The invention relates to a microporous membrane. The membrane can have an average thickness of 23 ?m or more, an air permeability in a range of about 20 sec/100 cm3 to 100 sec/100 cm3, a pin puncture strength of 2,450 mN or more, and a heat shrinkage ratio of 12% or less at 105° C. The membrane can be produced from a polyolefin solution made by combining a membrane-forming solvent and at least one polyolefin resin containing polyethylene having a viscoelastic angular frequency ??0#191 of at least about 0.01 rad/sec.