Phase separation and crystallinity in proton conducting membranes of styrene grafted and sulfonated poly(vinylidene fluoride)

A series of proton exchange membranes have been prepared by the preirradiat ion grafting method. Styrene was grafted onto a. matrix of poly(vinylidene fluoride) (PVDF) after electron beam irradiation. Part of the samples was c rosslinked with divinylbenzene (DVB) or bis(vinylphenyl)ethane (BVPE). Subs equent sulfonation gave membranes grafted with poly(styrene sulfonic acid) and marked PVDF-g-PSSA. It was found that the intrinsic crystallinity of th e matrix decreased in both the grafting and the sulfonation reaction in all the membranes. The graft penetration and the ion conductivity are influenc ed strongly by the crosslinker. The ion conductivity is considerably lower in crosslinked membranes than in noncrosslinked ones. Generally, the mechan ical strength decreases with crosslinking. The membranes show a regular pha se separated structure in which the sulfonated grafts are incorporated in t he amorphous parts of the matrix polymer. The phase separated domains are s mall, of the order of magnitude of 100-250 nm. These were resolved on trans mission electron micrographs and on atomic force images but could not be re solved with microprobe Raman spectroscopy. (C) 1999 John Wiley & Sons, Inc.