The dense aromatic imide polymer film which has been hitherto known to be usable as a gas-separating membrane is limited to a dense membrane produced from pyromellitic acid and an aromatic diamine in the following manner. A polyamic acid is prepared by the polymerization of pyromellitic acid with an aromatic diamine. The resultant polyamic acid is dissolved in an organic polar solvent so as to produce a solution usable as a dope solution for producing membranes or films. In the preparation of the membrane or film, the dope solution is spread on the smooth surface of a film-forming device so as to form a thin layer of the dope solution and then the solvent is evaporated from the dope solution layer at a predetermined temperature. During or after the evaporating procedure, the polyamic acid is converted to the corresponding imide polymer so as to form a thin, solid film or membrane of the imide polymer.
The above-mentioned process for producing the imide polymer membrane is rather complicated and, therefore, the quality of the resultant membrane is not consistent.
Also, it is known that the conventional dense membrane consisting of an aromatic imide polymer exhibits a poor permeating rate in the case of a gas or liquid. Therefore, the conventional dense membrane made of the aromatic imide polymer can not be used to separate a gas mixture or a liquid mixture or to concentrate a gas or liquid unless the membrane is made very thin.
Accordingly, there has been a great demand for a porous membrane having a large permeating rate in the case of a gas or liquid and being produced from an aromatic imide polymer having excellent heat resistance. However, the above-mentioned type of porous aromatic imide polymer membrane having a satisfactory permeating rate in the case of a gas or liquid can not be obtained by means of conventional processes.