improved novel single film bipolar membranes having an ion selectivity above 80 % in an electrolyte medium of about one mole , are prepared containing a relatively high amount , i . e . at least 15 % of an insoluble cross - linked aromatic polymer . before the styrenated sheet is chemically functionalized , the film surface is specifically treated , i . e . cleaned to make it more receptive to functional groups , so that the cationic - exchange groups and anion - exchange groups can be introduced homogeneously from each side of the surface - cleaned film , and a single - film bipolar membrane of superior durability , stability , and quality control results .