Investigation of shear stress effect within a spinneret on flux, separation and thermomechanical properties of hollow fiber ultrafiltration membranes

Abstract

The effect of shear rate within a spinneret during hollow fiber spinning on the morphology, flux, separation and thermomechanical properties of polyethersulfone (PES) hollow fiber ultrafiltration (UF) membranes with an outer dense skin made by high polymer concentration has been investigated in this work. Hollow fiber UF membranes were spun by use of a dope solution containing PES/ N-methyl-2-pyrrolidone (NMP)/diethylene glycol (DG) with a weight ratio of 21:40:39. This composition was very close to the cloud point; thus the coagulation of nascent fibers in water was rapid. Wet-spinning process was purposely chosen to make hollow fibers without extra stretching to minimize gravity and elongation stress and a strong precipitant, water, was employed as the external coagulant such that the orientation induced by shear stress within the spinneret could be frozen into the fibers. In order to decouple the effects of inner and outer skin resistance on separation performance, an optimized bore fluid with 86 wt.% NMP in water was used to obtain a hollow fiber membrane with a fully porous inner skin. A constant ratio of dope fluid flow rate to bore fluid flow rate was maintained while the shear rate within the spinneret was increased from 2413 to 19,117 s −1 to minimize the effect of bore liquid flow rate on fiber formation. The results of this study re-confirm our previous hypothesis that “the pore size in the outer dense skin of resulting hollow fiber UF membranes may decrease due to the enhanced molecular orientation when the shear rate within the spinneret increased”. As a consequence, a higher shear rate results in a lower pure water flux, coefficient of thermal expansion (CTE) and elongation of the final fiber, but a higher separation, storage modulus, loss modulus, tensile strength and Young’s modulus. However, contradictory to our previous work on the spinning of low concentration PES dopes, it was found that there was apparently no critical shear rate in both water flux versus shear rate and separation versus shear rate relationships for this high concentration dope. The surprising result may be due to the balance of shear-induced orientation and shear-thinning effect of a high concentration polymer dope on chain packing. These two factors offset each other; thus, no severe jump or drop could be noticed in separation versus shear rate or flux versus shear rate, respectively.