Formation of Poroas Membrane by Phase soparation with Supercritica/CO_2

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Microporous polystyrene membranes were prepared by the phase separation process using the supercritical CO_2 as a nonsolvent for the polymer solution. The thin polymer solution in a laboratory dish was located inside a cell and the supercritical CO_2 was introduced to induce the phase separation. The dry flat microporous membranes were obtained without collapse of the structure after the CO_2 pressure was diminished. Effects of the experimental conditions such as the CO_2 pressure, the polymer concentration and the temperature on the average pore size and membrane porosity were investigated.In the preparation of porous cellulose acetate membrane by phase separation with supercritical CO_2, five kinds of diluents were used and effect of the diluent on the membrane structure and performance was investigated. As the mutual affinity between diluent and supercritical CO_2 decreased, the membrane porosity and the average pore size near the center position increased and the thicker skin layer was formed at the top surface. In all cases, macrovoid was not formed, although the phase separation occurred instantaneously after CO_2 was introduced. This was a contrast to the macrovoid formation in the condition of instantaneous phase separation using water as nonsolvent. The membrane performances such as the solute rejection coefficient and the water permeability were measured for the obtained porous membranes.