Synthesis and alkaline stability of novel cardo poly(aryl ether sulfone)s with pendent quaternary ammonium aliphatic side chains for anion exchange membranes

Novel cardo poly(aryl ether sulfone)s containing pendent –[CH2CH2NMe3]+OH−, or –[CH2CH2CH2NMe3]+OH− groups, and partially fluorinated cardo poly(aryl ether sulfone) containing pendent –[CH2CH2CH2NMe3]+OH− groups were synthesized and examined with the focus of understanding how the polymer chemical structure affects their morphology, ion conductivity and alkaline stability. The resulting quaternized polymers exhibited outstanding solubility in polar aprotic solvents. The partially fluorinated cardo poly(aryl ether sulfone)s (PES-PF-OH) with ion-exchange capacity of 1.44 meq g−1 displayed the highest ion conductivities, varied from 3.0 × 10−2 to 4.1 × 10−2 S cm−1 over the range 20–60 °C. TEM revealed that PES-PF-OH membrane exhibited a distinct phase-separated morphology comprised of interconnected ionic clusters in size of 1–2 nm. The studies on alkaline stability of the membranes revealed that the PES-PF-OH polymer was not stable under strong basic conditions. The quarternized polymers containing pendent –[CH2CH2NMe3]+OH−, or –[CH2CH2CH2NMe3]+OH− groups mainly underwent Hofmann elimination and SN2 substitution reaction.