Characterizations of PDMS-graft copolyimide membrane and the permselectivity of gases and aqueous organic mixtures

- Polydimethylsiloxane grafted copolyimides are synthesized for separation membranes.
- The membrane become insoluble by thermal treatment over 150 °C.
- The thermal treatment would course a crosslinking reaction.
- The membrane shows the high gas permeability similar to silicone membrane.
- The efficient removal of VOCs from the aqueous mixtures is achieved by pervaporation.

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 °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 °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.

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