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

Abstract

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 1.2×10 5, 2.0×10 5, and 4.3×10 5, 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 N 2, O 2, CH 4, and CO 2 for the asymmetric membranes at 35 °C have been determined using a high vacuum apparatus. The permeances of N 2 and O 2 were measured at pressures up to 380 cmHg, and those of CH 4 and CO 2 were done at pressures up to 760 cmHg. The permeances of N 2, O 2 and CH 4 showed almost constant values during the permeation experiments carried out repeatedly. On the other hand, the CO 2 permeance of the asymmetric membrane with the ultrathin skin layer increased on exposure to a CO 2 pressure of 760 cmHg, and the membrane was plasticized by CO 2. However, the CO 2 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.