Swelling kinetics of poly(N-vinylimidazole-co-sodium styrenesulfonate) hydrogels

Hydrogels of N-vinylimidazole and sodium styrenesulfonate were synthesized by radical crosslinking copolymerization. Transient swelling measurements in water, at room temperature, reveal an unusual behaviour. For some gel compositions, the swelling time dependency is about a δ function: dry hydrogel pellets swell up readily to reach a high degree of swelling and, spontaneously, they deswell to collapse. Such peculiarity was interpreted in terms of several competing events in the swelling mechanism: water diffusion toward the gel, chain disentanglement, sodium-proton interchange through the gel-external bath boundary, approaching of chains to allow interaction of sulfonate groups with neighbouring protonated imidazole moieties and diffusion of water outside the gel. The results of cyclic swelling-deswelling following abrupt changes of composition in the external bath, from water to sodium hydroxide solution, support that mechanism.