Process for preparing aromatic polyimide semipermeable membranes

A process for preparing an aromatic polyimide semipermeable membrane, comprising the steps of:

• preparing a solution of an aromatic polyimide material consisting essentially of at least one aromatic polyimide comprising at least 90 molar % of at least one recurring unit of the formula (I):
• ##STR16## wherein R represents a divalent aromatic radical which is selected from the group consisting of those of the formulae (IV) and (V):
• ##STR17## wherein R.sup.1 and R.sup.2, respectively, represent, independently from each other, a member selected from the group consisting of a hydrogen atom, lower alkyl radicals having 1 to 3 carbon atoms and lower alkoxy radicals having 1 to 3 carbon atoms, and A represents a divalent bond selected from the group consisting of --O--, --S--, --CO--, --SO.sub.2 --, --SO--, --CH.sub.2 -- and --C(CH.sub.3).sub.2 --, and;
• ##STR18## wherein R.sup.1 and R.sup.2 are as defined above, in a solvent consisting of at least one phenol compound in the state of a liquid or melt;
• forming a thin layer of said aromatic polyimide solution;
• and
• coagulating said thin layer of said aromatic polyimide solution by bringing said thin layer into contact with a coagulating liquid to form a semipermeable membrane,
• said coagulating liquid consisting of at least one member selected from the group consisting of lower aliphatic alcohols having 1 to 5 carbon atoms, lower aliphatic ketones having 3 to 5 carbon atoms, ether compounds, amide compounds, dimethylsufoxide, and mixtures of at least one member of the above-mentioned compounds with water.