We prepared novel bipolar membranes (BPMs) consisting of cation and anion exchange layers (CEL and AEL) using radiation-induced asymmetric graft polymerization (RIAGP). In this technique, graft polymers containing cation and anion exchange groups were introduced into a base film from each side. To create a clear CEL/AEL boundary, grafting reactions were performed from each surface side using two graft monomer solutions, which are immiscible in each other. Sodium p-styrenesulfonate (SSS) and acrylic acid (AA) in water were co-grafted from one side of the base ethylene-co-tetrafluoroethylene film, and chloromethyl styrene (CMS) in xylene was simultaneously grafted from the other side, and then the CMS units were quaternized to afford a BPM. The distinct SSS + AA- and CMS-grafted layers were formed owing to the immiscibility of hydrophilic SSS + AA and hydrophobic CMS monomer solutions. This is the first BPM with a clear CEL/AEL boundary prepared by RIAGP. However, in this BPM, the CEL was considerably thinner than the AEL, which may be a problem in practical applications. Then, by using different starting times of the first SSS+AA and second CMS grafting reactions, the CEL and AEL thicknesses was found to be controlled in RIAGP.
Keywords: bipolar membrane; phase separation; radiation induced asymmetric grafting.