EFFECT OF SHEAR-STRESS WITHIN THE SPINNERET ON HOLLOW-FIBER MEMBRANE MORPHOLOGY AND SEPARATION PERFORMANCE

The effects of shear stress and shear experience within a spinneret du ring hollow fiber spinning on membrane morphology, gas separation perf ormance, and thermal and mechanical properties have been experimentall y determined. We purposely spun the hollow fibers using a wet phase in version process and water as the external coagulant with the belief th at the effect of gravity (elongational stress) on fiber formation can be significantly reduced and the orientation induced by shear stress w ithin the spinneret can be frozen into the wet-spun fibers. In additio n, 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 co upling 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 separati on were spun from a 37% poly(ether sulfone) (PES)/N-methyl-2-pyrrolido ne (NMP) dope solution using a spinneret with a L/Delta D (die length to flow channel gap) ratio of 17.5 that is much higher than the conven tional spinneret. Experimental results suggest that hollow fiber membr anes spun from this large L/Delta D die with high shear have a tighter molecular packing structure and therefore a higher selectivity that s urpasses the intrinsic value but a lower permeance. For example, the s electivity of H-2/N-2 for fibers spun with high shear rate is 4-fold o f the PES intrinsic value (292-307 vs 73.7). Hollow fibers spun from h igh shear have a lower coefficient of thermal expansion (CTE) and a hi gher loss modulus. Most surprisingly, we are not able to identify the nodular structure that has been observed previously in the as-cast fla t membranes or at the outer skin of the hollow fibers spun from the sp inneret with a small L/Delta D ratio. Clearly, the fully developed hig h shear stress within the spinneret has altered the thermodynamics of nodular formation, and the nodules either might not exist or become to o small to be detected or deform into ambiguous elliptical shape. For the first time, we have also observed a threadlike inner skin structur e in high-sheared membranes. In addition, the apparent dense layer thi ckness for the fiber spun with low hear is the thinnest that has ever been reported in the literature for hollow fiber membranes (450 Angstr om).