Design of gas separation membranes derived of rigid aromatic polyimides. 1. Polymers from diamines containing di-tert-butyl side groups

A series of polyimides has been obtained from the experimental diamine 1,4-bis(4-aminophenoxy)2,5-di-tert-butylbenzene (TBAPB) and three commercial dianhydrides, i.e. pyromellitic dianhydride (PMDA), 3,3′,4,4′-byphenyltetracarboxylic dyanhidride (BPDA) and 4,4′-hexafluoroisopropyliden diphthalic anhydride (6FDA), following classical polyimidation methods. Analogous polyimides with diamines 2,2-bis(4-aminophenyl)hexafluoropropane (6FpDA) and 1,4-bis(4-aminophenoxy)benzene (APB) have been also prepared for comparative purposes. All polyimides showed high thermal stability, with decomposition temperatures above 490 °C, and glass transition temperatures higher than 270 °C. The rigid, rod-like structure of PMDA, combined with bulky diamines TBAPB and 6FpDA, yielded polyimides with high fractional free volume (FFV), close to that of 6FDA–6FpDA. The high FFV of PMDA–TBAPB contrasts with its structure that has fairly large degree of short range order as evidenced by X-ray diffraction. The gas permeation properties were greatly dependent on the dianhydride moiety and, as a rule, they compared well with those of polyimides used for gas separation. The polymers from PMDA, PMDA–TBAPB and PMDA–6FpDA, showed the best gas productivity values. By means of molecular modelling calculations, it has been observed that monomer TBAPB preferably adopts a contorted, rotation-restricted conformation, what can explain the special characteristics observed in the reported polyimides.