Preparation of a poly(vinyl chloride) ultrafiltration membrane through the combination of thermally induced phase separation and non-solvent-induced phase separation

To regulate the polymer-diluent interaction and control the viscosity of the casting solution, diphenyl ketone (DPK) and a N,N-dimethylacetamide/N,N-dimethylformamide mixture were selected as a combined diluent. Poly(vinyl chloride) (PVC) utlrafiltration membranes, which had sufficient mechanical properties for their practical applications because of their bicontinuous spongy structure, were prepared by a combined process of thermally induced phase separation and non-solvent-induced phase separation. The phase-separation mechanism was analyzed. In an air bath, the cast nascent solution immediately transformed into a transparent gel, and liquid-liquid phase separation was induced by a sudden drop in the temperature before crystallization. An ice-water bath was used to coagulate the membrane. The effects of the DPK and PVC concentrations on the membrane structures and performances were mainly investigated. The results show that an increase in the DPK content made the membrane pores change from fingerlike to spongy. Fully spongy pores formed, and the pores size decreased with increasing PVC concentration.