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.