Dielectric studies of polyacrylamide hydrogels

The dielectric properties of polyacrylamide/water hydrogels, as a function of water content and temperature, are investigated using ac dielectric relaxation spectroscopy (DRS) in the frequency range 5 Hz-13 MHz and thermally stimulated depolarization currents (TSDC) in the temperature range 77-300 K. TSDC spectra reveal three peaks, the first two being attributed to the reorientation of free and loosely bound water molecules respectively, whereas the third one is a manifestation of the glass transition in the hydrogel constituents. DRS spectra display a dielectric dispersion which shifts gradually to lower frequencies with decreasing T. The water crystallization at T<0 /spl deg/C, as well as the non-reversibility of the freezing-thawing process, are clearly demonstrated from the evolution of ac conductivity spectra with temperature.