SORPTION ISOTHERMS OF GASES BY POLYMER MEMBRANES IN THE GLASSY STATE - AN EXPLANATION-BASED ON THE NONEQUILIBRIUM THERMODYNAMICS

Sorption isotherms of glassy polymers are concave to the pressure axis , and the absolute sorption levels are almost an order of magnitude hi gher than that of rubbery polymers on the relatively low-pressure side . There are several models to interpret this behavior, and the dual so rption model is the most widely accepted one among them. In the presen t work, sorption isotherms were first derived from permeation data wit h several gases for four polymer membranes which might be representati ve of the dual sorption model: two of them were in a glassy state (cel lulose acetate and polyamide), the third one was a block copolymer whi ch was composed of a glassy polymer and a rubbery polymer at the measu ring temperature, and the fourth one was a Nafion membrane which was t aken as a model membrane because of its channel structure with adsorpt ion sites of charged groups supported by a rubbery polymer. These resu lts did not necessarily support the dual sorption model. Subsequently, the validity of the underlying assumptions of two other sorption mode ls for glassy polymers, that is, the matrix model and the deformation model, were examined and a new equation for the sorption isotherm with an ordering parameter was derived, which implied that the glassy poly mer was in a nonequilibrium state and changed from the glassy to the r ubbery state by absorbing the gas. (C) 1998 Academic Press.