Synthesis and properties of side-chain-type ion exchange membrane PEEK-g-StSO3Na for bipolar membranes

Side-chain-type ion exchange membranes (PEEK-g-StSO₃Na) were prepared by grafting poly (ether ether ketone) (PEEK) containing propenyl groups with sodium sulfonic styrene (StSO₃Na) and KH570. PEEK was synthesized by the aromatic nucleophilic polycondensation reaction of 4,4′-difluorobenzophenone, bisphenol A and diallylbisphenol A. The synthesized copolymers with the -SO₃Na group on the side chain of polymers possessed high molecular weights. The cross-linking reaction was carried out through a sol-gel reaction of the trimethoxysilane group. The copolymer membranes exhibited excellent mechanical properties due to their aromatic structure extending through the backbone and flexible StSO₃Na aliphatic chains. The ion exchange capacities (IECs) of the membranes ranged from 2.27 to 2.50 mmol g⁻¹ and the water content ranged from 107.2 to 126.1%, with both parameters increasing with StSO₃Na grafting degree. The H⁺ permeability of copolymer membranes increased with increasing IEC, reaching value above 0.3056 mol/L at 2 h, which is higher than that of Nafion^® 117 at the same measurement condition. They displayed reasonably high H⁺ permeability due to the higher acidity of benzoyl sulfonic acid group, the larger interchain spacing, which is available for water occupation, and the lower AC impedance of the bipolar membrane.