Effect of wet and dry-jet wet spinning on the sheer-induced orientation during the formation of ultrafiltration hollow fiber membranes

We have demonstrated the effect of wet and dry-jet wet spinning on the sheer-induced orientation during hollow fiber membrane formation by characterizing the permeability, separation performance and thermomechanical properties of hollow fiber ultrafiltration membranes. Both wet-spun and dry-jet wet spun fibers were prepared by using the phase inversion process. An air gap of I cm was chosen for the dry-jet wet spinning process in order to minimize gravity effect. To generalize our conclusion, various hollow fiber UF membranes with different structures were prepared using six spinning dopes with different kinds of polymers, solvents and additives under different shear rates. Experimental results show that pure water flux, coefficient of thermal expansion (CTE) and elongation of the wet spun fibers are lower than that of the dry-jet spun fibers but separation performance. storage modulus, loss modulus and tensile strength of the wet spun fibers are higher. The results indicate that the wet spun fiber has smaller pore size and/or a denser skin than the dry-jet wet spun fiber. These results also confirm our hypothesis that the molecular orientation induced at the outer skin of the nascent fiber by sheer stress within the spinneret can be frozen into the wet-spun fiber but relax in a small air gap region for the dry-jet wet-spun fiber. Experimental results strongly indicate that an air gap of 1 cm does make significant impact to the membrane performance. This conclusion arises from the fact that wet spun fibers has the greater shear-induced molecular orientation (from spinneret) than the dry-jet wet spun fiber and there is strong molecular relaxation in air gap. A hollow fiber UF membrane with high flux of 1220 L/h m(2) bar can be prepared by the approach proposed in this paper. (C) 2001 Elsevier Science B.V. All rights reserved.