A series of commercial fluoropolymer films was irradiated with an electron beam, grafted with styrene, and sulfonated. The influence of the initial fluoropolymer on the grafting yields and the properties of the grafted and sulfonated membranes were investigated. The same synthesis procedure can be followed for most fluoropolymers and samples with a similar degree of grafting, and a homogenous polystyrene distribution can be prepared by varying the absorbed dose. The main difference among different fluoropolymer-based membranes is the water uptake from liquid water that has a roughly linear dependence on the crystallinity of the sample. The more amorphous the initial material, the greater the water uptake. Mechanical properties of the membranes at 50% relative humidity differ less than those of the starting materials and are comparable to those of Nafion 105.

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.

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ABSTRACT:We investigated the water present in a series of radiation-grafted fluoropolymers with similar poly(styrene sulfonic acid) (PSSA) contents with the aim of determining the influence of the initial fluoropolymer. Radiationgrafted membranes were compared with Nafion 117 and 105. Sorption curves and differential scanning calorimetry thermograms showed that all the membranes contained the same number of water molecules tightly bound to the sulfonic acid groups; this water did not freeze. In radiationgrafted membranes, the content of freezing water absorbed from the liquid-phase water varied according to the swelling abilities of the membrane, which were dependent on the initial fluoropolymer. Larger pores accompanied high water uptakes and high conductivity. The amount of water absorbed from the vapor phase was similar for all radiationgrafted membranes with similar PSSA contents, irrespective of matrix material. Nafion membranes had higher conductivities at intermediate hydration levels, and the relaxation times measured by NMR were longer than for the radiationgrafted materials. This suggests that the channels for water and proton conduction are different in the two types of materials.

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A novel series of polyaromatic ionomers with similar equivalent weights but very different sulphonic acid distributions along the ionomer backbone has been designed and prepared. By synthetically organising the sequence‐distribution so that it consists of fully defined ionic segments (containing singlets, doublets or quadruplets of sulphonic acid groups) alternating strictly with equally well‐defined nonionic spacer segments, a new class of polymers which may be described as microblock ionomers has been developed. These materials exhibit very different properties and morphologies from analogous randomly substituted systems. Progressively extending the nonionic spacer length in the repeat unit (maintaining a constant equivalent weight by increasing the degree of sulphonation of the ionic segment) leads to an increasing degree of nanophase separation between hydrophilic and hydrophobic domains in these materials. Membranes cast from ionomers with the more highly phase‐separated morphologies show significantly higher onset temperatures for uncontrolled swelling in water. This new type of ionomer design has enabled the fabrication of swelling‐resistant hydrocarbon membranes, suitable for fuel cell operation, with very much higher ion exchange capacities (>2 meq g–1) than those previously reported in the literature. When tested in a fuel cell at high temperature (120 °C) and low relative humidity (35% RH), the best microblock membrane matched the performance of Nafion 112. Moreover, comparative low load cycle testing of membrane ‐electrode assemblies suggests that the durability of the new membranes under conditions of high temperature and low relative humidity is superior to that of conventional perfluorinated materials.