High-productivity gas separation membranes derived from pyromellitic dianhydride and nonlinear diamines

• Pyromellitic anhydride and ortho-substituted meta-diamines give contorted structures.
• Membranes from these polyimides behave as polymers with intrinsic microporosity.
• PMDA polyimides give significantly better gas separation efficiency than 6FDA ones.
• Substituents size and asymmetry improve permeability without selectivity losses.

Two new polyimides have been obtained from commercial and inexpensive pyromellitic dianhydride and two affordable meta-substituted diamines bearing ortho methyl substituents (i.e., 2,4,6-trimethyl-1,3-phenylene diamine (TMPD) and 5,7-diamino-1,1,4,6-tetramethylindane (TMID)). Due to a combination of dianhydride rigidity and meta substitution, in addition to the influence of the ortho substituents on the orthogonal placement of the amine and imide planes, the two polyimides exhibited a high fractional free volume and a high rigidity. Therefore, these polyimides were able to produce gas separation membranes with an excellent balance between permeability and selectivity, and these membranes performed similar to many polymers with intrinsic microporosity (PIMs) and surpassed the 1991 Robeson upper bound for all gas pairs. The behavior is more favorable than that for similar polyimides based on 6FDA, which is considered to be the best dianhydride for gas separation polyimides.

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