Tracer mobility in aqueous poly(N-isopropylacrylamide) grafted networks: Effect of interactions and permanent crosslinks

Abstract

Using fluorescence correlation spectroscopy, we measured the translational mobility of three molecular tracers in aqueous poly(N-isopropylacrylamide) (PNIPAAm) grafted networks, under good solvency conditions. The influence of permanent crosslinks and the interaction between polymer and tracers was revealed. In contrast to the athermal tracer Alexa 647, we observed strong deviation from single Fickian diffusion for two interacting tracers, Alexa 488 and Rhodamine 6G. The dynamics of the latter tracers were represented by a double Fickian diffusion in PNIPAAm solutions and hydrogels for volume fractions up to 30 %. A different slowdown was observed for the slow process in hydrogels and solutions. For the hydrogels, it becomes virtually concentration independent above about 10 %, whereas for solutions it increases below this threshold with PNIPAAm concentration. A pure crowding effect, free of interactions, was observed for both Alexa 647 and for the fast diffusion process of the interacting tracers, whereas their slow process was attributed to the interaction between tracer and polymer. The elucidation of effects due to crowding as well as due to the strength and nature of interactions on the molecular diffusion in hydrogels is needed for both, fundamental and practical perspectives, as for example in biosensors.