Ultrathin, cross-linked polyimide pervaporation membranes prepared from polyelectrolyte multilayers

Alternating electrostatic adsorption of poly(amic acid) salts and polycations on porous alumina supports followed by heat-induced imidization yields ultrathin, polyimide membranes. Moreover, incorporation of additional carboxylic acid groups in the poly(amic acid) allows cross-linking via reaction of these groups with the amines of the polycation. FTIR spectroscopy confirms both full imidization and formation of amide cross-links after heating at 250 °C for 2 h, while scanning electron microscopy reveals uniform, ∼50 nm thick films based on 7.5-12.5 poly(amic acid)/polycation bilayers. Pervaporation was investigated as a function of cross-linking by varying either the polycation, i.e., poly(allylamine hydrochloride), polyethylenimine, or poly(diallyldimethylammonium chloride), or the number of cross-linkable groups in the poly(amic acid). Maximum cross-linkage and selectivities occur with imidized films prepared from poly(allylamine hydrochloride) and a poly(amic acid) that contains diaminobenzoic acid in each repeat unit. Such membranes exhibit water/alcohol selectivities of 1100 and 6100 for solutions containing 10 and 90% isopropanol, respectively, and the minimal thickness of the multilayer films still allows these selectivities to occur at fluxes of 11 and 2 kg m−2 h−1, respectively. Fluxes are essentially the same for water/ethanol solutions, but selectivities decrease to 100 and 500 for 10 and 90% ethanol, respectively.