Synthesis and properties of sulfonated polyarylene ether nitrile copolymers for PEM with high thermal stability

A series of sulfonated polyarylene ether nitrile copolymers (SPEN) were synthesized by the nucleophilic aromatic substitution polymerization of 2, 6-difluorobenzonitrile with different ratios of hydroquinonesulfonic acid potassium salt and bisphenol A in the presence of K₂CO₃. The synthesized SPEN exhibited good solubility in polar organic solvents such as N-methylpyrrolidone, N, N’-dimethylformamide, dimethylacetylamide and dimethylsulfoxide. These copolymers can be easily processed into membranes by a facile solution-casting method. The representative membranes in acid form were obtained and their chemical structures were characterized by Fourier transform infrared analysis. The thermal properties, mechanical properties, proton conductivity, water uptake and ion exchange capacity of copolymer membranes were also investigated. The results showed that they had high glass transition temperatures ranging from 189 °C to 245 °C and good thermal and thermo-oxidative stability with the 5 wt.% loss temperatures in the range of 328–378 °C in nitrogen and 329–379 °C in air, respectively. They also exhibited good mechanical property with the tensile strength in the range of 45–62 MPa in the dry state and 23–45 MPa in the wet state. Furthermore, these copolymer membranes exhibited good water uptake ranging from 9.7 % to 57.74 % and outstanding ion exchange capacity in the range of 0.64–1.99 mmol/g. Thus, the membranes had good proton conductivities in the range of 2.47 × 10⁻⁵–1.71 × 10⁻³ s/cm at room temperature and 55 % RH.