The morphological evolution of solvent-containing PMMA membranes in various solvent removal processes

In the present work poly (methyl methacrylate) (PMMA) membranes were fabricated by immersing PMMA/1,4-dioxane cast films into the coagulation bath of methanol, ethanol, n-propanol, or n-butanol. Because the alcohols are not strong nonsolvent to PMMA, it was not easy to remove enough solvent from the cast film in the coagulation bath, to avoid the effect of the residual solvent contained in the gelled cast films. Due to the residual solvent, redissolution of the gelled films could occur when they were air dried, and redissolution followed by redemixing was observed when the gelled films were immersed in water bath. The redissolution and redemixing in the water bath was strongly related to the amount of the residual solvent contained in gelled cast film. Also, it was found that, with a fixed time period in the alcohol bath (30 min), the amount of residual solvent in the gelled film was strongly related to the carbon number of the alcohol coagulant. The alcohol with higher carbon number resulted in lower solvent removal rate in the coagulation bath and higher amount of residual solvent. With methanol as the coagulant, the residual dioxane contained in the gelled film was too little to induce any redissolution or morphology change of the gelled film in the water bath. With ethanol, only partial redissolution was observed. For the case that n-propanol was the coagulant of the first bath, complete redissolution of the gelled film, followed by immediate redemixing, was observed in the second coagulation bath (water bath). And with n-butanol as the coagulant, the redemixing was further prolonged to allow for delayed redemixing. When redemixing occurred immediately after redissolution, membranes with a three-layer structure were formed, consisting of a lacy-like top porous layer, a dense middle layer, and sponge-like bottom layer. Mechanisms are also proposed to account for the formation of the three-layer structure.