Low-swelling proton-conducting copoly(aryl ether nitrile)s containing naphthalene structure with sulfonic acid groups meta to the ether linkage

• Published in: Polymer (Guilford) 2006, Vol.47 (3), p.808-816
• Main Authors: Gao, Yan, Robertson, Gilles P., Guiver, Michael D., Mikhailenko, Serguei D., Li, Xiang, Kaliaguine, Serge
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Nucleophilic polycondensation; Organic polymers; Physicochemistry of polymers; Poly(aryl ether ether nitrile); Polymers with particular properties; Preparation, kinetics, thermodynamics, mechanism and catalysts
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2005.12.015

Wholly aromatic poly(aryl ether ether nitrile)s containing naphthalene structure with sulfonic acid groups meta to ether linkage ( m-SPAEEN), intended for fuel cells applications as proton conducting membrane materials, were prepared via nucleophilic substitution polycondensation reactions. The incorporation of rigid naphthalene structure with meta-sulfonic acid groups was with the intent of improving the aggregation of hydrophilic and hydrophobic domains and to increase the acidity and conductivities. m-SPAEEN copolymers were readily synthesized by potassium carbonate mediated nucleophilic polycondensation reactions of commercially available monomers: 2,6-difluorobenzonitrile (2,6-DFBN), 2,8-dihydroxynaphthalene-6-sulfonate sodium salt (2,8-DHNS-6), and 4,4′-biphenol (4,4′-BP) in dimethylsulfoxide (DMSO) at 160–170 °C. The sulfonic acid group content (SC), expressed as a number per repeat unit of polymer, ranged from 0 to 0.6 and was readily controlled by changing the feed ratio of 2,8-DHNS-6 to 2,6-DFBN. High thermal stability of m-SPAEEN copolymers was indicated by observed glass transition temperatures ( T gs) ranging from 223 to 335 °C in sodium salt form and from 230 to 260 °C in acid form ( m-SPAEENH) and decomposition temperatures ( T d)s over 250 °C in acid form and over 350 °C in sodium form in both nitrogen and air. All m-SPAEENH copolymers exhibited reasonable flexibility and tensile strength in the range of 39–78 MPa, indicating they were mechanically stronger than Nafion ®117, which had an approximate value of 10 MPa under the same test conditions. As expected, m-SPAEENH copolymers showed considerably reduced moisture absorption compared to previously prepared sulfonated hydroquinone based poly(aryl ether nitrile). m-SPAEENH copolymers also showed improved proton conductivities. Proton conductivity curves parallel to that of Nafion 117 were obtained with proton conductivity of 10 −1 S/cm at equivalent ion exchange capacities (IEC) of 1.6 and 1.9, comparable to Nafion ®117. The best compromise combining PEM mechanical strength, water swelling and proton conductivity, was achieved at SC of 0.5 and 0.6.