WATER MOBILITY AND STRUCTURE IN POLY[2-HYDROXYETHYLMETHACRYLATE] HYDROGELS BY MEANS OF THE PULSED-FIELD GRADIENT NMR TECHNIQUE

The translational mobility of water in poly[2-hydroxyethylmethacrylate ] (pHEMA) hydrogels, cross-linked with ethyleneglycoldimethacrylate, w as studied by means of the pulsed field gradient (PFG) nuclear magneti c resonance (NMR) technique, which offers the opportunity to study the molecular displacements directly under well-defined equilibrium condi tions, resulting in a determination of the self-diffusion coefficient. It is possible to check whether coexisting water phases with differen t 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 f ound 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 meas urements have shown that the total water phase in a gel diffuses as on e homogeneous phase, which can be characterized by a diffusion coeffic ient. The self-diffusion coefficient is strongly dependent on the degr ee of hydration of the gel; the cross-linker concentration has no meas urable effect. The strong dependence of the diffusion of water on its concentration in the gel has consequences for the modelling of the swe lling and drug-release dynamics of pHEMA and necessitates a revision o f the present models describing these processes.