Mass transport in thermoresponsive poly(N-isopropylacrylamide-co-acrylic acid) hydrogels studied by electroanalytical techniques: Swollen gels

Steady-state voltammetry and chronoamperometry at microelectrodes were used to study mass transport properties of temperature sensitive poly(N-isoprop ylacrylamide-co-acrylic acid), NIPA-AA, hydrogels. 1,1 ' -Ferrocenedimethyl anol, Fc(MeOH)(2), and 2,2,6,6-tehamethyl-1-piperidinyloxy, TEMPO, were use d as electroactive probe molecules. The activation energy of diffusion of F c(MeOH)2 in aqueous solutions and in NIPA-AA hydrogels was found to be in t he range of 17-19 kJ/mol, which suggests that the local microscopic viscosi ty does not change significantly because of the gelation process, although the macroscopic viscosity of the gels is extremely large. It was found that the diffusion coefficients of Fc(MeOH)2 and TEMPO in NIPA-AA hydrogels in their swollen state are approximately 20%-50% smaller than those in aqueous solutions, and that the diffusion coefficient of probe molecules in these gels is inversely proportional to the concentration of copolymer in the hyd rogels. The "obstruction effect" and "hydration effect" were used to explai n this phenomenon, and experimental results were compared with predictions of the model.