Effect of shear stress within the spinneret on hollow fiber membrane morphology and separation performance

Abstract

The effects of shear stress and shear experience within a spinneret during hollow fiber spinning on membrane morphology, gas separation performance, and thermal and mechanical properties have been experimentally determined. We purposely spun the hollow fibers using a wet phase inversion process and water as the external coagulant with the belief that the effect of gravity (elongational stress) on fiber formation can be significantly reduced and the orientation induced by shear stress within the spinneret can be frozen into the wet-spun fibers. In addition, we chose 80/20 NMP/H2O as the bore fluid with a constant bore fluid to dope fluid flow rate ratio in order to minimize the complicated coupling effects of elongational stresses, uneven internal and external solvent exchange rates, and substructure resistance on fiber formation and separation performance. Asymmetric hollow fibers for gas separation were spun from a 37% poly(ether sulfone) (PES)/N-methyl-2-pyrrolidone (NMP) dope solution using a spinneret with a L/ΔD (die length to flow channel gap) ratio of 17.5 that is much higher than the conventional spinneret. Experimental results suggest that hollow fiber membranes spun from this large L/ΔD die with high shear have a tighter molecular packing structure and therefore a higher selectivity that surpasses the intrinsic value but a lower permeance. For example, the selectivity of H2/N2 for fibers spun with high shear rate is 4-fold of the PES intrinsic value (292-307 vs 73.7). Hollow fibers spun from high shear have a lower coefficient of thermal expansion (CTE) and a higher loss modulus. Most surprisingly, we are not able to identify the nodular structure that has been observed previously in the as-cast flat membranes or at the outer skin of the hollow fibers spun from the spinneret with a small L/ΔD ratio. Clearly, the fully developed high shear stress within the spinneret has altered the thermodynamics of nodular formation, and the nodules either might not exist or become too small to be detected or deform into ambiguous elliptical shape. For the first time, we have also observed a threadlike inner skin structure in high-sheared membranes. In addition, the apparent dense layer thickness for the fiber spun with low shear is the thinnest that has ever been reported in the literature for hollow fiber membranes (450 Å).