Sulfonated poly(ether ether ketone) (SPEEK) membranes were thermally treated at temperatures between 120 and 160°C. Water uptake measured at different relative humidity values or by full immersion in water between 25 and 145°C was found to depend very strongly on previous thermal treatment and casting solvent. Water-uptake coefficient values as low as 10-15 even upon immersion in water at 100°C were obtained with membranes treated at 160°C. This effect is related to cross-linking by SO 2 bridges between macromolecular chains. An important role is also played by the casting solvent: among the investigated solvents, dimethylsulfoxide (DMSO) gave the best results. A chemical kinetics model is outlined that permits the estimation of the relevant kinetic parameters, especially the activation energy of the cross-linking reaction, which was found to be about 60 kJ/mol. These results are of significant importance for the improvement of proton-exchange membrane fuel cells. IntroductionThe current trend toward environmentally friendlier and more efficient power production has shifted the bias from conventional fuels and internal combustion engines toward alternative fuels and power sources. Much interest is focused on developing proton-exchange membrane fuel cells (PEMFCs), which use a polymer membrane as the electrolyte. The future application of this type of technology depends greatly on the enhancement of membrane stability. The polymer electrolyte membrane must be improved in terms of durability; most importantly, it must be compatible with operation at temperatures of around 130°C (intermediate temperature) at low relative humidity (RH) for H 2 fuel cells, and it must present a reduced fuel crossover for direct methanol fuel cells (DMFCs).1,2 The objective is to reduce membrane swelling at high relative humidity and, in the framework of intermediate-temperature fuel cells, to reduce the degradation of properties observed during fuel cell operation at higher temperature.

a b s t r a c tThermal stability, hydration and mechanical properties of thermally cross-linked sulfonated aromatic polymers (SAP) with high ionic exchange capacity (IEC) were measured and compared to untreated samples. The formation of cross-linking greatly stabilizes sulfonated polyphenylsulfone (SPPSU) in terms of thermal, mechanical, and hydrolytic degradation: it can resist in water even at a temperature of 145• C with improved mechanical properties, while TGA experiments demonstrate that SPPSU membranes are stable well above 200• C. Sulfonated polyethersulfone (SPES) membranes show, instead, a hydrolytic instability.