MEASUREMENTS OF DIFFUSION-PROCESSES IN HEMA-DEGMA HYDROGELS BY ESR IMAGING

The diffusion coefficients of small paramagnetic tracers (nitroxide sp in probes) and spin-labeled poly(ethylene oxide) in hydrogels were mea sured at 300 K, using two-dimensional spatial-spectral ESR imaging (ES RI). The gels were prepared by copolymerization of 2-hydroxyethyl meth acrylate (HEMA) and 2-(2-hydroxyethoxy)ethyl methacrylate (DEGMA), in the presence of a fured molar concentration of glycol dimethacrylate a s the cross-linker and 4,4'-azobis(4-cyanopentanoic acid) as the initi ator. Variation of the amount of water in equilibrium with the gel was achieved by polymerizing different molar ratios of the monomers. Expe rimental concentration profiles of the diffusant were measured as a fu nction of time, and each profile was obtained by image reconstruction, from a complete set of projections. The diffusion coefficients were d etermined by simulating all experimental profiles, using the Fick mode l of diffusion for an initially confined tracer diffusing into a finit e system. The diffusion coefficients depend on the amount of water in the gels. The variation of the diffusion coefficients D with the weigh t fraction of the copolymer in the gel is consistent with the free vol ume model. The variation of D with the molecular mass M of the diffusa nt scales as M(-0.8) in gels containing greater than or equal to 67 wt % water and as M(-1.4) for the gel containing 55 wt% water. The advant ages and present limitations of 2D spatial-spectral ESRI for measuring transport properties in polymers are discussed.