Fabrication of fluoropolyimide/polyethersulfone (PES) dual-layer asymmetric hollow fiber membranes for gas separation

By using co-extrusion and dry-jet wet-spinning phase inversion techniques, we have developed delamination-free dual-layer asymmetric composite hollow fiber membranes for gas separation. Delamination-free is essential for dual-layer membranes to withstand high testing pressures. For concept demonstration, a 6FDA-durene-1,3-phenylenediamine (mPDA) (50:50) copolyimide was used to form the outer asymmetric separating layer, while polyethersulfone (PES) was employed to yield the inner interpenetrated porous supporting layer. A special assembly for outer-layer dope passage was firstly introduced into the dual-layer spinneret design in order to enhance the uniformity of dope distribution in nozzle orifice. A much thinner (≈ 10 μm) and uniform outer layer was achieved. The effects of spinning conditions, such as spinneret temperature, air gap, bore fluid chemistry, inner-layer dope concentration and formulation, and solvent exchange on the interface delamination between the dual layers were examined. Inner-layer dope concentration and bore fluid composition as well as the sequent solvent exchange were found to play important roles to produce delamination-free dual-layer membranes. Pure gas test results show that the resultant 6FDA-durene-mPDA/PES dual-layer membranes have an O₂/N₂ selectivity approaching to the intrinsic ideal selectivity value of 4.7 with a permeance of oxygen around 28 GPU (gas permeance unit) at room temperature, indicating the dual-layer hollow fiber membranes are apparently defect-free.