Gas permeation stability of asymmetric polyimide membrane with thin skin layer: effect of molecular weight of polyimide

The gas permeation stability of an asymmetric polyimide membrane with a thin and defect-free skin layer has been investigated. The molecular weights of the polyimides used in this study were 1.2×105, 2.0×105, and 4.3×105, and the asymmetric polyimide membranes were prepared by a dry/wet phase inversion. The apparent skin layer thickness of asymmetric membranes was controlled from 0.05 to 3.5 μm by changing the weight of the non-solvent in the casting solutions and the free standing time. The gas permeances of N2, O2, CH4, and CO2 for the asymmetric membranes at 35 °C have been determined using a high vacuum apparatus. The permeances of N2 and O2 were measured at pressures up to 380 cmHg, and those of CH4 and CO2 were done at pressures up to 760 cmHg. The permeances of N2, O2 and CH4 showed almost constant values during the permeation experiments carried out repeatedly. On the other hand, the CO2 permeance of the asymmetric membrane with the ultrathin skin layer increased on exposure to a CO2 pressure of 760 cmHg, and the membrane was plasticized by CO2. However, the CO2 permeation stability of the asymmetric polyimide membranes significantly depended on the molecular weight, and plasticization of the membrane prepared from the high molecular weight polyimide was not observed.