Sorption and diffusion of methanol and water in PVDF-g-PSSA and Nafion (R)117 polymer electrolyte membranes

Sorption and diffusion properties of poly(vinylidene fluoride)-graft-poly-( styrene sulfonic acid) (PVDF-g-PSSA) and Nafion(R) 117 polymer electrolyte membranes were studied in water/methanol mixtures. The two types of membran es were found to have different sorption properties. The Nafion 117 membran e was found to have a maximum in-solvent uptake around 0.4 to 0.6 mole frac tion of methanol, while the PVDF-g-PSSA membranes took up less solvent with increasing methanol concentration. The proton NMR spectra were recorded fo r membranes immersed in deuterated water/methanol mixtures. The spectra sho wed that the hydroxyl protons inside the membrane exhibit resonance lines d ifferent from the resonance lines of hydroxyl protons in the external solve nt. The spectral features of the lines of these internal hydroxyl groups in the membranes were different in the Nafion membrane compared with the PVDF -g-PSSA membranes. Diffusion measurements with the pulsed field gradient NM R (PFG-NMR) method showed that the diffusion coefficient of the internal hy droxyl groups in the solvent immersed Nafion membrane mirrors the changes i n the diffusion coefficients of hydroxyl and methyl protons in the external solvent. For the PVDF-g-PSSA membranes, a decrease in the diffusion coeffi cient of the internal hydroxyl protons was seen with increasing methanol co ncentration. These results indicate that the morphology and chemical struct ure of the membranes have an effect on their solvent sorption and diffusion characteristics. (C) 2000 John Wiley & Sons, Inc.