Factors controlling surface morphology of porous polystyrene membranes prepared by thermally induced phase separation

A special device for preparing porous polymer membranes through a thermally induced phase separation (TIPS) process was designed and machined; it included a solution container, a membrane-forming platform, a coldplate, a temperature-decreasing system and a temperature-supervising system. Polystyrene was selected as the model polymer from which to prepare porous membranes using the device due to its better understood TIPS and good biocompatibility with cells. The major factors controlling surface morphology and cell size, ie volume fraction of polystyrene (φ₂), quench rate and solvent-removing methods, were studied. Fixing the coldplate temperature, when φ₂ is as low as 0.045, provokes the formation of round pores on both the bottom and top surfaces of the membrane; when φ₂ = 0.16 no pores are formed on either surface; when φ₂ = 0.087 pores form on the top surface, but not on the bottom surface. When φ₂ = 0.087 the cell size is very small or no pores are formed on the bottom surface, whereas the top surface shows a regular decrease of the pore sizes and an increase of the pore number and pore area, along with a decrease of the coldplate temperature. The side near the coldplate is dense, and the dense layer aligns along the coldplate, while the side away from the coldplate is like a porous foam, the shape of which is isotropic and the surfaces are interconnected with each other three dimensionally. On the top surface of a membrane obtained by ethanol extraction, the cell size is enlarged and the cell number reduced, but the surface morphology and the whole area remained almost the same when compared to samples obtained by freeze drying in the same membrane-forming conditions. The isotropic, uniformly distributed and round pores suggest that the mechanism of phase separation is a spinodal liquid-liquid decomposition under our research conditions.