High molecular weight nitrile-functional, (hexafluoroisopropylidene)diphenol-based poly(arylene ether) copolymers with pendent sulfonic acid groups have been prepared by step copolymerization of 4,4'-(hexafluoroisopropylidene)diphenol, 2,6-dichlorobenzonitrile, and 3,3'-disulfonate-4,4'-dichlorodiphenylsulfone. Copolymers containing as much as 55 mole% disulfonated units were cast from dimethylformamide solutions to form tough ductile films. The films were converted from the salt to the acid forms with dilute sulfuric acid followed by deionized water. Dynamic TGA demonstrated that the well-dried, acidified, nitrile-containing copolymers had no weight loss up to 300degreesC in air. A systematically varied compositional series showed increased glass transition temperatures, protonic conductivities, and hydrophilicities as a function of disulfonation. Films containing greater than or equal to 20 mole% of the disulfonated repeat units had T-g's of 220degreesC and higher. At approximately equivalent ion exchange capacities (IEC), e.g. 1-1.6 meq g(-1), the protonic conductivities of these films were comparable to other disulfonated poly(arylene ether sulfone) copolymers investigated. The benzonitrile-containing disulfonated copolymers also had reduced moisture absorption (10-15 wt.%) compared to other disulfonated poly(arylene ether sulfone) copolymers with equivalent IECs. The copolymer with 35 mole% of the disulfonated comonomer had a protonic conductivity > 0.10 S cm(-1) at 110degreesC and 100% relative humidity. The protonic conductivities of the benzonitrile-containing copolymers decreased as expected as relative humidity was lowered. Atomic force microscopy in the tapping mode demonstrated that the acidified copolymer with 35 mole% disulfonated units was nanophase separated into an essentially co-continuous morphology of hydrophobic and hydrophilic domains. Further efforts are ongoing to translate these promising results into membrane electrode assemblies for proton exchange membrane fuel cell devices. (C) 2004 Elsevier B.V. All rights reserved.