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.

The durability testing of membranes for use in a polymer electrolyte fuel cell (PEFC) has been studied in situ by a combination of galvanostatic steady‐state and impedance measurements. The PEFC measurements, which are time consuming, have been compared to fast ex situ testing in 3% H2O2 solution. For the direct assessment of membrane degradation micro‐Raman spectroscopy and determination of ion exchange capacity (IEC) have been used. PVDF based membranes, radiation grafted with styrene and sulfonated, were used as model membranes. By using low degrees of grafting, below about 35%, the durability of this type of membrane can be increased. Degradation in the fuel cell was found to be highly localised. It was found that in situ measurements in the PEFC alone are not sufficient. Measurement of the cell resistance via impedance is not always a reliable indicator of changes in membrane resistance because other resistance changes in the cell can easily interfere and cannot be separated from those caused by the membrane. Micro‐Raman is an ideal complementary method to in situ testing, but it is time consuming. For fast pre‐screening of membrane durability mass loss measurements during exposure to 3% H2O2 solution combined with the determination of changes in the IEC can be performed.

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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.