Structure control of anisotropic and asymmetric polypropylene membrane prepared by thermally induced phase separation

Anisotropic (gradation in pore size) and asymmetric (dense surface skin) po lypropylene membranes were produced by the thermally induced phase separati on (TIPS) process and solute rejection by these membranes was investigated. Anisotropic membranes were obtained by inducing a polymer concentration gr adient in the polymer-diluent solution prior to phase separation. The conce ntration gradient was established by controlled evaporation of diluent from one side of the solution. The anisotropic membranes showed much higher rej ection coefficients for latex particles with diameter of 100 nm than an iso tropic polypropylene membrane prepared by the TIPS method without evaporati on. Asymmetric structures were achieved by imposing a cooling rate gradient across the membrane. The membrane had a skin layer of about 1 mum thicknes s at the surface that was rapidly cooled. The solute rejection coefficient reached more than 0.95 for lysozyme of molecular weight of 14 600. This res ult shows that ultrafiltration polypropylene membranes can be produced by t he TIPS process if asymmetry is induced during the membrane formation proce ss. (C) 2000 Elsevier Science B.V. All rights reserved.