Preparation of asymmetric porous membranes of poly(vinyl chloride)

The formation of the poly(vinyl chloride) (PVC) asymmetric porous membranes was investigated as a function of the evaporation time after casting PVC solutions in the mixture of tetrahydrofuran (THF) and water and in dimethylformamide (DMF), respectively. First, the phase diagram of the three components system, including the critical point, was obtained. In the case of the PVC/THF/water system, it was found that the particles composing the polymer lean phases grew by the coarsening in the spinodal decomposition, resulting in the continuous structure and rather homogeneous, non-circular pores were formed on casting from a 6.8% polymer solution. From a 1.8% polymer solution, the particles composed of the polymer-rich phase came out, but they could not make the continuous membrane. However, the nucleation and growth mechanism may hold in this case. After immersing the membrane into water, the membranes became asymmetric after drying. In the case where the PVC solution in DMF was cast, the phase separation occurred cooperatively by the evaporation of DMF and the absorption of water by DMF at high relative humidity conditions. The observation of the PVC solution in the mixture of THF and water under a microscope showed that the particles formed from the polymer-lean phase grew in the matrix of the polymer-rich phase, resulting in the pores surrounded by a continuous, compact structure with a similar morphology to the PVC/THF/water system, which is considered to come from the polymer-rich phase. The flux of methanol through the membrane and accordingly, the mean pore radius showed a maximum with the evaporation time, at the cloud point. The decrease in the mean pore radius after the cloud point seems to be due to the shrinkage of the membrane during the precipitation and the drying. (C) 1997 Elsevier Science Ltd.