Mass Transport in Thermoresponsive Poly(N-isopropylacrylamide-co-acrylic acid) Hydrogels Studied by Electroanalytical Techniques: Swollen Gels
Weimin Zhang, ChengSong Ma, and Malgorzata Ciszkowska*
Department of Chemistry, Brooklyn College, The City University of New York, Brooklyn, New York 11210-2889
Abstract:
Steady-state voltammetry and chronoamperometry at microelectrodes were
used to study mass transport properties of temperature sensitive poly(N-isopropylacrylamide-co-acrylic
acid), NIPA-AA, hydrogels. 1,1'-Ferrocenedimethylanol, Fc(MeOH)2,
and 2,2,6,6-tetramethyl-1-piperidinyloxy, TEMPO, were used as electroactive
probe molecules. The activation energy of diffusion of Fc(MeOH)2
in aqueous solutions and in NIPA-AA hydrogels was found to be in the range
of 17-19 kJ/mol, which suggests that the local microscopic viscosity 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 hydrogels. The "obstruction effect" and "hydration
effect" were used to explain this phenomenon, and experimental results
were compared with predictions of the model.