Kinking Matters: meta-Terphenyl Improves Hydroxide Conductivity of Mechanically Robust Fluorine-Free Poly(arylene piperidinium) Copolymers for Anion Exchange Membranes

Journal article, 2025

Poly(arylene piperidinium) (PAP) polymers have emerged as promising candidates for applications as anion exchange membranes (AEMs) and have seen some commercial use in the form of PiperION by Versogen; however, PiperION contains fluorinated units to balance its ionic content. Fluorine-free variants are environmentally more friendly alternatives as recycling is facilitated. Herein, we report a series of four fluorine-free PAP membranes that are mechanically robust and feature moderate water uptake yet high ionic conductivity. p-Quaterphenyl (pQP) is copolymerized with either m- or p-terphenyl (m/pTP) and N-methyl-4-piperidone under superacid-catalyzed polyhydroxyalkylation conditions. The molar ratios of the reactants are adjusted to maintain a balance of solubility and flexibility of the polymers and to reach ion exchange capacities between 2.53 and 2.66 mequiv g^-1. The polymers exhibit thermal stability of Td,95 > 260 °C, Young’s moduli between 0.7 and 1.0 GPa, and ultimate tensile stresses of 50-60 MPa in the dry state. Additionally, under submersion tensile deformation, the Young’s moduli and ultimate tensile stresses are in the range 200-320 MPa and 15-22 MPa, respectively. The sample with an equimolar ratio of pQP and mTP was found to exhibit a robust nature with elongation up to 170% when subjected to submersion tensile deformation, thus showing attractive mechanical properties under relevant working conditions. Wet membranes show an ionomer SAXS peak in the range of 5 nm, suggesting clustering of water and ionic parts of the chain. High hydroxide conductivity of up to 197 mS cm^-1 at 80 °C is observed. Such behavior is promising considering their water uptake of 85% at 80 °C as an upper limit, resulting in moderate areal and through-plane swellings of 100% and 55%, respectively. The results demonstrate that fluorine-free PAPs can be tuned to match important criteria of AEMs, including low water uptake, high dimensional and alkaline stability, and high hydroxide conductivity.