A major goal of current proton exchange membrane fuel cell research involves developing high temperature membranes that can operate at similar to 120 degrees C and low humidities. Multiblock copolymers synthesized from 100% disulfonated poly(arylene ether sulfone) (BPSH100) and naphthalene polyimide (PI) oligomers may be an alternative. The high temperature behavior of BPSH100-PI multiblock copolymers was evaluated in terms of morphology and water retention and compared to that of BPSH-based random copolymers, Nafion 112, and new recast Nafion (NRE211). Atomic force microscopy (AFM) performed at elevated temperatures with a hot stage demonstrated that at block lengths of similar to 15 kg/mol the multiblock copolymers displayed no morphological changes up to 120 degrees C or even higher. Water desorption was observed to decrease with increasing block length. The copolymers exhibited little to no water loss during a 200 degrees C isotherm in contrast to random BPSH copolymers and Nafion. A BPSH100-PI multiblock copolymer with large block length appears to have morphological stability and retain water (likely strongly bound) at temperatures exceeding 120 degrees C, suggesting its candidacy as a high temperature PEM. (c) 2008 Elsevier B.V. All rights reserved.