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Proton conducting membranes were prepared by irradiation grafting with styrene followed by sulfonation on matrices of poly(vinylidene ¯uoride), PVDF. Membranes crosslinked with divinylbenzene and/or bis(vinylphenyl)ethane were compared to non-crosslinked membranes. The ion conductivity of the crosslinked membranes is lower than that of the non-crosslinked membranes. This is due partly to the very inef®cient sulfonation of the crosslinked membranes below the graft penetration level, which in turn leads to a low water uptake at low degrees of grafting. The graft penetration level is lower in crosslinked membranes than in noncrosslinked membranes. This leads to a more compact structure of the crosslinked grafts within the matrix. The lower ion conductivity in the crosslinked membranes is therefore partly also due to restricted mobility of the ion clusters necessary for ion and water transport in the membranes.

We report on a confocal micro-Raman spectroscopic study of fuel-cell-tested proton conducting polymer membranes. The membranes are based on poly͑vinylidene fluoride͒, which is irradiated, grafted with styrene, and finally sulfonated. We have determined the composition of the membrane before and after fuel cell testing. The fuel cell tests result in a gradual loss of poly͑styrene sulfonic acid͒, which is also reflected in a decreasing conductivity. The degradation is an inhomogeneous process that differs both over the membrane surface and through the membrane depth. Most notably we detect an aggravated degradation at the cathode side of the membrane. Cross-linking the membranes with bis͑vinyl phenyl͒ethane or divinyl benzene does not prevent membrane degradation.

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