Gas permeation properties of hyperbranched polyimide membranes

A series of wholly aromatic hyperbranched polyimides were successfully prepared by condensation polymerization of a triamine monomer, tris(4-aminophenyl)amine (TAPA), and a series of commercially available dianhydride monomers such as 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride (DSDA) and pyromellitic anhydride (PMDA). Two types of hyperbranched polyimides (amine-terminated and anhydride terminated) were obtained by controlling monomer addition manner and monomer molar ratio. Films of these hyperbranched polyimides were fabricated by crosslinking treatment during film cast. Such a film formation technique is based on the chemical reaction between the terminal functional groups of a hyperbranched polyimide and a difunctional crosslinking agent, which leads to the connection of globular hyperbranched macromolecules via chemical bonds. The amine-terminated hyperbranched polyimides generally exhibited much higher gas permeability coefficients than the corresponding anhydride-terminated ones. Crosslinking had great effects on gas permeation properties: lower crosslinking density resulted in higher gas permeability coefficients, but the ideal selectivity hardly changed. For the same hyperbranched polyimide, the “rigid” crosslinking agent, terephthaldehyde (TPA), gave much higher gas permeability coefficients and very similar (CO2/N2) or only a little lower (CO2/CH4 and O2/N2) ideal selectivity than the “flexible” one, ethylene glycol diglycidyl ether (EGDE). TPA-crosslinked hyperbranched polyimide membranes displayed better separation performance than that of the linear analogues and many other linear polymeric membranes. The amine-terminated 6FDA-TAPA hyperbranched polyimide membrane crosslinked with TPA, for example, displayed fairly high CO2/N2 separation performance, i.e. PCO2=65 barrer and PCO2/PN2=30 at 1 atm and 35°C.