Polymer-water interactions in poly(hydroxyethyl acrylate) hydrogels studied by dielectric, calorimetric and sorption isotherm measurements

Poly(hydroxyethyl acrylate) hydrogels with water contents up to 0.96 (g water/g dry material) are studied with equilibrium and dynamic water sorption isotherm measurements, differential scanning calorimetry, thermally stimulated depolarization current measurements and broadband a.c. dielectric relaxation spectroscopy. Our main interest is focused on comparing results on the state of water in the hydrogel obtained with the different techniques on the same sample and on discussing in a quantitative way the relations among the different descriptions imposed by the different techniques. The results show that for water contents lower than about 0.20-0.30 the hydrogel is a homogeneous system in the temperature range down to -100 degrees C. At higher water contents phase separation occurs. Several critical water contents have been obtained by means of the different techniques used.