Hydration and conductivity studies of polymer-water interactions in polyacrylamide hydrogels

The conductivity and hydration mechanisms of polyacrylamide (PAA) hydrogels have been investigated by means of (a) equilibrium and dynamic water sorption isotherms, (b) thermally stimulated depolarization currents (TSDC) techniques in the temperature range 77 to 300 K and (c) broad band dielectric relaxation spectroscopy (DRS) in the frequency range 10 exp -2 to 10 exp 6 Hz and the temperature range 230 to 330 K. A systematic analysis of our combined results indicates that water molecules up to a critical hydration value, h sub c , of about 0.15 g of water/g of dry weight are irrotationally bound to primary hydration sites and do not contribute themselves to the dielectric spectra. This h sub c value is in accordance with equilibrium isothermal measurements on the same samples. Moreover, a reorganization of water in the hydrogels seems to take place for h > = 0.8. The Arrhenius behaviour of DC-conductivity, sigma sub dc , as well as of the loss tangent, tan delta , and electric modulus, M exp n , dispersions, with an activation energy of the order of 0.8 eV, suggests that the observed phenomena have a common origin and that the conductivity mechanism is not governed by the co-operative motion of the polymeric chains, the charge carriers (protons) motion occurring probably through the water-filled micropores of the hydrogel.