Patent ID: 8414663

Claim:
A method for making a microporous polyolefin membrane, comprising: (1) combining a membrane-forming solvent and a polyethylene resin having a viscoelastic angular frequency ω 0 greater than or equal to 0.01 rad/sec, to form a polyolefin solution, (2) extruding the polyolefin solution to form an extrudate, and cooling the extrudate to a temperature at or below the extrudate's gelation temperature to form a cooled extrudate, (3) stretching the cooled extrudate at a temperature of 118° C. or higher to form a stretched sheet, (4) removing at least a portion of the membrane-forming solvent from the stretched sheet to form a solvent-removed sheet, and (5) either (a) further stretching the solvent-removed sheet in at least one lateral direction of the sheet at a membrane temperature of 127° C. or lower from an initial length in the stretching direction L 1 to a final length in the stretching direction L 2 such that 1.3L 1 <L 2 <1.5L 1 and then heat-setting the re-stretched sheet at 127° C. or lower, or (b) further stretching the solvent-removed sheet in at least one lateral direction of the sheet at a membrane temperature of 127° C. or lower from an initial length in the stretching direction L I to a second length in the stretching direction L 2 such that 1.31L 1 <L 2 <2L 1 , and then annealing under conditions such that 1.3L 1 ≦L 3 ≦1.5L 1 , wherein L 3 represents the length in the stretching direction of the re-stretched sheet after annealing.