Preparation and Gas Separation Properties of Spirobichroman-Based Polyimides

Polymers with rigid and contorted structures are always preferred for use as gas separation membranes. Owing to the contorted nature of the spiro-center, the spirobichroman structure is an effective scaffold for impeding polymer chain packing. Two spirobichroman-based diamines, 6,6-bis(4-amino-2-trifluoromethylphenoxy)-4,4,4,4,7,7-hexamethyl-2,2-spirobichroman (FSBC) and 6,6-bis(4-aminophenoxy)-4,4,4,4,7,7-hexamethyl-2,2-spirobichroman (SBC), with or without CF3 side groups are prepared. The two diamines are separately reacted with 3,3,4,4-benzophenonetetracarboxylic dianhydride (BTDA) and 3,3,4,4-diphenylsulfonetetracarboxylic dianhydride (DSDA) to obtain four spirobichroman-based polyimides, denoted as BTDA-FSBC, BTDA-SBC, DSDA-FSBC, and DSDA-SBC. The different molecular structures among the four monomers are expected to alter the fractional free volume of the corresponding polyimides by impacting chain-packing conditions, and then controlling the gas transport properties. Chemical structures, thermal and physical properties of the polyimides are fully characterized. Two different methods, molecular dynamics simulation and Bondi's group contribution method are employed to characterize the fractional free volumes (FFVs) of these polyimides. In the end, it is found that FSBC-based polyimides exhibit relatively higher gas permeability than SBC-based polyimides.