The effect of shear rates on gas separation performance of 6FDA-durene polyimide hollow fibers

We have determined the effect of shear rates within a spinneret on gas separation performance of asymmetric 6FDA-durene polyimide hollow fiber membranes. We purposely wet-spun the hollow fibers at different dope flow rates without drawing and with a constant ratio of bore fluid flow rate to dope flow rate in order to study the effect of shear within a spinneret during hollow fiber spinning on fiber morphology and gas separation performance. For the first time, we found there is a V (down and up) pattern for permeance versus shear rate relationship, while a Λ (up and down) pattern for selectivity versus shear rate relationship. At low shear rates, the permeances of non-polar molecules such as H2, O2, N2, and CH4 decrease, while their relative selectivities increase with an increase in shear rates. Once a certain shear rate is reached, all permeances increase, while their selectivities decrease with an increase in shear rates. In low shear rate regions, the decrease in permeance or increase in selectivity with increasing shear rates arises from the better molecular orientation and chain packing induced by shear. With increasing shear in high shear rate regions, the increase in permeance or decrease in selectivity is mainly attributed to relatively porous skin structures induced by the low viscosity nature of a power-law spinning fluid at high shear rates, fracture, and modified thermodynamics and kinetics of phase inversion process. This work suggests there may exist an optimum shear rate to yield optimal membrane morphology for gas separation. To our surprise, an increase in CO2 permeance with increasing shear rates are possibly due to enhanced coupling effect between CO2 and the highly oriented and closely packed fluoropolyimide molecular chains induced by shear.