Formation of surface skin layer of asymmetric polyimide membranes and their gas transport properties

The fabrication and gas-transport properties of asymmetric 6FDA-APPS membranes have been described in this study. The apparent skin-layer thickness strongly depended on the evaporation time, which was of the utmost importance for the formation of an asymmetric membrane with high gas permeance. Surface morphology of the membrane has been determined by atomic force microscopy (AFM). The surface roughness parameters of the membranes analyzed from AFM micrographs increased with a decrease in the apparent skin-layer thickness. Permeance and selectivity of CO2, O-2, N-2, and CH4 for the asymmetric membranes at 35 degrees C and at pressures up to 760 cmHg have been measured. The selectivities for (O-2/N-2) and (CO2/CH4) in the membrane with a skin layer of 34 nm were 6.1 at an O-2 permeance of 5.6 x 10(-5) cm(3)(STP)/cm(2) s cmHg and 39 at a CO2 permeance of 2.7 x 10(-4) cm(3)(STP)/cm(2) s cmHg, respectively, without the necessity of an additional coating process. The asymmetric membrane exhibited gas selectivities equal to or even higher than those determined for the dense membrane. (C) 1997 Elsevier Science B.V.