novel single film bipolar membranes , of substantially improved efficiency and durability , i . e . having an ion selectivity above 80 % in an electrolyte medium of about one mole , are prepared from pre - swollen films containing a relatively high amount , i . e . at least 15 % of an insoluble cross - linked aromatic polymer . under controlled conditions , high dissociable cationic - exchange groups are chemically bonded to the aromatic nuclei to a desired depth of the film from one side only ; subsequently , highly dissociable anion - exchange groups are chemically bonded to the unreacted aromatic nuclei on the other side of the film . the functionalized , densely structured single film ion exchange membrane undergoes negligible degradation , is blister free , and is uniquely suited for electrodialysis particularly water - splitting operations due to its low electrical resistance and low salt leakage . the membrane operates with improved current efficiencies at both high electrolyte concentrations and high current densities , for time periods not previously achievable .