Fourier transform infrared analysis.
Chemical composition of the four hydrogels was analyzed by FTIR (Figure 4 ). As a result, some differences among each spectrum were observed and the main peaks are identified in Table 1 . Three bands are consistently present in the four spectra: those related to C-O and C-O-C stretching: 1017-1022 cm-1 and 1079-1082 cm-1, respectively; and the one about 3300 cm-1, which is usually associated to the presence of -OH group (Henao, 2018). These bands have been previously observed in FTIR spectra of chitosan (Brugnerotto, 2001), PVA (Peng, 2019), and starch (Xu, 2005).
In the case of the band at 3300 cm-1, it is noticeably increased in the presence of chitosan. This effect can be a consequence of -OH groups reinforcement, but also could be related to the impact of N-H stretching from chitosan molecules (Brugnerotto, 2001). This band shows differences between physically and chemically obtained hydrogels, suggesting a rearrangement of -OH and -NH2 groups. This is comprehensible, since the chemical mechanism for hydrogel synthesis involves the presence of new molecular bonds that do not exist in physical hydrogels (Gulrez, 2011). The stretching of C=O bonds in amides showed signals at 1550 cm-1 and 1639 cm-1. Both were observable in the presence of chitosan, as a consequence of glucosamine units in chitosan molecules (Henao, 2018).
Interestingly, spectra from starch-PVA and starch hydrogel were quite different, suggesting strong interactions between both polymers that modified starch spectrum. The -CH2 symmetric stretching at 2359 cm-1 is an identifying feature of starch-PVA hydrogel, due to a PVA contribution (Ghabboun, 2012). These differences in the FTIR spectra are associated with the structural variations observed between both hydrogels and will surely be reflected on the properties of each one.