Water sorption and dielectric relaxation spectroscopy studies in hydrated Nafion® (-SO3K) membranes

Dynamic and equilibrium sorption isotherm measurements were carried out on hydrated perfluorosulfonate polymer membranes in potassium salt form (Nafion(R)-SO3K, 1190 EW), at 22 and 40 degreesC respectively. Despite the glassy state of the membrane the diffusion process is Fickian with an average diffusion coefficient D in the range 2.8-30 x 10(-9) cm(2)/s. The equilibrium sorption isotherms may be quantitatively analyzed according to the GAB equation for multilayer sorption processes up to the activity value of alpha = 0.79, which corresponds to 1.0 site-bound water molecules per (-SO3K) group. The analysis of the equilibrium sorption isotherm, using the method of Zimm and Lundberg, suggests clustering of the water molecules for activities alpha greater than or equal to 0.79. The apparent polymer-water interaction parameter chi.(Flory parameter), was found to have a maximum at alpha = 0.79 (1.07 < chi less than or equal to 1.95). Also, dielectric relaxation studies of the same hydrated Nafion(R) membranes have been carried out in the frequency range of 10(-2) - 5 x 10(6) Hz and in the temperature range of 223-295 K. Two relaxation mechanisms were observed which both sift to higher frequencies by increasing the water content. The low frequency relaxation mechanism is due to interfacial polarization which takes place in the hydrated ionic regions. The high frequency relaxation mechanism, is a dipolar mechanism which may be attributed to the rotation of (-SO3-) group-water complexes at the end of the side chains. (C) 2000 Kluwer Academic Publishers.