Gas separation of asymmetric 6FDA polyimide membrane with oriented surface skin layer

A novel asymmetric polyimide membrane with an oriented surface skin layer was prepared by a dry-wet phase inversion process at different shear stresses: The surface morphology and the structure of the membranes were characterized by scanning electron microscopy and atomic force microscopy. Molecular orientation in the membranes was investigated using polarized ATR-FTIR spectroscopy. We found that the shear rate controlled by a doctor blade affects the thickness and morphology of the surface skin layer and the structure of the membrane. The gas permeances of a novel asymmetric polyimide membrane with an oriented surface skin layer were measured using a high-vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg. Both the gas permeance and selectivity of the asymmetric polyimide membranes increased with an increase in the shear rate. This may be due to the fact that the molecular orientation induced in the membrane influenced the diffusion of gas molecules through the oriented skin layer and that the gas diffusivity selectivity increased with decreasing skin layer thickness. Finally, the gas selectivity in the asymmetric polyimide membrane was enhanced. Discussions were carried out on the correlation between the shear rate and the membrane formation or the gas permeability.