Sorption/desorption properties of water vapour in poly(2-hydroxyethyl methacrylate): 1. Experimental and preliminary analysis

Abstract

The sorption and desorption of water vapour into and from a hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) membrane were examined with the gravimetric method in an integral manner at 37°C. The sorption isotherm of water in PHEMA at 37°C followed Henry's law at lower activities and Flory-Huggins type sorption at higher activities. The water content which leads to a glass-rubber transition within the polymer matrix at the isotherm temperature [ C g( T)] was found to be 0.079, which also coincided with the transition region between the Henry's law sorption and the Flory-Huggins type sorption. When the equilibrium water content was below the C g( T), the sorption and desorption kinetics followed two-stage and pseudo-Fickian, respectively. The anomalies could be interpreted by either a slow relaxation of polymer networks or a reversible immobilization reaction coupled with the diffusion process. When the equilibrium water content was beyond the C g( T), both the sorption and desorption kinetics seemed to be sigmoidal. Such behaviour could be a result of the glass-rubber transition, and the prevailing states of water might further complicate the processes of sorption and desorption.