(The) nature of water in tactic poly(2-hydroxyethyl methacrylate) hydrogels

Abstract

The hypothesis that three classes of water exist in hydrogels, namely X water (free water-like), Z water (bound water-like), and Y water (interfacial water-like), has been verified and generally accepted. To further check the validity of this hypothesis and to study the nature of X, Y, and Z water as conformation changes, several experiments have been done using tactic poly (2-hydroxyethyl methacrylate) (P-HEMA) gels. Thermal expansivity data for tactic P-HEMA gel was obtained. In each case of isotactic and syndiotactic P-HEMA, the higher water content gels showed an extremely sharp volume change at $0^\circ C$, indicating the presence of normal free water-like. Lower water content gels showed no anomalous change in thermal expansion, indicating that the water is bound water-like. The medium water content gels exhibited intermediate behavior. These results were also confirmed by bulk gel conductivity measurements. The differential scanning calorimeter (DSC) experiment was simply introduced to further verify the bound water-like quantities which was obtained by the method of dilatometry and specific conductivity. Observing the amounts of X, Y, and Z water with the change of tacticity, the similar content of bound water-like may be due to the same primary structure of isotactic and syndiotactic polymer and the difference in free and interfacial water-like content may be due to the difference in secondary and tertiary structure of tactic polymer. Therefore, as the polymer conformation varies the free and interfacial water-like content will be varied. In order to demonstrate these concepts, Russel``s $CPK^R$ space-filling molecular models of isotactic and syndiotactic P-$HEMA^{(32)}$ was utilized.