Analysis about the Behavior and Modeling of pH-Sensitive Hydrogels with Different Ratios of Chitosan and Polyvinylpyrrolidone

Abstract

Smart material can be defined as a material that can dock or convert energy between physical domains or as a material that can generate a response, in their characteristics, properties or geometries, when submitted to an external stimulus, for example, to heat, water presence, light, etc. In this paper, the second definition will be approached. Hydrogels are crosslinked materials that can absorb a big amount of water. They generally can be considered as smart materials once they exhibit sensibility to external stimuli like to pH variation, as will be approached in this paper. Thus, chitosan/polyvinylpyrrolidone hydrogels of three different ratios between these two polymers (1:1, 7:3 and 3:7) were synthesized and putted in aqueous solution with different pHs. The pH was adjusted adding drops of NaOH and HCl, slowly. After the collection of results and in order to understand the phenomena in a visual way, models of the molecules were also elaborated using the Avogadro software. Therefore, it was possible to realize that the greater the ratio of chitosan in the hydrogel, the greater its sensitivity to pH. Such characteristic is associated with the amino (-NH 2 ) groups in it structure, which are capable of protonating and deprotonating (depending of the pH), generating charges under the chemical structure of the material, which will expand its volume in order to minimize the repulsion between charges. In addition, it was also noted that the hydrogel expansion is inversely proportional to the pH increase. By practical tests, it was possible to conclude that chitosan/PVP hydrogel with ratio 7:3 is the most interesting once it presented a greater quantity of chitosan in its composition, what implied in more rigidity than the others and greater ease of handling, resulting in more reliable results. This hydrogel also showed higher sensitivity to pH.