Porous aromatic imide polymer membrane and a process for its manufacture

A process for producing a porous aromatic imide polymer membrane, comprising the steps of:

• (1) dissolving 3 to 30% by weight of an aromatic imide copolymer which has a degree of imidization of at least 90% and comprises at least 80 molar % of at least one type of recurring unit selected from the group consisting of those of the formulae (I) and (II):
• ##STR9## wherein R represents a member selected from divalent aromatic radicals, in 70 to 97% by weight of a solvent consisting essentially of a homogeneous mixture of:
• (A) 100 parts by weight of a base solvent consisting essentially of at least one polar organic compound, which base solvent is capable of dissolving therein said aromatic imide polymer in a concentration of 5% by weight of more therein and of evaporating at a temperature of from 50.degree. to 400.degree. C., and
• (B) 5 to 150 parts by weight of an additional liquid consisting essentially of at least one polar organic compound, which additional liquid is incapable of dissolving therein 3% by weight or more of said aromatic imide polymer and is capable of evaporating at a smaller evaporating rate than that of said base solvent at the above-mentioned evaporating temperature so as to provide a dope solution;
• (2) forming a thin layer of said dope solution;
• and
• (3) gradually and completely removing said solvent from said thin layer of said dope solution by means of a dry-evaporating method at a temperature of from 50.degree. C. to 400.degree. C., to form a porous aromatic imide polymer membrane.