An integrally skinned asymmetric polyimide membrane, wherein at least a fraction of the imide groups of the polyimide are crosslinked using ammonia, hydrazine or amines, such as aliphatic amines, aliphatic-aromatic amines or aromatic amines and the polyimide is impregnated with a conditioning agent, such as synthetic oils, mineral oils, vegetable oils, higher alcohols, such as decanol, dodecanol, heptadecanol, glycerols and glycols, is used for nanofiltration of a feed stream solution comprising a solvent and dissolved solutes. The polyimide is at least one copolymer derived from (a) the co-condensation of benzophenone 3,3',4,4'-tetracarboxylic acid dianhydride and a mixture of (i) di(4-aminophenyl) methane and toluene diisocyanate; or (b) the condensation of 1H,3H-Benzo[1,2-c:4,5-c']difuran-1,3,5,7-tetrone with 5,5'-carbonylbis[1,3-isobenzofurandione], 1,3-diisocyanayo-2-methylbenzene and 2,4-diisocyanato-1-methylbenzene. Also disclosed is a process for forming the membrane and use of the membrane for nanofiltration in the presence of organic solvents.
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