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.

a b s t r a c tThe mechanical properties of sulfonated aromatic polymers (SAPs: SPEEK and SPPSU) are studied by tensile stress-strain tests and dynamic mechanical analysis (DMA). The elastic moduli are generally above 1 GPa with tensile strength between 25 and 80 MPa and elongation at rupture between 7 and 50%. These properties are consistent with polymers below their glass transition temperature. The glass transition and elastic moduli are strongly increased by thermal treatments of the SAP membranes, due to formation of cross-links between macromolecules. The cross-linking is observed "in situ" during DMA experiments on thermally untreated SPPSU. These data show that previously neglected SAPs might become very interesting PEM fuel cell membranes, if previously thermally treated.

Mechanical properties (elastic modulus, ultimate strength, and elongation at rupture), water sorption isotherms, and chemical diffusion coefficients of water are reported for composite polymers made from a majority polymer (93 wt %), sulfonated (DS ) 0.9) poly-ether-ether-ketone (S-PEEK), and a minority polymer (7 wt %), sulfonated (DS ) 2) and/or silylated poly-phenyl-sulfone (PPSU). S-PEEK is responsible for proton conductivity of the membranes, whereas the minority component is added to maintain the mechanical and morphological stability. It is shown that the addition of sulfonated PPSU reduces the mechanical strength and leads to important membrane swelling. In contrast, addition of silylated PPSU highly improves the membrane strength and reduces considerably membrane swelling, indicating a suitable strategy to improve polymer electrolytes for PEM fuel cells. The calculated water uptake coefficients and diffusion coefficients are consistent with those of other members of the S-PEEK polymer family.