Microporous Poly(methyl methacrylate) Membranes: Effect of a Low-Viscosity Solvent on the Formation Mechanism

The effect of a relatively low viscosity solvent, tert-butyl alcohol, on the formation of microporous poly(methyl methacrylate) (PMMA) membranes through the thermally induced phase separation (TIPS) method has been studied. Both cloud point and spinodal curves of a secondary standard PMMA (M_w = 93 300, M_n = 46 400) in tert-butyl alcohol were determined by optical density methods and thermal analysis. A rather flat cloud point curve was observed for this system which may be associated with hydrogen-bonding effects. In comparison to more viscous systems, it was determined that phase separation by nucleation and growth is more favorable during the quench through the metastable region into the unstable region of a low viscosity system such as PMMA/tert-butyl alcohol. In the case of the PMMA/tert-butyl alcohol system studied here, except near the critical composition, it was not possible to avoid any effects of nucleation and growth during quenching into the unstable region until the PMMA concentration was increased to 25 wt %. In terms of designing a well-defined porous structure with good material strength through the TIPS process, this study illustrates the importance of choosing a system viscous enough to avoid phase separation by nucleation and growth during quenching to the unstable region.