INVESTIGATION OF HYDROGEL WATER IN POLYELECTROLYTE GELS USING DIFFERENTIAL SCANNING CALORIMETRY

Characterization of the water structure in hydrogels is of the utmost importance in understanding their dynamic and equilibrium swelling behaviour as well as in analysing solute transport and other diffusive properties of such systems. Polymer-water interactions coupled with the physical property changes of the water associated with the hydrogel structure can determine the permeability characteristics of solutes through these hydrogels. The water structures inside ionic copolymeric networks of poly(2-hydroxyethyl methacrylate-co-methacrylic acid) and poly(2-hydroxyethyl methacrylate-co-acrylic acid) were investigated as a function of the pH and ionic strength of the swelling medium and the type of counterion in the swelling medium using differential scanning calorimetry. The free water content increased with an increase in pH and a decrease in ionic strength. The bound water content was found to be independent of the pH of the external swelling medium. The Donnan ionization equilibrium concept was used to explain the observed trends. This study is useful in correlating the hydrogel water structure with solute diffusion through these hydrogels.