Polyimide blends for gas separation membranes

A method of separating one or more gases from a gas mixture comprising:

• (a) providing a gas separation membrane comprising a blend of at least one polymer of a Type 1 copolyimide and at least one polymer of a Type 2 copolyimide, in which the Type 1 copolyimide comprises repeating units of formula I
• in which R2 is a moiety having a composition selected from the group consisting of formula A, formula B, formula C, and a mixture thereof,
• Z is a moiety having a composition selected from the group consisting of formula L, formula M, formula N, and a mixture thereof;
• and
• R1 is a moiety having a composition selected from the group consisting of formula Q, formula S, formula T, and a mixture thereof,
• in which the Type 2 copolyimide comprises the repeating units of formulas IIa and IIb,
• in which Ar is a moiety having a composition selected from the group consisting of formula U, formula V, and a mixture thereof, and
• in which X, X1, X2, X3 independently are hydrogen or an alkyl group having 1 to 6 carbon atoms, provided that at least two of X, X1, X2, or X3 on each of U and V are an alkyl group, Ar′ is any aromatic moiety, Ra and Rb, each independently have composition of formulas A, B, C, D or a mixture thereof, and
• Z is a moiety having composition selected from the group consisting of formula L, formula M, formula N, and a mixture thereof;
• (b) contacting the gas mixture with one side of the gas separation membrane, thereby causing more preferentially permeable gases of the mixture to permeate the membrane faster than less preferentially permeable gases to form a permeate gas mixture enriched in the more preferentially permeable gases on the opposite side of the membrane, and a retentate gas mixture depleted in the more preferentially permeable gases on the one side of the membrane;
• and
• (c) withdrawing the permeate gas mixture and the retentate gas mixture separately from the membrane.