Effect of shear rate within the spinneret on morphology, separation performance and mechanical properties of ultrafiltration polyethersulfone hollow fiber membranes

Abstract

The effect of shear rate within a spinneret during hollow fiber spinning on the morphology, permeability and separation performance, and mechanical properties of ultrafiltration polyethersulfone hollow fiber membranes has been investigated. Hollow fiber UF membranes were prepared from a dope solution containing Polyethersulfone (PES)/N-methyl-2-pyrrolidone (NMP)/Diethylene Glycol (DG) with a weight ratio of 13/45/42. This dope formulation was very close to its cloud point (binodal line) in order to speed up the coagulation of nascent fibers as much as possible so that the relaxation effect on molecular orientation was reduced. The wet-spinning process was purposely chosen to fabricate the hollow fibers without drawing and water was used as the external coagulant. Therefore, it is in the belief that the effects of gravity and elongation stress on fiber formation could be significantly reduced and the orientation induced by shear stress within the spinneret could be frozen into the wet-spun fibers. In order to minimize the complicated coupling effects of elongation stress and uneven external solvent exchange rate on fiber formation and the effect of inner skin resistance on separation performance of final fibers while the shear rate within the spinneret was increased from 245 to 2568 s −1, an optimized bore fluid of 86 wt% NMP in water was employed and a constant ratio of dope fluid flow rate to bore fluid flow rate was kept. Experimental results showed that a higher shear rate in the spinneret apparently resulted in a hollow fiber UF membrane with a thicker and/or a denser skin due to a greater molecular orientation. As a consequence, when the shear rate increased, pure water flux, coefficient of thermal expansion (CTE) and elongation of the final fibers decreased, but storage modulus, tensile strength and Young's modulus increased. For the first time, it was found that there was a certain critical value of shear rate below which the separation performance of fibers increased obviously while the flux decreased dramatically with an increase in shear rate but above which the separation decreased slightly while the flux did not change. The results suggested that it was possible to dramatically enhance the production efficiency of hollow fiber UF membranes for the separation of high Mw (>40 K) solutes with the same fiber dimension and similar separation performance by the approach proposed in this paper.