Effect of the Volume Phase Transition on Diffusion and Concentration of Molecular Species in Temperature-Responsive Gels: Electroanalytical Studies

Effect of the volume phase transition on the diffusion and concentration of molecular probes was studied in temperature-responsive polymeric hydrogels. Results were compared for two gels, poly(N-isopropylacrylamide), NIPA, and poly(N-isopropylacrylamide-co-acrylic acid), NIPA-AA. These gels undergo discontinuous, reversible volume phase transition as a response to temperature changes; this transition results in release of approximately 40% and 90% of the solution/solvent from NIPA-AA and NIPA gel phases, respectively. 1,1 -Ferrocenedimethanol, Fc(MeOH)2, served as an electroactive and spectroscopic probe. Diffusion of Fc(MeOH)2 was investigated using voltammetry and chronoamperometry with platinum disk electrodes, while UV-vis supported electroanalytical techniques to determine the concentration of that probe in the gel. The diffusion coefficient of Fc(MeOH)2 was inversely proportional to the concentration of the polymer in both NIPA and NIPA-AA swollen gels for temperatures below the volume phase transition, and differed from that in an aqueous solution. The diffusion coefficient was 16 and 46% smaller than that in an aqueous solution for 3% NIPA and NIPA-AA gels at 25 C, respectively. As a result of the volume phase transition, after the gel collapses, no significant changes in the diffusion coefficient values were observed for NIPA-AA gels. However, for NIPA gels, the diffusion coefficient of a probe decreases approximately two orders of magnitude. The volume phase transition also resulted in a change of the concentration of a probe in the collapsed gel phase. The concentration of Fc(MeOH)2 in collapsed NIPA gel was as much as 4.5 times higher than that in the original swollen gel, while for NIPA-AA gel that increase in concentration was only 20%.