Interfacial morphology between the two layers of the dual-layer asymmetric hollow fiber membranes fabricated by co-extrusion and dry-jet wet-spinning phase-inversion techniques

The dual-layer asymmetric hollow fiber membranes were fabricated with Matrimid® 5218 as the functional material by using co-extrusion and dry-jet wet-spinning phase-inversion techniques. Different polymers polysulfone (PSf), polyethersulfone (PES) and polyetherimide (PEI), were used as support materials. Different organic solvents N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and dimethylformamide (DMF) were used as solvents in the inner layer spinning dopes. The precipitation order of the two layers was estimated from the cloud point curves for the respective systems. The purpose of this paper was to give out a method to control the interface structure of the dual-layer asymmetric hollow fiber membranes. The experimental result shows that if the inner spinning dope demixes prior to the outer spinning dope, dense skin will form both on the outer surfaces of the two layers, while if the outer spinning dope demixes prior to the inner spinning dope, the dense skin on the outer surface of the outer layer will prevent the formation of the dense skin on the outer surface of the inner layer. The skin morphology investigated by scanning electron microscopy (SEM) proves this result. The double dense skin structure of membranes increases the mass transfer resistance and decreases the gas permeance.