Patent ID: 8008417

Claim:
A method for producing a microporous membrane comprising the steps of: combining a diluent or solvent and a first polyolefin composition to prepare a first polyolefin solution, the first polyolefin composition comprising greater than about 90 mole % propylene monomer and having one or more of the following: i. an intrinsic viscosity greater than about 10 dl/g; ii. a heat of fusion greater than about 108 J/g, a melting point of 165° C. or greater; iii. a molecular weight greater than about 1.5×10 6 ; iv. a molecular weight distribution of from about 2.5 to about 7; v. a Melt Flow Rate at 230° C. of less than about 0.01 dg/minute; vi. an amount of extractable species of 0.5 wt. % or less based on the weight of the polyolefin composition; vii. a meso pentad fraction of greater than about 96 mol % mmmm pentads; and viii. an amount of stereo defects less than about 50 per 10,000 carbon atoms; extruding the first polyolefin solution through at least one die to form an extrudate; removing at least a portion of the membrane-forming solvent from the cooled extrudate or extrudate to form a solvent-removed sheet; and drying the sheet to form the microporous membrane.