ELECTRICAL-CONDUCTIVITY OF PH-RESPONSIVE HYDROGELS

The electrical conductivity of pH-responsive hydrogels based on cross- linked poly(2-hydroxyethyl methacrylate-co-dimethylaminoethyl methacry late) copolymers has been studied as a function of pH over the range 5 -10, for copolymers containing up to 20 mol% of the amine-containing c omonomer. The conductivities of membranes equilibrated in buffered pot assium chloride solutions were determined by measuring the electrical resistance of a membrane mounted between the chambers of a modified si de-by-side diffusion cell. The conductivity, expressed as a fraction o f the conductivity of the buffer in which the gels were equilibrated, ranged from 1% for the gels in the collapsed state to 70% for the most highly swollen gels. The observed results are qualitatively consisten t with a proposed model in which the ion concentrations in the fluid p hase of the gel are described by Donnan partitioning, and the ion mobi lities by the free-volume theory of Yasuda. The results suggest that c onductivity measurements may provide an alternative to diffusive trans port studies for characterizing moderately swollen hydrogel membranes.