Absence of Schroederʹs paradox: Experimental evidence for water-swollen Nafion® membranes

Solvent sorption into polymeric membranes may be analysed by equilibrating the polymer sample either with the liquid or with the gaseous solvent. For a number of polymer/solvent systems, however, measurements at the point of saturation have revealed differences in solvent uptake from the liquid versus the corresponding vapour phase (Schroederʹs paradox). Attempts at explanation are numerous, but still far from comprehensive. Yet, insight into the sorption behaviour is of crucial importance as it not only controls membrane performance, but further affects the development of accurate mass transfer models, especially when applications such as membrane pervaporation or solid polymer electrolyte fuel cells are concerned where a selective membrane is simultaneously exposed to a gaseous and a liquid phase. Using Nafion® ionomer membranes as the most prominent example of Schroederʹs paradox, the present study aims at the comparative examination of water sorption for samples equilibrated with the gaseous and the liquid solvent respectively, covering a wide range of solvent activities. For the determination of liquid-phase sorption isotherms, a versatile method was developed, employing a suitably pretreated polymeric deswelling agent which is barred from penetration into the sample network. Sorption data obtained for both equilibration modes suggests that, strictly identical experimental conditions provided, Schroederʹs paradox is resolved.