Porous membrane and production process thereof
 The present invention relates to a porous membrane which has excellent durability and is useful for ultrafiltration. The porous membrane of the present invention has heterogeneous structure, which consists essentially of a phenylenesulfide-based copolymer of the formula: (Ph--S--Ph--X--).sub.q -- (wherein Ph represents phenylene group, X represents --SO.sub.2 -- or --CO--, and g represents an integer from 80-400 which membrane has a dense layer on one or both surfaces of the membrane, the dense layer controlling the separation performance of the membrane, and a porous layer next to the dense layer, which porous layer has a porosity higher than that of the dense layer. 
Toray Industries, Inc.
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Hideaki Tetsuo Tadahiro Masaru Yoshio
Tanisugi Watanabe Uemura Kurihara Himeshima