Process for forming microporous membranes

A method for forming a microporous membrane sheet having a microstructure, the method comprising:

• a) providing a dope formulation comprising a polymeric material and a solvent;
• b) providing a first coagulant formulation comprising a first coagulant and a first coating adjuvant;
• c) simultaneously casting the dope formulation to provide a dope formulation layer having a first surface and the first coagulant formulation to provide a first coagulant formulation layer under conditions that form an interface between the first coagulant formulation layer and the dope formulation layer, the interface being opposite the first surface of the dope formulation layer, the first coagulant formulation diffusing into the dope formulation through the interface effecting a first phase inversion;
• d) contacting the first surface of the dope formulation layer with a second coagulant which is a vapor simultaneously with the dope formulation and the first coagulant formulation;
• the second coagulant diffusing into the dope formulation layer through the first surface effecting a second phase inversion, thereby forming the membrane sheet having a first microstructure and a second microstructure, wherein the first microstructure is different than the second microstructure and the method is accomplished in the absence of a separate step of exposing the dope formulation layer to a coagulation bath, and
• e) removing the solvent.