Patent ID: 8026005

Claim:
A microporous polyolefin membrane, whose pore size distribution curve obtained by mercury intrusion porosimetry has at least two peaks, the polyolefin comprising (1) a polyethylene resin containing 7% or less by mass of ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 1×10 6 or more, or (2) a mixture of 75% by mass or more of the polyethylene resin and 25% or less by mass of polypropylene wherein the membrane 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 corrsponding to at least one sub-peak in a range of more than 0.08 μm and 1.5 μm in the pore size distribution curve, and wherein the dense domains and the coarse domains are irregularly entangled.