EFFECT OF SURFACTANTS ON THE STRUCTURE OF PMMA MEMBRANES

In this work, the role played by surfactants in the formation of PMMA membranes was investigated. For a wet inversion process with PMMA as p olymer, acetone as solvent, and water as coagulant, delayed demixing w as observed and the resulting membrane possessed a sponge-like structu re. Adding 1.8 vol% of Tween 80 in the casting solution (PMMA + aceton e) could shift the PMMA/acetone/water system from delayed demixing to instantaneous demixing and could induce macrovoids with small pin-hole s on the membrane surface. It was found that the affinity of surfactan t for coagulant is crucial for the enhancement of the formation of mac rovoids. When water was the coagulant, adding the surfactants with hig h water solubility in PMMA/acetone solution can enhance the formation of macrovoids while adding the surfactants with low water solubility h as no effect. On the other hand, for a lipophilic coagulant such as n- hexane, lipophilic surfactants were more effective in changing membran e structure than hydrophillic surfactants. In addition, a mechanism de scribing the role of surfactants in the formation of macrovoids is pro posed: the addition of appropriate surfactants can enhance the affinit y between solvent and coagulant, resulting in a shift from delayed dem ixing to instantaneous demixing, and macrovoids can then be induced.