Structure and properties of sulfonated poly[(vinylidene fluoride)-g-styrene] porous membranes

The possibility of developing poly(vinylidene fluoride) (PVDF)-based membranes as proton conductors has been investigated. Using electron beam radiation grafting and functionalisation, the composition of the membranes can be controlled. Membranes have been prepared from porous films of PVDF, and their structure and properties have been studied with Raman spectroscopy, wide angle X-ray Scattering (WAXS), small angle X-ray scattering (SAXS), swelling tests, and by impedance spectroscopy. The grafting reaction of styrene, initiated in the amorphous regions and at the surfaces of the crystallites in the partly crystalline PVDF matrix, is very efficient, and high degrees of grafting can be achieved: Grafts are formed both from C-H and C-F branch points. Sulfonation can be accomplished to ca. 100%, and occurs mainly at the para-position of the phenyl rings. The hydration number was found to be independent of the degree of grafting, degree of sulfonation and crystallinity. The overall crystallinity decreased in the structure with the degree of grafting and sulfonation, partly owing to the diluting effect of the grafts, partly owing to efficient penetration of the grafts in the crystallites. The conductivity increased with the content of sulfonic acid groups and, in particular, with decreasing crystallinity. In the case of fully sulfonated membranes, a levelling-out of the conductivity was found at a degree of grafting of around 200%. Conductivities up to 120 mS cm(-1) at room temperature were achieved.