Hydrophobic membrane materials for filter venting applications

A method for preparing a membrane, the method comprising:

• providing a casting dope comprising a solvent and a terpolymer of tetrafluoroethylene, vinylidene fluoride, and hexafluoropropylene;
• casting the dope to form a thin film;
• exposing the film to a humid atmosphere for a period of time sufficient to allow formation of surface pores;
• quenching the film in a quenching bath;
• and
• recovering from the quenching bath a flat sheet membrane, the membrane comprising a terpolymer of tetrafluoroethylene, vinylidene fluoride, and hexafluoropropylene, the membrane having a first porous face having a first average pore diameter, a second porous face having a second average pore diameter, and a porous supporting structure therebetween wherein the supporting structure comprises a reticulated network of flow channels, and wherein the porous faces and the porous supporting structure comprise a network of structural surfaces capable of contacting a filter stream.