Patent ID: 8273279

Claim:
A microporous membrane comprising from about 75 to about 99% of a 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 10 to about 100, and from about 1 to about 25% polypropylene having a weight average molecular weight of from about 1×10 4 to about 4×10 6 , a heat of fusion of 80 J/g or higher, and a molecular weight distribution of from about 1 to about 100, percentages based on the mass of the membrane, wherein the membrane comprises pores characterized by a pore size distribution curve determined by mercury intrusion porosimetry, the curve representing a number of pores on a y-axis and pore size on an x-axis and having at least two peaks and 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 course domains corresponding to at least one sub-peak in a range of more than 0.08 μm to 1.5 μm or less in the pore size distribution curve such that the dense and coarse domains are irregularly entangled in any cross section of the membrane viewed in both longitudinal and transverse directions.