Water vapour transport mechanism in naphthalenic sulfonated polyimides

The morphology and the water transport properties of a large series of sulfonated copolyimides synthesised with a naphthalenic dianhydride, a sulfonated diamine and various non-sulfonated diamines were studied. For a sulfonated diamine content in the total diamine amount higher than 30%, the copolymers are composed by a continuous ionic phase and a dispersed non-sulfonated phase. The water sorption isotherms are of BET type II for all the copolymers and they result from the contribution of both the sulfonated and non-sulfonated phases. For the sulfonated phase, a two step sorption mechanism has been identified. It consists at low activity in sorption on ionic sites but also in unrelaxed holes formed during the triethylammonium/H+ ionic exchange and at high activity in a clustering process. The contribution of the non-sulfonated phase to the transport phenomenon can be modulated by the hindrance brought by the non-sulfonated diamine and by its more or less hydrophobic character.