Blended crosslinked polybenzoxazole (PBO) membranes derived from phenolphthalein-based polyamide and polyimide for CO2/CH4 gas separation

Designing membrane materials with excellent gas separation performance is always a goal of membrane researchers. However, to be able to deliver stable performance and maintain membrane integrity under actual working conditions, a membrane material must be mechanically strong. In the last two decades, thermally rearranged polybenzoxazole (TR-PBO) polymers exhibit out-standing gas transport properties and great anti-plasticization behavior. But the TR reaction often lead to catastrophic deterioration in roughness of the polymer. To overcome this problem, we developed a polymer blending system consisting of a phenolphthalein-based polyamide (PHA, 6FC-DAP) and polyimide (API, OH-6FDA-DAP). Because of their similar chemical structures and the formation of inter-molecular hydrogen bonds, homogenous polymer blended membranes were obtained at molar ratios from 1:3 to 3:1 of PHA and API. Importantly, the B11 (PHA to API molar ratio of 1:1) derived PBO-based membranes exhibited superior mechanical and gas separation performance compared to the B31 (PHA: API=3:1) and B13 (PHA: API=3:1) derived PBO-based membranes. Note that, when the blended membrane B11 was fully converted into PBO, its tensile strength and elongation at break are 5.89 times and 3.27 times higher than those of the API derived TR-PBO. In addition, the B11–350 and B11–400 (thermal treatment at 350 °C and 400 °C in air atmosphere for 2 h, respectively) were not plasticized at a CO2 pressure of 30 atm. The excellent mechanical properties with appealing gas separation properties of these TR-PBO polymers have potential for CO2 relative gas separation applications.