Increased facilitated transport related to microstructural changes in heat-treated ion-exchange membranes

Abstract

Poly(perfluorosulfonic acid) ionomer films were subjected to a heat treatment, with a low volatility polar solvent, which swells the film. These swollen films were used as supports for liquid membranes using water as a solvent. The diffusive and facilitated transport of acid gases was measured and found to be greatly increased in these membranes versus untreated ones. Microstructural analysis of the films, using small angle X-ray scattering, indicates that there is an increase in the size of the ionic (hydrophilic) clusters. This increase in ionic cluster size correlates with the observed increase in permeability of the acid gases. The effective diffusion coefficient of the carbon dioxide carrier complex (determined from a model for facilitated transport) has increased by 1-2 orders of magnitude, while that of the carbon dioxide alone has increased only 4-fold over the same range of heat treatment conditions. The microstructural changes persist over extended periods of time, and therefore the heat treatment is possibly a practical means of increasing permeability for this polymer in commercial applications. In limited tests it has been previously confirmed that the selectivity of the treated membranes for carbon dioxide versus carbon monoxide remains high, with separation factors of 126 to 70.