Nature of water in synthetic hydrogels. I. Dilatometry, specific conductivity, and differential scanning calorimetry of polyhydroxyethyl methacrylate

Three classes of water may exist in certain hydrogels. We have previously labeled these as X water (bulk water), Z water (bound water) and Y water (intermediate forms we call interfacial water). Bulk gel conductivity data for poly (2-hydroxyethyl methacrylate) (PHEMA) were obtained. The activation energy for specific conduction was calculated. A plot of the activation energy versus wt percent of water in the gel clearly indicated three different zones, showing three possible classes of water in the gels. These results were confirmed by thermal expansion measurements. The high water content gels (50%) demonstrated an extremely sharp volume change at 0°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 semiquantitative analysis of the three classes of water is presented. A further verification of these results was obtained by differential scanning calorimetry (DSC) studies. The low water content gel (20%) consists mainly of bound water, which exhibited no phase transitions over the range −15 to 24°C. The high water content gels showed phase transitions near 0°C. The medium water content gels show gradual shifts of the phase transition temperatures near 0°C.