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 sorpt ion isotherms, (b) thermally stimulated depolarization currents (TSDC) tech niques in the temperature range 77 to 300 K and (c) broad band dielectric r elaxation spectroscopy (DRS) in the frequency range 10(-2) to 10(6) Hz and the temperature range 230 to 330 K. A systematic analysis of our combined r esults indicates that water molecules up to a critical hydration value, h(c ), of about 0.15 g of water/g of dry weight are irrotationally bound to pri mary hydration sites and do not contribute themselves to the dielectric spe ctra. This h(c) value is in accordance with equilibrium isothermal measurem ents on the same samples. Moreover, a reorganization of water in the hydrog els seems to take place for h greater than or equal to 0.8. The Arrhenius b ehaviour of DC-conductivity, sigma(dc), as well as of the loss tangent, tan delta, and electric modulus, M ", dispersions, with an activation energy o f the order of 0.8 eV, suggests that the observed phenomena have a common o rigin and that the conductivity mechanism is not governed by the co-operati ve motion of the polymeric chains, the charge carriers (protons) motion occ urring probably through the water-filled micropores of the hydrogel. (C) 19 99 Elsevier Science B.V. All rights reserved.