We have prepared a series of new copolyimide membranes by varying the ratio of flexible siloxane diamines to rigid fluorinated aromatic diamines, and have examined the influence of the diamine ratio on the physical and pervaporation properties of trichloroethylene (TCE) from water. The membranes were synthesized from 4,4'-(hexafluoroisopropylidene) diphthalic anhydride, bis(3-aminopropyl) terminated poly(dimethylsiloxane) (SIDA), and 2-(perfluorohexyl) ethyl- 3,5-diaminobenzoate (PFDAB) using a one-step thermal imidization process. Nuclear magnetic resonance and ATR/IR observations confirmed that the polyimides with different SIDA/PFDAB ratios were successfully prepared, and thermal analysis showed that the copolyimides exhibited two glass transition temperatures, indicating that they were micro-phase separated. The crystallographic d spacing, fractional free volume, and surface free energy of the polyimides were measured as a function of SIDA content. The effect of the SIDA content on the membranes' sorption, diffusion, and permeation properties of TCE/water mixtures was also investigated. (C) 2004 Elsevier B.V. All rights reserved.