Gas transport properties of polyimide membranes bearing phenyl pendant group

Polyimides (PIs) with single phenyl pendant substitution were prepared based on three diamines containing phenyl pendant group, namely, 2,5-bis(4-aminophenoxy) biphenyl, 2-phenyl-4,4'-diaminodiphenyl ether, and 2,5-diaminobiphenyl (p-PDA), with the dianhydride component of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride, respectively. The physical properties of the membranes were examined, including thermal properties, fractional free volume (FFV), solubility, and morphological structures, and were compared with the analogues without phenyl pendant. Gas transport properties of the membranes were investigated and discussed from the viewpoint of structure-property relationship. For 6FDA-derived PI membranes, gas permeability increased as the degree of PI backbone rigidity leveled up. Gas transport properties were not improved by the incorporation of phenyl pendant group for 6FDA type containing ether linkage and marginally improved as compared between PI (6FDA/p-PDA) and PI (6FDA/p-phenylenediamine (PDA)). To increase the phenyl substitution density of 6FDA/PDA-type backbone, a novel diamine bearing two phenyl pendant groups, that is, 2,6-diphenyl-1,4-diaminobenzene (p, p-'PDA) was synthesized, and PI derived from 6FDA and p, p'-PDA was prepared. The gas permeability coefficients of PI (6FDA/p, p'-PDA) were remarkably larger than those of PI (6FDA/p'-PDA) and PI (6FDA/PDA).