Patent ID: 8021789

Claim:
A microporous membrane comprising (a) from about 74 to about 99% of a first polyethylene having a weight average molecular weight of from about 2.5×10 5 to about 5×10 5 and a molecular weight distribution of from about 5 to about 100, (b) from about 1 to about 5% of a second polyethylene having a weight average molecular weight of from about 5×10 5 to about 1×10 6 and a molecular weight distribution of from about 5 to about 100, and (c) from 0 to about 25% of a polypropylene having a weight average molecular weight of from about 3×10 5 to about 1.5×10 6 , a molecular weight distribution of from about 1 to about 100, and a heat of fusion of 80 J/g or higher, percentages based on the mass of the membrane, wherein the membrane 1) comprises pores characterized by a pore size distribution curve, wherein differential pore volume is plotted on a y axis and pore diameter on an x axis, having at least two peaks, and 2) contains dense domains corresponding to a main peak in a range of 0.01 μm to 0.08 μm in the pore size distribution curve and coarse domains corresponding to at least one sub-peak in a range of more than 0.08 μm and 1.5 μm or less in the pore size distribution curve.