Patent ID: 8147732

Claim:
A method of producing a microporous polymer film comprising: dissolving a polymer having a polymer free surface energy in a low surface tension liquid to form a polymer solution; forming an approximately saturated solution by adding, to the polymer solution, a high surface tension liquid having a surface tension equal to or in excess of the polymer surface free energy; the forming being conducted at a temperature less than or equal to the lower of a boiling temperature of the approximately saturated solution or a melt temperature of the polymer; placing the approximately saturated solution in a layer at the forming temperature on a generally planar substrate having a substrate surface free energy at least equal to the surface tension of the low surface tension liquid; urging the low surface tension liquid into a non-wetting relation to the polymer by cooling the approximately saturated solution to increase surface tension of the solvent to form a gelled polymer having a surface tension which exceeds the surface free energy of the polymer material, causing the low surface tension liquid to tend to bead within the gelled polymer; and evaporating the low surface tension liquid from the gelled polymer to form a film defining a plurality of pores, such that sufficient low surface tension liquid is partially removed from the gelled polymer to form the film defining the plurality of pores, the low surface tension liquid being evaporated at temperatures not to exceed the lower of a boiling temperature of the approximately saturated solution and of the melt temperature of the polymer.