Water mobility and structure in poly[2-hydroxyethylmethacrylate] hydrogels by means of the pulsed field gradient NMR...

The translational mobility of water in poly[2-hydroxyethylmethacrylate] (pHEMA) hydrogels, cross-linked with ethyleneglycoldimethacrylate, was studied by means of the pulsed field gradient (PFG) nuclear magnetic resonance (NMR) technique, which offers the opportunity to study the molecular displacements directly under well-defined equilibrium conditions, resulting in a determination of the self-diffusion coefficient. It is possible to check whether coexisting water phases with different mobilities (on a timescale of ca. 10 ms) are present. The dependence of the diffusion coefficient of water on the degree of hydration and the cross-linker concentration was measured. Magnetic interaction is found to cause cross-relaxation between the protons of water and those of the polymer matrix. This affects the data, rendering the evaluation by the standard equation invalid. An equation taking cross-relaxation into account has been derived. Amplitude measurements have shown that all the water in the gels contributes to the NMR signal. The PFG measurements have shown that the total water phase in a gel diffuses as one homogeneous phase, which can be characterized by a diffusion coefficient. The self-diffusion coefficient is strongly dependent on the degree of hydration of the gel; the cross-linker concentration has no measurable effect. The strong dependence of the diffusion of water on its concentration in the gel has consequences for the modelling of the swelling and drug-release dynamics of pHEMA and necessitates a revision of the present models describing these processes.