Solvent self-diffusion dependence on the swelling degree of a hydrogel

This article describes high-field nuclear magnetic resonance experiments using pulsed gradient spin echo methods to investigate water mobility inside homogeneously swollen polyacrylamide hydrogel beads. The NMR data permitted determining the solvent self-diffusion coefficient dependence on either the swelling time or the polymer volume fraction. The sensitivity to the structure of the hydrogel network could be tuned to a certain extent via controlling the diffusion probing time. Relaxation measurements have helped substantiate a theoretical description for these self-diffusion dependencies based upon a fast-exchange two-site model. A tortuosity-porosity dependence is extracted from the model and then compared to other known tortuosity regimes. The diffusion dependence on the degree of swelling obtained here corroborates the existence of a gel swelling-front observed during inhomogeneous solvent imbibition experiments. Finally, we stress that the NMR experimental results reported here are of great value on performing theoretical modeling of gel swelling via solvent imbibition.