Synthesis of Copoly(aryl ether ether nitrile)s Containing Sulfonic Acid Groups for PEM Application

Abstract

Wholly aromatic poly(aryl ether ether nitrile)s containing sulfonic acid groups (SPAEEN)s, intended for fuel cells applications as proton conducting membrane materials, were first prepared via nucleophilic substitution polycondensation reactions. SPAEEN copolymers were synthesized by potassium carbonate-mediated nucleophilic polycondensation reactions of commercially available monomers: 2,6-difluorobenzonitrile (2,6-DFBN), potassium 2,5-dihydroxybenzenesulfonate (sulfonated hydroquinone SHQ), and a third monomer 4,4‘-biphenol or hydroquinone in N-methyl-2-pyrrolidone (NMP) at 160 °C. The sulfonic acid group content (SC), expressed as a number per repeat unit of polymer, ranged from 0.5 to 1.0 and was obtained by changing the feed ratio of SHQ to the unsulfonated bisphenol. Membrane films in potassium salt and acid forms were obtained by casting N,N-dimethylacetamide (DMAc) solution of SPAEEN copolymers, followed by immersing in 2 N sulfuric acid at room temperature. FT-IR confirmed the structure of polymer in both salt and acid forms. NMR was used to determine the obtained SC values of the SPAEEN series. Glass transition temperatures (Tgs) of SPAEEN copolymers (potassium form) determined by differential scanning calorimetry (DSC) ranged from 308 to 371 °C. Decomposition temperatures (Tds) of SPAEEN were around 300 °C for acid form and over 400 °C for potassium form. Water uptake and swelling ratio values increased with SC and temperature. All SPAEENH copolymers were mechanically stronger than Nafion117 and exhibited a reasonable flexibility. The proton conductivities of acid form membrane at different SC values were close to or higher than that of Nafion117 and reached 10-1 S/cm. The best compromise on PEM mechanical strength, water swelling, and proton conductivity was achieved at SC ranging from 0.5 to 0.7.