Sulfonated polyimides as proton conductor exchange membranes.: Physicochemical properties and separation H+/Mz+ by electrodialysis comparison with a perfluorosulfonic membrane

The properties of new sulphonated polyimide membranes (SP) - ion exchange isotherms, electrical conductivity, selectivity and proton-cation electrotransport - are compared with that of perfluorinated Nafion(R) membrane. Both membranes when in contact with H+/Mz+ aqueous solutions (MzM+=Na+, CU2+, Cr3+) present an affinity to cations which increases with their valencies; however the affinity of one of the SP membranes for protons is approximately 10% higher than that of Nafion(R). The proton transport number is also 10% higher for this SP membrane than for Nafion(R). Using SP membranes for electrodialysis of H+/Cu2+ solutions produces solutions about 10% more concentrated in H+ and about 40% less concentrated in CU2+ ions than with Nafion(R) membranes in the same conditions. The difference in conductivity may be explained by differences in ionic clustering because of differences in the polymer structure of the two membranes, block copolymer for SP and statistic copolymer for Nafion(R). The difference in properties for the two SP membranes is explained in terms of the chemical structure of the non-sulfonated diamine groups used in the polymer synthesis. These SP membranes seem to be interesting materials for electromembrane processes in acid media. (C) 1999 Elsevier Science B.V. All rights reserved.