Formation of anistripic membrane by the hybrid process with thermally induced phase separation and nonsolvent induced phase separation

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Anisotropic membranes were produced by the hybrid process with both thermally induced phase separation and dry process. To induce an anisotropic structure, diluent was evaporated from one side of the polymer-diluent melt-blended, thereby creating a concentration gradient in the nascent membrane prior to quenching and inducing phase separation. The system used to prepare these membranes was isotactic polypropylene (iPP) in diphenyl ether.In the case of no evaporation, nearly isotropic structures were obtained. This indicates that the cooling rates at both surfaces were controlled to be nearly equal. The evaporation process was found to be useful in producing anisotropic structures. The effects of evaporation time and initial polymer concentration on the anisotropic membrane structure were investigated. The pore size at the top surface decreased by increasing evaporation time.The effects of thermal history, such as cooling rate and quench temperature on membrane structure were investigated. When cooled in air, decreasing the quench temperature brought about an increase in the cooling rate, which led to a decrease in the average cell diameter at the bottom surface. The combined use of a thermal gradient and concentration gradient produced pronounced asymmetric structures with a skin layer at the top surface.The evaporation process was analyzed by solving appropriate mass transfer and heat transfer equations. The agreement between the calculated results and the experimental data on the membrane weight loss and the membrane thickness was satisfactory. The anisotropic structures were discussed in detail based on the calculated polymer volume fraction profiles in the membranes.