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Abstract

Microporous films of the semicrystalline polymer according to the present invention are obtained by stretching semicrystalline polymer sheets extruded through a die with the phase separation between a semicrystalline polymer resin and a diluent, of which sheet is comprised of a crystalline region, a pore region, and a non-crystalline region which is a swollen region swollen by the diluent, and extracting the diluent. The pore region has irregular sizes and shapes, has an average diameter of 0.01 .mu.m to 2 .mu.m, is connected in three dimensions, penetrates the thickness of the sheet, has gas permeability, and has a volume ratio with respect to the volume of the entire composition of 10% to 40%. The swollen non-crystalline region has a swelling ratio of 200% or greater and is a region making micropores of which average diameter is 0.1 .mu.m to 1 .mu.m as the region is split and pores are generated during the process of stretching. Thus manufactured microporous films are characterized by having a gas permeability of 1.3.times.10.sup.-5 Darcy or greater as well as a puncture strength of 0.1 N/.mu.m or greater even without tearing destroying of the pores during stretching.

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First Claim