Synthesised spirobichroman‐based polyimide functionalized by pyridine: Effects of substituent position on gas separation and thermal properties.

Summary: Spirobichroman‐based polymers have garnered considerable attention as promising gas separation membrane materials. Herein, two spirobichroman‐based diamines were synthesised using a pyridine heterocyclic ring with methyl substituents at different positions. The diamines were reacted with 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and bis‐(3‐phthalyl anhydride) ether (ODPA) to achieve four polyimides functionalised by pyridine. The polymers performances were characterised by experimentally and molecular dynamics simulations. They showed high molecular weights of Mn = 9.4 to 14.0 × 104, excellent thermal stability, good solubility in a series of common organic solvents, and easy processability to form membranes. The membrane showed high gas pair selectivities of αCO2/CH4>30 and αCO2/N2>24, which were attributed to turnstile‐like rotary motions of the methyl and basic group of pyridine. The gas permeability results suggested that the substituents position of the methyl has little effect on the rigid molecular chain, unlike the densely packed flexible molecules chains. [ABSTRACT FROM AUTHOR]