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Abstract

A process for producing a relatively thick microporous ultra-high-molecular-weight polyolefin membrane, including the steps of: (1) preparing a solution of an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 5.times.10.sup.5 or more; (b) extruding the solution from a die while rapidly cooling it in advance to its gelation temperature or below, thereby forming a gel-like sheet; (c) removing at least 10 wt % of the solvent from the gel-like sheet so that the gel-like sheet contains 10 to 90 wt % of the ultra-high-molecular-weight polyolefin; (d) stretching the gel-like sheet at a temperature equal to or lower than that which is 10.degree. C. above the melting point of the ultra-high-molecular-weight polyolefin; and (e) removing the residual solvent from the stretched product.

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