Thermal and pulse NMR analysis of water in poly(2-hydroxyethyl methacrylate)

Hydrophilic three‐dimensional methacrylate polymer networks (hydrogels) were prepared from 2‐hydroxyethyl methacrylate (HEMA) monomer and tetraethylene glycol dimethacrylate (TEGDMA) as crosslinker. The nature and states of water in these hydrogels were studied by differential thermal analysis and pulse NMR relaxation spectroscopy. The thermal studies showed no endotherm peak for ice melting in the lower water content (bound water region); there are two endotherms peaks for higher water content hydrogels near 0°C. The amounts of bound water, intermediate water, and bulklike (free) water in the hydrogels were determined from a quantitative analysis of the endotherms of the water melting transitions. The water structure ordering in the hydrogels were discussed in terms of the fusion entropy and enthalpy obtained from the endotherm. Nuclear magnetic relaxation spectroscopy was also used to understand the mobilities of the water protons in the hydrogels and the interaction of water molecules with the gel networks. The measured spin‐lattice relaxation time (T1) values for water protons in the hydrogels are greatly reduced compared to that of liquid water. The measured values of spin–spin relaxation times (T2) of water protons in the hydrogels are approximately 10 times less than that of T1 and are almost constant in the region of bound water content. Beyond the bound water content region in the hydrogels, the T2 values rapidly increase as the water content increases.