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Abstract

A micro-porous membrane of ultra-high-molecular-weight alpha-olefin polymer having a weight-average molecular weight greater than 5.times.10.sup.5, the micro-porous membrane having through holes 0.01 to 1 micrometer in average pore size, with a void ratio from 30 to 90% and being oriented such that the linear draw ratio in one axis is greater than two and the areal draw ratio is greater than ten. The micro-porous membrane is obtained by forming a gel-like object from a solution of an alpha-olefin polymer having a weight-average molecular weight greater than 5.times.10.sup.5, removing at least 10 wt % of the solvent contained in the gel-like object so that the gel-like object contains 10 to 90 wt % of alpa-olefin polymer, orientating the gel-like object at a temperature lower than that which is 10.degree. C. above the melting point of the alpa-olefin polymer, and removing the residual solvent from the orientated product. A film is produced from the orientated product by pressing the orientated product at a temperature lower than that of the melting point of the alpha-olefin polymer.

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