Selective permeable membranes and process for preparing the same

A process for preparing a selective permeable membrane which comprises:

• applying to a supporting substrate a membrane solution prepared by dissolving in an organic solvent a polyimide polymer comprising principally a repeating unit represented by the general formula (I):
• ##STR9## wherein R.sup.1 represents a divalent organic group, and a liquid swelling agent having a coagulation value with respect to said polyimide polymer of about 50 to about 200 and a boiling point of about 50.degree. C. to about 120.degree. C. under atmospheric pressure, wherein the coagulation value of the swelling agent is defined as the minimum amount in ml of the swelling agent added to 50 ml of a 2% by weight solution of said polyimide polymer of N-methylpyrrolidone causing white-turbidity due to separation of the polyimide polymer at 25.degree. C.,
• evaporating at least a part of said swelling agent, and
• coagulating said polyimide polymer by contacting the polyimide polymer with a coagulating solvent which does not dissolve said polyimide polymer but is compatible with said organic solvent and said swelling agent, wherein the membrane forming solution comprises about 10 to about 40% by weight of said polyimide polymer and about 30 to about 300 parts by weight of said swelling agent based on 100 parts by weight of said polymer in an organic solvent.