Patent ID: 8304114

Claim:
A monolayer microporous polyolefin membrane comprising (a) from about 73 to about 97% high density 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 3 to about 20% polypropylene having a weight average molecular weight of from about 3×10 5 to about 1.5×10 6 , a heat of fusion of 80 J/g or higher, and a molecular weight distribution of from about 1 to about 100, and (c) from about 0 to about 7% ultra-high molecular weight polyethylene resin having a weight average molecular weight of about 5×10 5 or higher, percentages based on the mass of the polyolefin membrane composition, wherein the membrane comprises 1) pores measured by mercury intrusion porosimetry and characterized by a pore size distribution curve determined by the number of pores on a y-axis and pore size on an x-axis and having at least two peaks and 2) dense domains corresponding to a main peak in a range of 0.01 μm and 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 such that the dense and coarse domains are irregularly entangled.