The sorption, diffusion and swelling properties of methanol vapor in a short-side-chain perfluorosulfonic acid ionomer (PFSI) membrane of equivalent weight 860 gpol/(mol(SO3H) were investigated at different activities and at temperatures between 35 and 65°C. The solubility isotherms can be well represented by a BET-type of equation as a function of the penetrant activity.
The effect of temperature on the methanol uptake is very weak and the molar uptake of methanol, at fixed activity, is actually comparable to that of water, the difference being mainly due to the different molecular binding to SO3H sites. Methanol diffusivity increases with temperature and remarkably it shows a maximum with increasing methanol concentration, similarly to what is observed in the case of water vapor diffusion.
The swelling behavior has also been determined as a function of vapor activity: different trends can be observed in the directions parallel and perpendicular to the extrusion lines, and the volumetric dilation is slightly higher than what is predicted by a simple additive law. The trend of methanol partial molar volume with methanol content parallels that of diffusion, indicating that the diffusion behavior is tightly related to the dilation response of the matrix.
The pure methanol vapor permeability has also been estimated and shows values lower than those obtained, at the same activity, in the liquid phase in case of aqueous solutions.&'
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