Cardo polyimides/TiO2 mixed matrix membranes: Synthesis, characterization, and gas separation property improvement

This paper reports the synthesis of a novel cardo type polyimide PI(BTDA-BAPF-DMMDA) based on 3,3′,4,4′-benzophenone-tetracarboxylic dianhydride (BTDA), 9,9′-bis (4-aminophenyl) fluorine (BAPF) and 4,4′-biamino-3,3′-dimethyldiphenyl-methane (DMMDA) using the low temperature solution copolymerization method, and cardo polyimide/TiO2 mixed matrix membranes (MMMs) were prepared by blending of TiO2 sol and the PI solution. The incorporation of the cardo groups was achieved through the reaction between the free -COOH of polyamide acid (PAA) and the -NH2 of the BAPF. The chemical structure and physical properties of membrane material were characterized using FTIR, 1H NMR, DSC and XRD. Results showed that the MMMs containing 24 wt% of TiO2 sol were nano-composite membranes with strong interactions between inorganic TiO2 phase and organic PI phase. Compared with PI(BTDA-DMMDA) membrane, the addition of BAPF and TiO2 greatly improved the gas separation properties of cardo PI membranes and MMMs. The gas permeation (PO2) and ideal separation factor (aO2/N2) of the cardo polyimide PI/TiO2 membrane with a BAPF: BTDA molar ratio of 0.15:1 and TiO2 content of 24 wt% were 4.5 Barrer and 15.8 respectively, which were a respective 9.4 and 4.6 times higher than that of the PI(BTDA-DMMDA) membrane. The PO2 and aO2/N2 of the cardo polyimide/TiO2 MMMs prepared in this work were beyond Robeson's trade-off upper bound, highly promising for future gas separation application.