Radiation grafted and sulfonated poly(vinylidene fluoride), PVDF, membranes have been studied by thermal analysis and X-ray diÂraction to determine the changes in membrane crystallinity and structure during preparation. Commercial PVDF films were irradiated with an electron beam, grafted with styrene and finally sulfonated. Both the crystallinity and the size of the crystallites of PVDF decrease in the grafting reaction. A further decrease in crystallinity is observed in the sulfonation reaction. The residual crystallinity of PVDF was considerable (10-20%) even in membranes which had been subjected to severe reaction conditions. These results can be explained by assuming that the grafting takes place mainly in the amorphous region of the PVDF, and close to the surfaces of the crystals, but that grafts do not penetrate into the crystals. The proton conductivity of the grafted and sulfonated PVDF membranes reached values comparable to those of Nafion membranes.

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The state of water in proton exchange membranes prepared by pre-irradiation (electron beam, 100 kGy) grafting of styrene onto poly(viny1idene fluoride) films (PVDF-g-PS), followed by sulfonation (PVDF-g-PSSA), has been studied with thermal analysis, Raman spectroscopy and small angle X-ray diffraction (SAXS). Raman spectra show that, in addition to free liquid water in the membranes, single water molecules are weakly bound to the polymer backbone. Thermal analysis shows that there are three types of water molecules in the membrane; non-freezable water associated with the ionic sites, freezable free water, and freezable bound water. The amount of non-freezable water is around 10 H20/S03H (mol/mol), and is independent of the degree of grafting (d.0.g.). The amount of freezable water is strongly dependent on the d.0.g. as long as the grafting has not penetrated the whole of the film, and reaches a value of around 40 H20/S03H (mol/mol) above a d.0.g. of 50%. The conductivity of membranes containing only the non-freezable water is low, i. e. the ionically bound water alone does not form the domains necessary for proton and water transport. SAXS measurements show that water/sulfonic acid clusters in hydrated PVDF-g-PSSA membranes with a Bragg distance of 25 A are formed; these form the ion conducting channels in the membrane.