Electrochemical studies of volume phase transitions of temperature-responsive polymeric gels

The objective of this project is to study volume phase transition of polymeric gels utilizing electroanalytical techniques. Two types of gels, poly(N-isopropylacrylamide), (NIPA), and poly(N-isopropylacrylamide- co-acrylic acid), (NIPA-AA), were studied in our project.; The oxidation of electroactive probes, which include 1,1′ -ferrocenedimethanol (Fc(MeOH)₂), 4-hydroxy-tempo (TEMPO), Ferrocenylmethyltrimethylammonium ion (FcTMA+) and Hexaammineruthenium(III) ion (Ru(NH₆)3+) was studied in aqueous solutions and swollen state NIPA and NIPA-AA gels using Pt microdisk electrodes. The diffusion coefficient (D) of Fc(MeOH)₂ in swollen state of gels is only about 15%–55% smaller than that in aqueous solution. However, the viscosity of these gels is approximately 5 orders of magnitude greater than that of aqueous solution. The value of activation energy ( E_a) for the diffusion of Fc(MeOH)₂ in aqueous solution and in NIPA and NIPA-AA gels was almost identical.{09}These data suggest that microscopic viscosity of solvent trapped in swollen gel network is similar to that in aqueous solutions.; A decrease of the D of a probe in swollen gels was analyzed in terms of “obstruction effect” and “hydration effect”. A new model was developed to explain this phenomenon and experimental results were compared with predictions of that model.; For NIPA gels, after the volume phase transition, the D of Fc(MeOH)₂ in collapsed gels decreased approximately 2 orders of magnitude when the gel collapsed. D of Fc(MeOH)₂ in collapsed NIPA-AA gels does not change significantly.; Voltammetric experiments were also carried out for positively charged FcTMA+ and Ru(NH₃)₆3+ in NIPA-AA gels with and without supporting electrolyte. After the volume phase transition occurs, a decrease of the D of both probes was observed. Such decrease is more pronounced in NIPA-AA gel without supporting electrolyte than that with supporting electrolyte.; UV-vis spectroscopy was used to study the distribution of Fc(MeOH) ₂ between the collapsed polymer phase and released solution during the volume phase transition. The estimated concentration of Fc(MeOH)₂ in collapsed phase is 4.5 times higher than that in the original swollen gel and 6 times higher than that in released liquid. The concentration of Fc(MeOH)₂ in collapsed NIPA-AA gel is about 20% higher than that in original swollen gels.