Comparison of PEM Properties of Copoly(aryl ether ether nitrile)s Containing Sulfonic Acid Bonded to Naphthalene in Structurally Different Ways

Abstract

The study represents a comparative investigation of three series of high molecular weight aromatic poly(aryl ether ether nitrile) copolymers (SPAEEN-n), containing different sulfonated naphthalene groups, where n (in the range 0−80 mol %) refers to molar percentage of repeat units containing the sulfonic acid function. The three series of SPAEEN-n copolymers were synthesized via potassium carbonate-mediated nucleophilic polycondensation reactions of commercially available monomers: 2,6-difluorobenzonitrile (2,6-BFBN), one of three dihydroxynaphthalenesulfonate (DHNS) monomers, and unsulfonated 4,4‘-biphenol (BP), incorporated for the purpose of adjusting the sulfonic acid content. The incorporation of the sulfonated repeat unit into copolymers and their structures were confirmed by 1H NMR and FT-IR spectroscopy. Products were solvent-cast into membranes of varying ionic content as proton exchange membrane (PEM) materials for fuel cells applications. Ion exchange capacity (IEC), thermal stabilities, swelling, and proton conducting properties of the membranes were investigated in relation to their structures and compared with those of perfluorinated ionomer (Nafion 117). Films of SPAEENH copolymers in acid form all showed high thermal stabilities with glass transition temperatures (Tg) > 224 °C and decomposition temperatures (Td) > 264 °C. In general, the SPAEENH copolymers exhibited low moisture absorption and in some instances high proton conductivities (from 10-2 to 10-1 S/cm) comparable to that of Nafion 117 at similar water uptake, swelling, and H2O/SO3H ratio.