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Anisotropic (gradation in pore size) and asymmetric (dense surface skin) polypropylene membranes were produced by the thermally induced phase separation (TIPS) process and solute rejection by these membranes was investigated. Anisotropic membranes were obtained by inducing a polymer concentration gradient in the polymer-diluent solution prior to phase separation. The concentration gradient was established by controlled evaporation of diluent from one side of the solution. The anisotropic membranes showed much higher rejection coefficients for latex particles with diameter of 100nm than an isotropic polypropylene membrane prepared by the TIPS method without evaporation. Asymmetric structures were achieved by imposing a cooling rate gradient across the membrane. The membrane had a skin layer of about 1μm thickness at the surface that was rapidly cooled. The solute rejection coefficient …