Micro-Raman investigations of PVDF-based proton-conducting membranes

Proton-conducting membranes of poly(vinylidene fluoride), PVDF, grafted wit h styrene and thereafter sulfonated, were investigated by Raman spectroscop y. The depth and surface distribution of polystyrene grafts and crosslinker , as well as the sulfonation efficiency, were determined by using confocal micro-Raman spectroscopy. Highly grafted samples show homogeneous distribut ion of grafted material and homogeneous sulfonation. Depth profiles reveal uneven graft distribution for samples with low and intermediate degrees of grafting. In crosslinked samples, the crosslinker concentration in the inte rior of the film is found to be roughly 50% of the surface concentration. I n contrast to what has previously been reported, the sulfonation efficiency is poor at low graft levels and is further inhibited by the presence of di vinyl benzene, DVB, as crosslinker. At degrees of grafting above similar to 60%, the crosslinker does not affect the sulfonation efficiency and a leve l of 70-90% sulfonation is reached. The matrix polymer structure is found t o be largely retained after irradiation, grafting, and sulfonation. (C) 199 9 John Wiley & Sons, Inc.