Water sorption in physically crosslinked poly(vinyl alcohol) membranes: An experimental investigation of Schroeder's paradox

The sorption characteristics of crosslinked polymers may be analysed by equilibrating the sample either with the liquid or with the gaseous solvent. However, for a number of polymer/solvent systems, measurements at the point of saturation have revealed differences in solvent uptake from the liquid versus the vapour phase (Schroeder's paradox). Attempts at explanation are numerous, but far from comprehensive. Yet, understanding the sorption behaviour of crosslinked polymers is of crucial importance for the development of accurate mass transfer models, especially when applications such as membrane pervaporation or solid polymer electrolytes are concerned where membranes are simultaneously exposed to gaseous and liquid phases.The present study aims at the systematic examination of water sorption into physically crosslinked poly(vinyl alcohol) membranes in both the gaseous and the liquid phase, covering a wide range of solvent activities. For the determination of liquid-phase sorption isotherms, an appropriate method was devised, employing a suitably pretreated polymeric deswelling agent which does not penetrate into the sample network. Sorption data obtained for both equilibration modes suggest that, identical thermal histories of the membrane material provided, Schroeder's paradox is resolved.