Macrovoids in Hybrid Organic/Inorganic Hollow Fiber Membranes

Large, characteristically tear- or finger-shaped voids found in polymer hollow fiber membranes, termed macrovoids, have been known since the early development of asymmetric membranes. These voids are undesirable as they decrease the mechanical integrity of the hollow fiber membrane, limiting the use of high pressure feeds for separations. Moreover, if such macrovoids penetrate the selective layer they become and even more serious problem for membrane performance. With current membrane technology moving toward a mixed-matrix hybrid format with inorganic and carbon molecular sieves embedded in a polymer matrix, additional complications caused by the presence of particulates within the spinning dope toward the formation of macrovoids must be considered. While numerous hypotheses have been suggested regarding the formation of macrovoids formed in polymer-only membranes, no mention so far has been made of such macrovoids initiated in hollow fiber membranes by particles present in the spinning dope. This paper provides evidence for the presence of such macrovoids for such a spinning dope and presents a hypothesis for their formation. This hypothesis is presented in terms of a similar basic mechanism for macrovoid formation in mixed-matrix membranes that benefits from an understanding of macrovoid formation in conventional polymer-only asymmetric membranes. Moreover, decreasing the sieve particle size tends to suppress macrovoid formation tendency, which is consistent with the proposed mechanism.