Cationic Side-Chain Attachment to Poly(Phenylene Oxide) Backbones for Chemically Stable and Conductive Anion Exchange Membranes

A series of chemically stable and ionically conductive side-chain anion exchange membranes (AEMs) based on poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) backbones and alkyltrimethylammonium cations are reported in this work. Two alkyltrimethylammonium groups with n-propyl (C-3) and n-pentyl (C-5) alkyl chains were tethered onto PPO backbones through secondary amine moieties, resulting in two side-chain AEMs, that is, NC(3)Q-PPO and NC(5)Q-PPO and NC5 Q-PPO. In comparison to benzylic QA groups (e.g., benzyltrimethylammonium cations in quarternized PPO (QPPO) and benzylalkyldimethylammonium cations in comb-shaped PPOs (i.e., QC(3)-PPO and QC6-PPO)), the alkyltrimethylammonium cations of the side-chain PPOs, which do not possess highly reactive benzylic protons adjacent to both the aromatic ring and the cation, showed superior alkaline stability. After 30 days of aging in 1 mol/L NaOH solution at 80 degrees C, the retention of the conductivities of NC(3)Q-PPO (IEC = 2.17 mmol/g), NC(5)Q-PPO-40 (IEC = 2.03 mmol/g), and NC(5)Q-PPO-60 (IEC = 2.57 mmol/g) were 73.1%, 89.9%, and 81.2% compared with 39.8%, 41.2%, and 56.5% for the QPPO-40 (IEC = 2.27 mmol/g), QC(3)-PPO-40 (IEC = 2.22 mmol/g), and QC6-PPO-40 (IEC = 2.13 mmol/g) samples, respectively. In addition to good stability, the side-chain NC(5)Q-PPO-40 and NC(5)Q-PPO-60 with longer spacers between the aromatic polymer backbone and the cation exhibited high conductivities of 73.9 and 96.1 mS/cm at 80 degrees C in liquid water, while the swelling ratios were limited to 15% and 28%. The flexible linear spacer in NC(5)Q-PPO membranes induced distinct hydrophilic/hydrophobic microphase separation, which enhanced the physical properties of the membranes. Thus, we believe that the NC(5)Q-PPO-type AEMs derive their superior performance from both their unique chemical structures with n-pentyl cationic tethers and the microphase-separated morphologies of the materials driven by the side chain architecture.