(The) nature of water in poly(2,3-dihydroxypropyl methacrylate)hydrogels

Abstract

The hypothesis that three classes of water exist in hydrogels, namely X water(bulk water), Z water (bound water) and Y water(interfacial water), has been verified and generally accepted. To further check the validity of this hypothesis and to study a more concrete concept of X, Y and Z water, several experiments have been done using poly (2, 3-dihydroxypropyl methacrylate) (PDHPMA) gels. Bulk gel conductivity data for PDHPMA gel was obtained. The activation energy for specific conduction was obtained from the specific conductivity curve at various temperatures. A plot of the activation energy vs. volume \% of water in the gels clearly indicated that three different zones, showing three classes of water in the gels. These results were confirmed by thermal expansion measurements. The high water content gels(60\%) showed an extremely sharp volume change at 0$^\circ$C, indicating the presence of normal bulk water. Lower water content gels(20\%) showed no anomalous change in thermal expansion, indicating that the water is bound. The medium water content gels exhibited intermediate behavior. A semi-quantitative analysis of the three classes of water is presented. The third method in this research was the dielectric relaxation study which gave new insights into the structure of water in hydrogels. At low frequencies, the dielectric constant of PDHPMA gels is much higher than that of water. It decreased continuously as the frequency increases, tending to level off at about $10^8$ HZ. The dielectric constsnt is lower the higher the concentration of polymer. Results are most reasonably explained by assuming the more structured phase of water the lower the dielectric constant in terms of X, Y and Z water at high frequencies.