Synthesis, Chemical Modifications and Characterizations of 6FDA-NDA/Daba Polyimide Membranes for Pervaporation Dehydration of Ethanol-Water Mixture

A novel polyimide, copoly(1,5- naphthalene/3,5-benzoic acid-2,2'-bis(3,4-dicarboxyphenyl) hexafluoropropanedimide (6FDA-NDA/DABA) has been synthesized and modified via various cross-linking modifications for ethanol dehydration via pervaporation. Three different cross-linking approaches — thermal, diamino and diol cross-linking modifications are employed; their reaction mechanism as well as their effects on membrane morphology, physicochemical properties and separation performance are investigated. Thermal cross-linking is induced by decarboxylation; creates a higher d-space among polymer chains, facilitates the formation of charged transfer complexes (CTCs) and leads to an impressively high flux and a comparable separation factor for ethanol dehydration. Diamino cross-linking modification changes imide groups in the polyimide into amide groups but post heat-treatment converts them back. These combined effects result in the formation of CTCs and the enhanced hydrophilicity, which bring out the improvement in membrane performance. Diol cross-linking also constrains the membrane swelling and results in a higher d-space by the cross-linking spacer, and therefore leading to a higher flux but a lower separation factor than membranes modified by the above two other methods. This work may provide useful insights for polyimide synthesis and cross-linking modifications for biofuel separation.