Ionic conductivity of proton exchange membranes

Proton conductivity of a series of proton exchange membranes based on sulfonated alpha beta beta -trifluorostyrene-co-substituted-alpha beta beta beta -trifluorostyrenes (BAM(R)) (equivalent weight, EW, 735-407 g mol(-1)) and sulfonated styrene-(ethylene-butylene)-styrene triblock copolymers (DAIS-Analytical) (EW 585-1062 g mol(-1)) was determined by ac impedance spectroscopy. These data are compared to previously published data on Nafion(R) 117 and ethylenetetrafluoroethylene-g-polystyrene sulfonic acid membranes. A correlation exists between the membrane's conductivity, its water content, the ratio of water to ionic sites, and the overall proton concentration. These factors are determined by the membrane's equivalent weight and chemical structure. SAXS spectra for the sulfonated styrene-(ethylene-butylene)-styrene triblock copolymer membranes were featureless. BAM(R) membranes, however, show distinct scattering indicative of ionic aggregates. An interparticle model fit the scattering intensities. The volume fraction of ionic aggregates in the film was a maximum with membranes of intermediate equivalent weight but in all cases much lower than the water content of the membrane, i.e. phase separation of hydrophilic and hydrophobic domains is not extensive. TEM analysis of dry membranes also indicates that phase separation, although evident, is not extensive. These results lead to the conclusion that large majority of sulfonic acid sites are dispersed homogeneously through the membrane. (C) 2001 Elsevier Science B.V. All rights reserved.