FORMATION MECHANISM OF THE MACROVOIDS INDUCED BY SURFACTANT ADDITIVES

In the present work, it is shown that addition of surfactants in the c asting solution can induce or suppress macrovoids in polymeric membran es, depending on the miscibility between the added surfactant and the coagulant. Dual-bath experiments, in which casting solution was immers ed in one coagulant for a very short time (<2 s) and then removed to a second bath containing different coagulants, were performed to invest igate the formation process of macrovoids induced by surfactants. The results suggest that the initiation of macrovoids is completed in the first bath and the growth of macrovoids begins in the first bath but c ontinues in the second bath. It was observed that the size of macrovoi ds is insensitive to the coagulant used in the second bath, which can be accounted for by proposing that the growth of macrovoids is mainly due to imbibing solvent from the surrounding polymer solution. To veri fy the above proposal, results from optical study and analysis of the growth speed of macrovoids are also presented. (C) 1998 Elsevier Scien ce B.V.