TECHNOLOGY FOCUS - POLYMERS - Preferred Structure: The repeating units (A) of polyimide polymer are of formulae (Ia,Ib). The amount of repeating units (A) of formula (Ib) is 1-99%, preferably 30-50% with respect to total repeating units (A).R1=same as defined above, preferably group of formula (A) is 10-30% (preferably, 20%) and group of formula (B) is 70-90% (preferably, 80%).Preferred Composition: When the blend polymer comprises polyamide-imide polymer, the ratio of polyimide polymer with respect to polyamide-imide polymer is at least about 1, preferably at least about 2. When the blend polymer comprises polyamide polymer, the ratio of polyimide polymer to polyamide polymer is at least about 1, preferably at least about 2. The membrane contains 20-80% of polyimide polymer, which is formed by polycondensation of aromatic diisocyanate and dianhydride. The aromatic diisocyanate is chosen from 2,4-toluene diisocyanate, 2,6-toluene diisocyanate and 4,4'-methylene-bis(phenyl isocyanate). The dianhydride is chosen from pyromellitic dianhydride, benzophenone tetracarboxylic dianhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, and/or 3,3',4,4'-biphenyl tetracarboxylic anhydride. The elastic modulus of mixed polymer of polyimide polymer and blend polymer is 10% higher than the elastic modulus of polyimide polymer. The yield stress of mixed polymer of polyimide polymer and blend polymer is higher than the yield stress of polyimide polymer. The polyimide polymer, polyamide polymer, and polyamide-imide polymer have average molecular weight of 20000-400000, preferably 50000-300000. The polyamide polymer is chosen from P84 and/or P84HT. The fluid mixture used in separation process, contains carbon dioxide and methane. Preferred Membrane: The membrane is chosen from asymmetric membrane, uniformly dense membrane, hollow fiber membrane and composite membrane. Preferred Conditions: The pressure gradient across the membrane is 100-2000 psi, preferably 1000-2000 psi. EXAMPLE - A fiber was prepared from a spin dope formulation containing polymer blend (30 wt.%) of P84 (polyimide polymer) and polymer (2-phenyl APB-144:3APS (1:1)/iso- and tere-phthaloyl chloride) in weight ratio of 4:1, tetramethylene sulfone (9 wt.%) and acetic anhydride (1.5 wt.%) in N-methyl-2-pyrolidone. The obtained fiber was tested for its separation performance of mixed gas containing carbon dioxide and methane, at 23degreesC and 50 psi pressure. The carbon dioxide permeability and carbon dioxide/methane selectivity of the fiber were 19 GPU and 40, respectively. The fiber had helium permeability of 116 GPU and helium/nitrogen selectivity of 7, when used for separation of gas containing helium and nitrogen at 23degreesC and 100 psi pressure.