Some of the physical aspects of pristine, sulfonated chitosan and PEC films are shown in Table 1. Each film was measured using a caliper of 0.005 mm precision. All films were transparent and homogeneous with a smooth surface, although some opacity occurred when the Sapitinib content was increased, indicating possible fiber formation due to the interaction of opposite charges on the groups of the chitosan and carrageenan. Sulfonated chitosan films presented a yellowish color, which was produced by the addition of FFSA, probably due to remaining imino bonds that were not reduced by sodium borohydride, providing preliminary evidence that the reaction had occurred. The thickness of sulfonated chitosan increased, signifying that sulfonate groups were increased in the void space between the chitosan chains, due to the charge effects. The thickness of PECs decreased due to a combination of the polymer chain, with the exclusion of water and the consequent reduction of the volume occupied by the polymers in the film. In our study, we used a higher acid concentration than that described in other reports for soluble PEC chitosan/kappa-carrageenan, which could lead to a higher protonation of chitosan and a protonation of the sulfonate anionic groups of carrageenan, strengthening the electrostatic repulsion, avoiding their chain interaction and hence, preventing precipitation .