The contact angle between water and the surface of perfluorosulphonic acid membranes

Abstract

Recently studies aimed at clarifying the effect of perfluorinated sulphonic acid membrane surface properties on their behaviour in fuel cells were initiated. These polymers generally consist of a poly(tetrafluoroethylene) (PTFE) backbone with sulphonateterminated sidechains. A primary influence of membrane surface properties in fuel cells could be through surface effects on membrane hydration. The difference in the timescale for membrane equilibration with water in liquid and vapour phases may be explained by the effect of surface properties on membrane hydration. A membrane of thickness 178/~m equilibrates with liquid water within, at most, a few tens of seconds [1], whereas it takes much longer for this membrane to completely equilibrate with vapour phase water [2, 3]. (This is not simply because of the low concentration in the gas phase; a simple calculation shows that, at 1 atm vapour pressure, the membrane would be saturated with water in less than a second if the process were limited by the number of collisions of gas-phase water molecules with the membrane surface.) This difference in hydration dynamics may cause the apparent difference in equilibrium water uptake by ionomers from liquid and saturated vapour phases [3]. Surface effects may also account for the differences in water diffusion coefficients measured by (i) classical vapour-phase sorption methods, which involve transport of isotopically labelled water into, across, and out of a membrane sample, and (ii) N M R methods, which probe transport within the membrane. The vapourphase sorption measurement yields apparent water diffusion coefficients significantly lower [4] than those determined by N M R [5]. The self-diffusion coefficients measured by the sorption method are weakly dependent on membrane water content while those measured by N M R depend strongly on water content. A hydrophobic membrane surface could control the flux observed in the sorption experiment and likely acts as a barrier to water transport into, and possibly out of, the membrane. Our work described here aims to determine whether or not such a hydrophobic 'skin' exists. We report here on water contact angles measured on the perfluorosulfonate ionomer membranes to obtain some information on the hydrophilicity of the ionomer surface.