Large pore synthetic polymer membranes

A method for preparing a polymer membrane, said membrane comprising a first porous skin surface, a second surface, and a porous supporting structure therebetween, wherein the first surface comprises a relatively open pore structure and the second surface comprises a more open pore structure and wherein the supporting structure comprises an isotropic region adjacent the first porous skin surface, the isotropic region having substantially constant pore size, the porous support further comprising an asymmetric region adjacent the isotropic region, the asymmetric region possessing a high degree of asymmetry and comprising from about 50% to about 85% of a thickness of the membrane, the method comprising:

• preparing a metastable casting dispersion comprising a polymer and a solvent for the polymer, and wherein the dispersion comprises a polymer-rich phase and a polymer-poor phase at a selected casting temperature;
• casting the dispersion into a cast layer at the casting temperature;
• contacting the cast layer with a pore forming atmosphere for a period of time sufficient to form pores in the first surface of at least about 1.2 microns;
• quenching the cast layer with a non-solvent quench liquid in which the solvent is miscible and in which the polymer is substantially insoluble to precipitate the polymer as an integral membrane;
• and
• recovering the membrane from the quench liquid.