Influence of composition on the proton conductivity and mechanical properties of sulfonated poly(aryl ether nitrile) copolymers for proton exchange membranes

A series of sulfonated poly(arylene ether nitrile) (SPEN) copolymers with controlled degrees of sulfonation were successfully synthesized by the direct copolymerization of hydroquinonesulfonic acid potassium salt (SHQ), 2,6-difluorobenzonitrile (DFBN), and different proportions of bisphenols with different structures. Five bisphenols (bisphenol A, phenolphthalein, phenolphthalin, biphenol, and hydroquinone) were investigated for the syntheses of novel copolymers with controlled degrees of sulfonation and different compositions. The composition and structures of the SPEN copolymers were characterized by Fourier transform infrared spectroscopy. Due to their different structural units, the derived copolymers showed different glass transition temperatures of 171–199 °C, and also exhibited high thermal stability, with their 5 % weight loss temperatures ranging from 277 °C to 327 °C. Moreover, they all showed good flexibility and film-forming properties along with excellent tensile strengths of 51–67 MPa in the dry state and 18–41 MPa in the wet state. Solubility tests confirmed that the SPEN copolymers possess good solubility in polar solvents such as NMP, DMAc, DMF, and DMSO. Furthermore, these copolymer membranes exhibited good water uptake values ranging from 30.1 % to 71.2 %, and outstanding ion exchange capacities of 1.27–2.32 mmol g⁻¹. Thus, the membranes presented good proton conductivities of 2.2 × 10⁻⁴ to 4.3 × 10⁻³ S cm⁻¹ at 25 °C and 100 % RH. Furthermore, the SPEN copolymer membranes showed much lower methanol permeabilities and higher selectivities than Nafion 117. All of these attributes indicate that these SPEN copolymers are promising candidates for application in high-temperature proton exchange membranes.