures of thermoplastic starch and polycaprolactone during short storage periods (5 and 26 days). Furthermore, the author found that the retrogradation of starch-based polymers occurs more rapidly when the materials are stored in conditions of high RH, which also negatively impacts their mechanical performance.

#### also negatively impacts their mechanical performance. *3.6. Thermal Behavior by DSC*

The results of the thermal behavior of the biodegradable films are presented in Table 2. DSC experiments have shown that the melting temperature (*Tm*) of the neat corn starch film decreases with a significant increase in enthalpy (∆*Hm*), indicating a gain in the ordering of the polymeric structures involved. In contrast, chitosan films exhibited an increase of 15 ◦C after 16 months of storage. The enthalpy values for this biopolymer decreased as a function of the evaluated time. Notably, the mixture of starch and chitosan (F127 0%) exhibited a thermal behavior in which starch transitions predominated. The incorporation of the poloxamer in the starch-chitosan matrix ensured gelatinization, with the absence of the band at ~62 ◦C (*TP*, gelatinization peak). A decrease was observed in the melting temperature (56.1 ◦C) corresponding to neat F127. The increase in poloxamer

content resulted in an increase in the melting temperature and a decrease in enthalpy after 16 months, with respect to its nonfunctionalized homologous (F127 0%). This allowed for demonstrating the plasticizing effect generated by the poloxamer due to the destabilization of the crystalline regions. The fusion of intra- and inter-molecular double helices and the partial recovery of the crystalline structure of amylopectin (Figure S1, Supplementary Material) can be observed [28]. The crystals formed during the starch retrogradation process are less orderly and homogeneous than native starch; this is reflected in their lower melting temperatures. This effect is similar to that observed in other starches, such as sago [29]. The DSC parameters of starch in biodegradable films are summarized in Table 2, and this result is in accordance with that obtained from the XRD analysis. *Polymers* **2021**, *13*, x FOR PEER REVIEW 7 of 10

**Figure 4.** X-ray diffraction patterns of biodegradable films of corn starch-chitosan with pluronic F127 at ratios of 0%, 1%, 3%, and 5% stored for 0 and 16 months. **Figure 4.** X-ray diffraction patterns of biodegradable films of corn starch-chitosan with pluronic F127 at ratios of 0%, 1%, 3%, and 5% stored for 0 and 16 months.

*3.6. Thermal Behavior by DSC*  The results of the thermal behavior of the biodegradable films are presented in Table **Table 2.** DSC parameters of biodegradable films of corn starch-chitosan with pluronic F127 at ratios of 0%, 1%, 3%, and 5% stored for 0 and 16 months.


respect to its nonfunctionalized homologous (F127 0%). This allowed for demonstrating – negligible.

**Film Sample** 

#### the plasticizing effect generated by the poloxamer due to the destabilization of the crystalline regions. The fusion of intra- and inter-molecular double helices and the partial re-*3.7. Water Solubility (WS)*

covery of the crystalline structure of amylopectin (Figure S1, Supplementary Material) can be observed [28]. The crystals formed during the starch retrogradation process are less orderly and homogeneous than native starch; this is reflected in their lower melting temperatures. This effect is similar to that observed in other starches, such as sago [29]. The DSC parameters of starch in biodegradable films are summarized in Table 2, and this result is in accordance with that obtained from the XRD analysis. **Table 2.** DSC parameters of biodegradable films of corn starch-chitosan with pluronic F127 at ratios of 0%, 1%, 3%, and 5% stored for 0 and 16 months. The WS of biodegradable materials is a crucial parameter because it is important that they can decompose in both terrestrial and aquatic environments after being discarded, where they can potentially contaminate the environment on several occasions. A significant increase in the water solubility capacity of the starch-chitosan-based materials was observed (Table 1) at all poloxamer contents (1%, 3%, and 5%) after 16 months of storage with water solubility values of 18%, 24%, and 57%, respectively, compared to the materials evaluated at time zero [11]. Shaker, Elbadawy [30] found that by using different types of poloxamers, such as pluronic F127 and F68, the degree of solubility and the dissolution rate of drugs improved, thus facilitating their usage in pharmaceutical production.

> **Month 16**

Corn starch 63.13 -- -- 141.03 117.75 6.0 18.7

Chitosan -- -- -- 104.68 119.47 28.0 19.6

F127 0% 62.06 -- -- 138.96 106.63 6.4 10.3

**Tp (°C) Tm1 (°C) Tm2 (°C) ∆Hm2 (J g−1)** 

**Month 0** 

**Month 16** 

**Month 0** 

**Month 16** 

F127 56.10 56.10

**Month 0** 

**Month 0** 
