Abstract: Aromatic polyimide hollow fibers having an enhanced mechanical strength are produced by a process comprising the steps of: preparing a spinning dope solution of at least one aromatic polyimide having 90 molar % or more of a recurring unit of the formula (I). ##STR1## wherein R is a divalent aromatic radical, in a phenolic solvent; converting the dope solution into at least one filamentary stream thereof; and introducing the filamentary dope solution stream into a coagulating liquid having a coagulating rate coefficient of 1.5 microns/sec for the dope solution, to provide rigid hollow filaments.

Abstract: An aromatic polyimide composite membrane having an excellent gas-separating property is produced by a process comprising the steps of: coating at least one surface of a substrate consisting of an aromatic polyimide porous membrane with a solution of 0.05% to 10% by weight of an aromatic polyamic acid in a polar organic solvent, which is not capable of dissolving the polyimide porous membrane, at a temperature not exceeding 140.degree. C.; after a during drying the resultant thin coating layer of the polyamic acid solution, heating the resultant polyamic acid layer at a temperature of 150.degree. C. or more so as to convert the polyamic acid layer to the corresponding polyimide layer.

Abstract: A polyimide semipermeable membrane is prepared by the process comprising the steps of:preparing a solution of a 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): ##STR1## wherein R represents a divalent aromatic radical, in a solvent consisting of at least one phenol compound in the state of a liquid or melt;forming a thin layer of the polyimide solution, and;coagulating the thin layer of the polyimide solution by bringing the thin layer into contact with a coagulating liquid to form a semipermeable membrane.

Abstract: A gas separating material having excellent heat and chemical resistance, mechanical strength and gas-separating function, comprises a gas separating layer consisting of at least one dense film having a thickness of from 0.1 to 20 microns and comprising at least one aromatic imide polymer which comprises at least 80 molar % of at least one type of recurring unit of the formula (I): ##STR1## wherein R represents a divalent aromatic radical.

Abstract: Aromatic polyimide fibers having an enhanced mechanical strength are produced by a process comprising the steps of: preparing a spinning dope solution of at least one aromatic polyimide having 90 molar % or more of a recurring unit of the formula (I): ##STR1## wherein R is a divalent aromatic radical, in a phenolic solvent; converting the dope solution into at least one filamentary stream thereof; solidifying the filamentary dope solution stream by removing the solvent therefrom to provide at least one undrawn filament, and; drawing the undrawn filament at a temperature of 20.degree. to 600.degree. C. at a draw ratio of 1.5 to 5.0.

Abstract: Carbon fibers having excellent modulus of elasticity and tensile strength are produced in a high yield by a process wherein polybutadiene fibers are prepared by melt-spinning polybutadiene having at least 85% of vinyl structure, a melting point of at least 100.degree. C and a reduced viscosity of 0.3 to 1.2 dl/g; the fibers are then treated in a solution or suspension of a Lewis acid in an inert organic liquid, whereby the pollybutadiene is cyclized and cross-linked; the polybutadiene fibers treated as mentioned above are further treated in a melted sulphur or a sulphur-containing solution at a temperature of 170 to 300.degree. C, whereby the cyclized and cross-linked polybutadiene is dehydrogenated and aromatized; and the fibers treated as mentioned above are carbonized by raising the temperature thereof to 750.degree. C to 3000.degree. C to form carbon fibers or graphite fibers. If so desired, the ordinary carbon fibers which have been produced at 750 to 1500.degree.