Steady state analysis of water vapor transport in ionomers

Abstract

Diffusion coefficients for water in high permeability ionomers are determined from steady state permeation measurements, corrected for boundary layer resistance and change in vapor concentration along sample length, to circumvent problems due to heat of condensation and structural relaxation. Values for water in the perfluoroionomer, Nafion, converted to solvent self-diffusion coefficients, D1, are consistent with the NMR based free volume (fv) relation for Nafion at high concentrations. At low concentrations D1 decreases more rapidly than fv values, an effect attributed to ion hydration involving up to 6 water molecules per sulfonate group. The reduction in D1 due to concentration averaging in permeation, estimated using the fv relation, is about 23%. Water vapor sorption isotherms and steady state values of D1, for two different types of styrene hydrogenated butadiene triblock ionomers and Nafion can be superimposed using morphology dependent vertical scaling factors. The simple correspondence between results for the hydrocarbon ionomers and Nafion, is attributed to a common transport mechanism based on the restriction of water to ionic pathways. There are a number of claims in the literature that water uptake by highly permeability polymers is greater from the liquid phase than from saturated vapor at unit activity. This paper demonstrates that the disparity, known as Schroeder's paradox, does not occur in sorption under equilibrium conditions or in permeability measurements, if proper corrections for boundary layer resistance and vapor concentration profile are applied.