Six-membered ring copolyimides as novel high performance membrane materials for gas separations

A series of organo-soluble six-membered ring copolyimides have been synthesized by one-step random copolymerization of 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 2,4,6-trimethyl-1,3-phenylenediamine (TrMPD) and 2,2'-bis(trifluormethyl) benzidine (BTFBz). The resulting copolyimides exhibit excellent thermal resistance (Tg > 450 degrees C) and fairly high storage moduli (5.7-7.1 GPa). XRD patterns suggest that they are amorphous and have large segmental distances (0.61-0.67 nm). The gas permeation properties of these copolyimide membranes and their five-membered ring analogue PMDA-TrMPD/BTFBz(1/1) (PMDA is the abbreviation of pyromellitic dianhydride, '1/1' refers to molar ratio) are investigated. Among the NTDA-based copolyimide membranes, the gas permeability coefficient increases with an increase in the molar ratio of TrMPD to BTFBz. Moreover, except NTDA-TrMPD/BTFBz(3/1) the NTDA-based copolyimide membranes show significantly higher gas permeability coefficients and roughly similar ideal selectivities to the PMDA-TrMPD/BTFBz (1/1). For example, the NTDA-TrMPD/BTFBz(1/1) exhibits approximately one order of magnitude higher gas permeability coefficients but roughly similar selectivity values to that of PMDA-TrMPD/BTFBz(1/1) despite their exactly the same diamine composition. The NTDA-TrMPD/BTFBz(1/1) membrane exhibits excellent separation performance for CO2/CH4 (P-CO2 = 930 Barrer, 1 Barrer = 1x10(-10) cm(3) (STP) cm(-1) s(-1) cm Hg-1; P-CO2/P-CH4 = 21.6) gas pair which almost reaches the latest upper bound reported by Robeson in 2008.