Effect of structure of HEMA-DEGMA hydrogel matrix on diffusion coefficients of PEG tracers. Variation of hydrogel crosslink density by change of polymer concentration

Dependence of diffusion coefficients of spin-labeled poly(ethylene glycol)s (PEGs, Mw = 1500, 3000, and 6000) determined by One-Dimensional Electron Spin Resonance Imaging (1D ESRI) on the properties of hydrogel matrices prepared by photopolymerization of 2-hydroxyethyl methacrylate (HEMA) and 2-(2′-hydroxyethoxy)ethyl methacrylate (DEGMA) was studied. The hydrogels were characterized by degree of crosslinking, νgel, by hydrodynamic correlation length, ξgel, and by self-diffusion coefficient of water, DH2O. The tracer diffusion coefficients were analyzed in the frame of scaling reptation and hydrodynamic models. Molecular weight dependence of the tracer diffusion coefficients follow expression D ∼ Mwx with exponent x = −1.2 ± 0.1 which excludes contribution of reptation to the diffusion. Steep decrease of the tracer diffusion coefficients with increasing polymer concentration was observed. Decrease of the tracer diffusion coefficients with increasing ratio of the hydrodynamic radius of the tracer to the hydrodynamic correlation length of the hydrogel matrix, RH/ξgel, was well fitted with predicted stretched exponential expression. Dependence of the tracer diffusion coefficients on the degree of crosslinking νgel suggested as a new scaling variable was found to closely follow empiric stretched exponential dependence in the whole νgel range studied.