Ion clustering in quaternary ammonium functionalized benzylmethyl containing poly(arylene ether ketone)s

Quaternary ammonium functionalized poly(arylene ether ketone)s (QA-PAEKs) with mass-based ion exchange capacities (IEC_g) ranging from 1.12 to 2.88 mequiv g^-1 were synthesized via condensation polymerization of a newly designed highly benzylmethylated bisphenol, subsequent bromination of the benzylmethyl groups, and then quaternization with trimethylamine. The quaternary ammonium groups were densely and selectively anchored on the bis(3,5-dimethyl-4-hydroxyphenyl)-3,5-dimethylphenylmethane residues in the QA-PAEK backbone. ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to confirm the chemical structure of the samples. The morphology, mechanical properties, thermal stabilities, water uptake, swelling ratio, and bromide (Br^-) conductivity of the QA-PAEK membranes were investigated. It was found that QA-PAEK samples had much higher Br^- conductivity than a randomly functionalized quaternary ammonium Radel (QA-Radel) membrane at similar IEC, which was attributed to the existence of distinct ion clusters in the QA-PAEK materials as evidenced by small-angle X-ray scattering (SAXS). The Br^- conductivity of the QA-PAEK membranes increased with increasing quaternary ammonium ion concentration, water uptake, temperature, and conducting volume fraction. The results indicated that ion clustering was important for enhanced ion conductivity, and QA-PAEKs are promising anion exchange membranes for use in energy and water treatment applications.