Charles E. Reineke

Water Transport in Ionic Polymers

Hydrophilic polymers such as perfluorinated ionomers with sulfonic acid functional groups have high water permeabilities and good selectivities for water over most gases and organic liquids. Understanding the water transport mechanism in these water plasticized membranes is necessary in order to effectively utilize them for dehydration applications. Water diffusion in these polymers is non-Fickian. Water permeabilities and solubility coefficients are a function of water vapor pressure. A careful analysis is required to arrive at the concentration dependent diffusion coefficients. An approach for calculating thermodynamic diffusion coefficients based on steady state permeabilities and equilibrium sorption measurements is presented in this paper. At very low water vapor pressures, strong ion-water interactions give rise to high solubility coefficients and low diffusion coefficients. At water activities of 0.1 to 0.6, the solubility coefficient decreases and the diffusion coefficient increases with water activity. At very high water activities (≥0.6), the diffusion coefficient appears to attain a limiting value while the solubility coefficient sharply increases. An anomalous thickness effect was also observed. Thickness normalized permeabilities increased with increasing membrane thickness. Several mechanisms were considered.