NOVELTY - Membrane (M1) for fluid separation made of a material comprises a blend of polyimide polymer, and a polyimidazole polymer; where the polyimide polymer is containing several repeating units and the polyimidazole polymer containing repeating units.    USE - For separating fluid from fluid mixture, where the fluid mixture comprises air; the permeate fluid is enriched in oxygen; and the retentate fluid mixture is depleted in oxygen. The fluid mixture comprises a mixture of carbon dioxide and methane; the permeate fluid is enriched in carbon dioxide; and the retentate fluid mixture is depleted in carbon dioxide. The fluid mixture comprises a mixture of carbon dioxide and nitrogen; the permeate fluid is enriched in carbon dioxide; and the retentate fluid mixture is depleted in carbon dioxide. The fluid mixture comprises a mixture of hydrogen and methane; the permeate fluid is enriched in hydrogen; and the retentate fluid mixture is depleted in hydrogen. The fluid mixture comprises a mixture of hydrogen and carbon monoxide; the permeate fluid is enriched in hydrogen; and the retentate fluid mixture is depleted in hydrogen. The fluid mixture comprises a mixture of water and 1-4C alcohol selected from ethanol, propanol, isopropanol, and/or butanol; the permeate fluid is enriched in water; and the retentate fluid mixture is depleted in water. The fluid mixture comprises a mixture of unsaturated 2-4C hydrocarbon and branched or straight chain 2-4C paraffin; the permeate fluid is enriched in the unsaturated hydrocarbon; and the retentate fluid mixture is depleted in the unsaturated hydrocarbon (claimed) found in various process streams in oil refineries and petrochemical plants; for economical purification of the organic or for overcoming an azeotropic composition.    ADVANTAGE - The membrane exhibits resistance to interaction of the material with the process and the resulting plasticizing of the membrane; having superior mechanical properties, resulting in superior performance when exposed to high membrane differential pressures and high process temperatures; is capable of processing into membranes of a wide variety of desired configurations such as a hollow fiber membrane; having good balance of high productivity and selectivity for the fluids; provides persistently good separation performance despite long-term contact with aggressive process composition, pressure and temperature conditions.    DETAILED DESCRIPTION - Membrane (M1) for fluid separation made of a material comprises a blend of polyimide polymer, and a polyimidazole polymer; where the polyimide polymer containing several repeating units of formula (I), and the polyimidazole polymer containing repeating units of formula (IIa) and (IIb).    R2=1,3-di-yl-2-methyl-benzene (i), 2,4-di-yl-1-methyl-benzene (ii), or bis(4-yl-phenyl)methane (iii);    R1=1,2,4,5-tetra-yl-benzene, 3,4,3',4'-tetra-yl-biphenyl, or formula (iv);    Z1=group of formulae -C(=O)-, -O-, or -S(=O)2-;    R3=aromatic group;    R4=aromatic group;    R5=aromatic, aliphatic or heterocyclic group.    An INDEPENDENT CLAIM is included for separating at least one fluid from fluid mixture involving providing the fluid separation membrane (M1); contacting a fluid mixture with a first side of the fluid separation membrane by causing a preferentially permeable fluid of the fluid mixture to permeate the fluid separation membrane faster than a less preferentially permeable fluid to form a permeate fluid mixture enriched in the preferentially permeable fluid on a second side of the fluid separation membrane and a retentate fluid mixture depleted in the preferentially permeable fluid on the first side of the fluid separation membrane; and separately withdrawing the permeate fluid mixture and the retentate fluid mixture.