Na Peng, Chemical and Biomolecular Engineering, National university of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore, Singapore, Natalia Widjojo, Biomolecular and Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore, 117602, Singapore, and Tai-Shung Chung, Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent,, Singapore, 119260, Singapore.
Since the phase inversion process was developed to fabricate membranes in late 1950s, how to fabricate macrovoid-free and defect-free hollow fiber membranes with an ultra-thin dense selective layer for gas separation has always been a scientifically interesting topic. To the present, most literatures on fabricating hollow fiber membranes mainly focus on the effects of process parameters such as dope formulation, coagulant chemistry, dope extrusion rate, elongation stress, as well as air gap distance on membrane morphology and separation performance. However, membrane scientists must not ignore the importance of a spinneret's internal design and annulus dimension, because they play an utmost important role in determining not only the molecular orientation, chain packing, macro- and micro-pore formation of the resultant membranes, but also the membranes' separation performance and long term stability. Therefore, the main objective of this work is to investigate how a spinneret's internal design and annulus gap dimension affect 1) the macrovoid formation and 2) gas separation performance of the resultant hollow fiber membranes. Several spinnerets with different dimensions were designed and several interesting phenomena have been observed.
As for the morphological aspect, four types of macrovoids (i.e. inward- and outward-pointed, elliptical, and tear-drop shapes) can be observed simultaneously in the P84 copolyimide hollow fiber membranes. Hollow fibers spun from thinner spinneret annulus gaps show only inward-pointed long, tear drop, and elliptical shape macrovoids, while those spun from thicker annulus gaps have both inward and outward-pointed macrovoids. In addition, the number of inward-pointed macrovoids increases, while the number of outward-pointed macrovoids decreases with an increase in air gap distance. In terms of the effect of spinneret dimension in the formation of hollow fiber membranes for gas separation, defect-free Torlon® hollow fiber membranes with ultra-thin dense selective layers have been successfully fabricated without any post treatments. It is observed that narrower annular gap of the spinneret facilitates the formation of macrovoid-free hollow fibers, while broader one produces membranes with higher permselectivity and thinner dense selective layer. The highest ideal O2/N2 permselectivity is 9.06, with a dense layer of 540 Ǻ. The average selectivity of CO2/CH4 mixed gas is around 40.
This is a pioneer work in studying the effect of spinneret dimension in the formation mechanism of hollow fiber membranes. Besides providing an integrated understanding of elongation induced molecular orientation and shear induced molecular orientation in different spinnerets for hollow fiber formation, it is also believed our idea in spinneret design may start a new step in improving hollow fiber performance, not only for gas separation but also for all industrial applications.