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.