1H-NMR studies on water in methacrylate hydrogels. I



The behavior of water contained in a hydrophilic hydrogel poly(hydroxyethyl methacrylate) membrane and a hydrophobic hydrogel poly(methyl methacrylate) membrane was studied by proton nuclear magnetic resonance (NMR) spectroscopy. Significant differences between the two hydrogels were observed in their relaxation times, the temperature dependence, and frequency dependence of these relaxation times, and changes of signal intensity of water protons on freezing and thawing. The water in these hydrogels was classified into three states: free, intermediate, and bound states. Poly(hydroxyethyl methacrylate) contains relatively more water in bound and intermediate states than poly(methyl methacrylate). The mobility of water decreases in the order, free water, intermediate water, bound water. The correlation times of the three states were roughly estimated to be 10−11, 10−9, and 10−6 s, respectively. The mobility of water in poly(methyl methacrylate) is several orders of magnitude higher than that in poly(hydroxyethyl methacrylate). In poly(hydroxyethyl methacrylate), the exchange between intermediate and free water is slow in the NMR time scale: the apparent exchange rate and the activation energy were estimated to be 3.6 × 102 s−1 (28°C) and 2.0 kcal/mol, respectively.