Synthesis, cross-linking modifications of 6FDA-NDA/DABA polyimide membranes for ethanol dehydration via pervaporation

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 as a pervaporation membrane. Three different cross-linking approaches – thermal, diamino and diol cross-linking modifications – are employed; their effects on membrane morphology and separation performance are investigated with the help of various characterization techniques. Thermal treatment at high temperature (425 °C) facilitates the decarboxylation-induced cross-linking, therefore restricting the membrane swelling, creating a higher d-space among polymer chains, and contributing to high permeation fluxes and comparable separation factors of the resultant membranes for ethanol dehydration. Diamino cross-linking modification changes imide groups in the polyimide into amide groups but post heat-treatment converts them back. However, the interactions among polymer chains are irreversibly changed by the formation of chemical cross-linking and charge transfer complexes, as well as 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.