## 3 Results and Discussion  
### Composition and Structure of CHT Films  
#### 3.1.1 FTIR Spectroscopy  
FTIR spectra of films of pristine chitosan (CHT), pristine sodium hyalurone (HNa), and mixtures of CHT/HNa, CHT/HNa-PEGDE are shown in Figure 1(a) (15%HNa) and Figure 1(b) (30%HNa), (for clarity high quality FTIR individual spectra is reported in supplementary information, S1). Pristine chitosan (Figure 1a) showed the typical broad absorption between 3677 cm\({}^{-1}\) and 3000 cm\({}^{-1}\) (peaks at 3350 cm\({}^{-1}\) and 3289 cm\({}^{-1}\)), which correlate with the O-H (from chitosan and water) and N-H vibration in N-H\({}_{2}\) groups. At 2921 cm\({}^{-1}\) and 2852 cm\({}^{-1}\) absorptions corresponding to stretching C-H in C-H\({}_{2}\) and C-H\({}_{3}\) were observed. The absorption band at 1647 cm\({}^{-1}\) was related to C=O vibration in the amide I. The band at 1558 cm\({}^{-1}\) was attributed to C-N and N-H\({}_{2}\) bending in amide II region. At 1419 cm\({}^{-1}\), bands associated to C-H\({}_{2}\) bending were observed. At 1314 cm\({}^{-1}\), the amide III appeared. Skeletal vibrations characteristic of the chitosan structure appeared at 1068 cm\({}^{-1}\) and 1050 cm\({}^{-1}\) (pyranose ring). Sodium Hyaluronate (Figure 1a) showed a broad band with a maximum at 3307 cm\({}^{-1}\) for films and 3294 cm\({}^{-1}\) for the powder, which were assigned to N-H and O-H stretch absorptions. It was also observed a peak at 2905 cm\({}^{-1}\) and 2868 cm\({}^{-1}\) related to CH\({}_{2}\). Absorptions at 1734 cm\({}^{-1}\) was assigned to carboxylic acid/carboxylate while at 1606 cm\({}^{-1}\) Amide I was observed, and Amide II observed as a small shoulder at 1557 cm\({}^{-1}\). Bands at 1408, 1374, 1348, 1295, 1252 can be assigned to N-H and C-H bending and C-N stretching. Two peaks at 1078 cm\({}^{-1}\) and 1033 cm\({}^{-1}\) were attributed to C-O-C stretch as previously reported [27; 29]. Finally, absorptions at 944 cm\({}^{-1}\) and 841 cm\({}^{-1}\) were assigned to the pyranose ring. Chitosan and hyaluronic acid are polysaccharides, so both spectra were similar, however when blended might exhibit some differences. The cationic nature of chitosan allows it to interact with negatively charged polymers to form a polyelectrolyte complex (PEC) through ionic bonds. Ionic interactions between chitosan and hyaluronic acid can occur between the amine groups of chitosan and the carboxyl groups of hyaluronic acid. As expected, when the amount of HNa increased in the uncrosslinked blend, peaks from HNa tend to be more evident. For instance, a single 3300 cm\({}^{-1}\) band was observed for NH and OH stretching absorptions but the bands at 2870 (CH stretching), 1552 (amide II), and 1370 cm\({}^{-1}\) increased in agreement with previous works [30]. The FTIR spectra of uncrosslinked CHT/HNa 15% (Figure 1a) films showed a small band, which can be related to the carboxylate of HNa located at 1740 cm\({}^{-1}\) but it was not observed in CHT/HNa 30%. The infrared spectra of the CHT/HNa/PEGE crosslinked films are depicted in Figure 1a,b. When the CHT/HNa was crosslinked with PEGDE a low concentration it showed a single broad band peaking at 3340 cm\({}^{-1}\). As HNa increased the band at 2871 cm\({}^{-1}\) increased while the absorption at 2919 cm\({}^{-1}\) was reduced. Amide I appeared at 1617 cm\({}^{-1}\), i.e., between CHT and HNa while Amide II was displaced to 1570 cm\({}^{-1}\). Finally, the band at 1375 cm\({}^{-1}\) increased as HNa increased. The main effect of crosslinking with PEGDE was two-fold. First, at both HNa concentrations, Amide II was reduced whereas Amide I increased. Bands located at 1552 cm\({}^{-1}\) for PEGDE2 and PEGDE3 crosslinked samples can also be assigned to the deformation vibration of the amine. Therefore, the decrease in the intensity of this band, when the concentration of PEGDE increases, occurs since the epoxy groups of PEGDE react with the amino groups of chitosan and with the carboxyl groups of HNa molecules, decreasing the concentration of amino groups.