New type asymmetric membranes having almost defect free hyper-thin skin layer and sponge-like porous matrix

Abstract

We have developed a new type of asymmetric membrane that has a hyper-thin skin layer and sponge-like porous matrix. 6FDA-BAAF polyimide was used as one of the membrane materials. The skin layer had hardly any defects and the thickness of the membrane was 40–60 nm. The skin layer and sponge-like porous matrix, which had a finger-void-free morphology, were formed as one body. The interface between them was found to be clearer than with other asymmetric membranes and the skin layer did not form as a graded density skin. It is suggested that the formation of such hyper-thin skin layers depends on the dissolution property between a dope solvent and water as a solidification solvent. Namely, solvents such as diethylene glycol dimethyl ether (DGDE; used in this study) are miscible in water, but a sharp interface between DGDE and water forms for a while like between water and oil. The hyper-thin skin layer having hardly any defects is possibly formed by solidification at the sharp interface. It is obvious that such solvent properties might depend on the dielectric constant and/or dipole moment. The selectivity and permeation rate of the 6FDA-BAAF polyimide asymmetric membrane had an α of 27 for CO 2 CH 4 and a permeation rate of 1.3 [Nm 3/m 2 h atm] (=5.5×10 −4 [cm 3(STP)/cm 2 s cmHg]) for CO 2. (The maximum value had an α of 35 for CO 2 CH 4 and a permeation rate of 1.6 [Nm 3/m 2 h atm] (=5.8×10 −4 [cm 3(STP)/cm 2 s cmHg])).