Polydimethylsiloxane (PDMS)-graft copolyimides were synthesized by polycondensation of 3,5-bis(4-aminophenoxy) benzyloxypropyl-terminated PDMS macromonomer with 4,4'-hexafluoroisopropylidene diphthalic anhydride (6FDA) followed by chemical imidization to investigate the effects of PDMS segment length of copolyimides on the membrane characteristics, the gas permeability and the pervaporation performance. The copolyimide membranes, which were prepared by solvent-casting method from the chloroform solutions, became insoluble in any solvents after the thermal treatment over 150 degrees C in vacuo. It was confirmed from TEM observation of these membranes that the phase separation of flexible PDMS domains and hard polyimide backbones. The gas permeability coefficients of copolyimide membranes were increased as the PDMS segment length increased, but decreased after thermal treatment at 200 degrees C. It would be due to the chain scission and the rearrangement of PDMS segments in the side chain to become a cross-linked structure, which make the membranes insoluble. From the results of pervaporation of dilute aqueous solutions of organics, it was found that the copolyimide membranes exhibited the excellent permselectivity toward several organics, such as ethanol, acetone, benzene, chloroform and dichloromethane with a high and stable permeation. Furthermore, it was confirmed that the removal of dichloromethane from its saturated aqueous solution was efficiently achieved by the pervaporation. (C) 2016 Elsevier Ltd. All rights reserved.