Roles of phase separation mechanism and coarsening in the formation of poly(Methyl methacrylate) asymmetric membranes

The effects of phase separation mechanism and flow properties on asymmetric poly(methyl methacrylate) (PMMA) membranes formed by thermally induced phase separation have been studied. Both cloud point and spinodal curves of a secondary standard PMMA (M_w = 93 300, M_n = 46 400) in sulfolane were determined by optical density methods and differential scanning calorimetry. In the absence of ripening/coalescence, an excellent agreement between the conditions of phase separation (in either the metastable or unstable region) and the membrane morphology observed by scanning electron microscopy is obtained. During the very early stage of phase separation by spinodal decomposition, a membrane with lacy structure results. In contrast, a structure composed of strings of small beads is a consequence of the early stage of nucleation and growth. The effect of ripening or coalescence has been demonstrated to be significant in determining the final membrane morphology for phase separation by both spinodal decomposition and nucleation and growth. A lacy structure can also be obtained for nucleation and growth due to the ripening or coalescence effect.