Abstract
Copolymers of hydrophobic diglycidyl ether of bisphenol A (DGEBA) vinyl ester (VE) and hydrophilic 2‐acrylamido 2‐methyl 1‐propane sulfonic acid (AMPS) were evaluated as proton conducting membranes for fuel cell applications. Membranes were synthesized using free radical copolymerization in the presence of a common solvent for both monomers, dimethyl formamide (DMF), followed by solvent removal by supercritical CO~2~ to induce porosity. Micrographs revealed pore sizes below 60 nm with porosity proportional to the initial solvent fraction used. Studies on the states of water showed that the presence of this pore volume significantly altered the freezable water fraction at equivalent AMPS concentrations. Comparison of the moles of water per mole of sulfonic acid (λ) between copolymer membranes and AMPS monomer solutions showed that the nonfreezable water (λ|~nonfr~) was depressed at high AMPS concentrations, suggesting that differences in interatomic distances between sulfonic acid groups might alter λ|~nonfr~. The highest average through plane conductivity of membranes was determined to be 30 mS/cm and was comparable to that of Nafion®117 (27 mS/cm). The effective proton mobility, μ~eff~, was calculated and suggested to be a parameter used to capture the effects of membrane structure and swelling while acting as a comparison between different membrane types. Fuel cell tests on membranes at low ion exchange capacities were compared to Nafion®117, with suggestions on improvements of copolymer structures for improved performance. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1245–1255, 2010