Sorption of water in hydrophilic polymers—I Sorption isotherms in copolymers of hydroxyethyl methacrylate and hydroxyethoxyethyl methacrylate

Abstract

Sorption isotherms of water vapour were determined for crosslinked poly-2-hydroxyethyl methacrylate (PHEMA), poly-2-(2′-hydroxyethoxy)ethyl methacrylate (PHEOEMA) and statistical copolymers at 35°. In the case of PHEMA the amount sorbed does not depend on the porosity of structure: sorption is influenced by the crosslinking parameters only at higher activities. The isotherm of PHEMA is S-shaped, while that of PHEOEMA, is convex; at lower activities, the sorption in mol/mol is higher in PHEMA than in PHEOEMA, although the former has a lower content of polar groups per monomer unit. It seems that the differences between isotherms could be explained by the fact (in principle identical with Kargin's hypothesis) that PHEMA is in the glassy state at the temperature of measurement, while the state of PHEOEMA is viscoelastic. The sorption data were used to calculate the parameters of the B.E.T. equation modified by Anderson; the concentrations of the sorption sites thus determined do not oppose the view that strongly bound water molecules are sorbed between two hydroxyl groups. The dependence of the Flory-Huggins interaction parameter χ on the volume fraction of the polymer exhibits a marked change of slope at a concentration roughly corresponding to the water content needed to transform the polymer from the glassy into the viscoelastic state at 35°. The Zimm clustering function indicates, at a higher water content, a considerable tende`ncy towards clustering; however, for samples in the glassy state and at low amounts sorbed, this function assumes negative values, suggesting mutual isolation of the molecules of the sorbate.