Synthesis and Alkaline Stability of Solubilized Anion Exchange Membrane Binders Based on Poly(phenylene oxide) Functionalized with Quaternary Ammonium Groups via a Hexyl Spacer

Abstract

Anion exchange membranes (AEMs) with hexyl pendant chains were synthesized by Friedel–Crafts acylation of 6-bromo-1-hexanoyl chloride on poly(phenylene oxide) (PPO), followed by the reduction of the ketone with triethylamine, and quaternization with a tertiary amine (trimethylamine, quinuclidine, or 1-methylimidazole). 1H-NMR, 13C-NMR, COSY and HMQC NMR spectroscopies were used to prove the formation of the brominated polymers and to assess the alkaline stability of the AEMs. The alkaline stability of trimethylamine (TMA) based AEMs was evaluated using two separate, complementary experiments. In the first experiment, the AEMs were dissolved in 1 M NaOD+DMSO-d6 and kept at 60°C. 1H NMR spectra revealed signs of AEM degradation (same qualitative result for all the cationic groups) after 2 days in alkaline solution. The ion exchange capacity of TMA-C6-PPO, as calculated by integration of the corresponding NMR peaks, decreased 39% after 30 days (from 1.8 ± 0.1 to 1.1 ± 0.1 mmol/g). In the second experiment, TMA-C6-PPO AEM was immersed in nitrogen degassed 1 M KOH at 60°C for 30 days. The ion exchange capacity of this AEM decreased 33% confirming previous results. The finding that PPO-based AEMs with hexyl spacers degrade in alkali suggests the hypothesis that alkyl spacers can be used to increase the alkaline stability is not valid for all backbones and conditions.