Formation and evolution of a bicontinuous structure of PMMA membrane during wet immersion process

In this study, a bicontinuous structure of poly(methyl methacrylate) (PMMA) membrane was obtained via wet immersion method by coagulating PMMA/1,4-dioxane casting film into alcohol bath. Freeze-dried method was utilized to investigate the evolution of membrane structure at various coagulation times. The FTIR-microscope, scanning electronic microscope (SEM), ternary phase diagram as well as light transmission were used to investigate the formation and structure evolution mechanism of PMMA membrane. The results show that the bicontinuous structure can be formed on the top surface of PMMA membrane as contacted the PMMA/1,4-dioxane casting film with n-propanol only 2 s. Furthermore, it also can be found that as increasing the immersion time in n-propanol bath over than 1.5 min, the bicontinuous structure would be broken up to form nodules morphology due to capillarity effect, or led to a matrix/disperse domain morphology by coalescence effect. The analysis of FTIR-microscopy shows that the PMMA polymer concentration in the top layer of nascent membrane was decreased as immersed the casting film into n-propanol bath. The lower PMMA polymer concentration in the top layer could cause the composition path entered into the unstable region, resulting in the formation of bicontinuous structures.