Highly Sulfonated Multiblock Copoly(ether sulfone)s for Fuel Cell Membranes

Highly sulfonated multiblock copoly(ether sulfone)s applicable to proton electrolyte fuel cells (PEFCs) were synthesized by the coupling reaction of corresponding hydroxyl-terminated oligomers in the presence of highly reactive decafluorobiphenyl (DFB) as a chain extender, followed by postsulfonation with concentrated sulfuric acid. Their molecular weights were reasonably high as determined by viscosity measurement (eta(inh) = 0.72-1.58 dL/g). It was also confirmed that postsulfonation selectively took place in hydrophilic segments to yield highly sulfonated multiblock copolymers (IEC = 1.90-2.75 mequiv/g). The resulting polymers gave transparent, flexible, and tough membranes by solution casting. The 4b membrane, as a representative sample, demonstrated good mechanical strength in the dry state regardless of high IEC value (2.75 mequiv/g). The 4a-c membranes with higher IEC values (IEC = 2.75-2.79 mequiv/g) maintained high water uptake (13.7-17.7 wt %) at 50% RH and it was still high (7.4-8.5 wt %) at 30% RH. Proton conductivity of all membranes at 80 degrees C and 95% RH was higher than that of Nafion 117. Furthermore, the 4a membrane showed high proton conductivity, comparable with Nafion 117 in the range of 50-95% RH, and maintained high proton conductivity (2.3 x 10(-3) S/cm) even at 30% RH. Finally, the surface morphology of the membrane was investigated by tapping mode atomic force microscopy, which showed well-connected hydrophilic domains that could work as proton transportation channel. This phase separation and the high water uptake behavior probably contributed to high and effective proton conduction in a wide range of relative humidity. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2757-2764, 2010