Insights into the relationship between structure and properties of Spirobichroman-based polyimides: Effects of substituents on molecular structure and gas separation.

Four spirobichroman-based polyimides (6FDA-F, 6FDA-O, 6FDA-P, and 6FDA-M) were successfully obtained via polyreaction between diamines of different substituents (-H, -OCH 3 , -N, and -CH 3) and 4,4′-(hexafluoroisopropylidene)-diphthalic anhydride (6FDA). The molecular weights, thermophysical, dihedral angle of the molecules, fractional free volume, d-spacing, Brunauer–Emmett–Teller surface area, and gas separation performance were studied through a combination of experiments and molecular dynamics simulations. These polymers show high molecular weights, with number-average molecular weights (M n) in the range of 5.0–15.3 × 104, and excellent thermal stability, with weight loss temperatures (T 5%) of over 420 °C. The pure-gas (O 2 , N 2 , CO 2 , and CH 4) permeation results indicated that the four polymers selectivity of CO 2 /CH 4 over 31 and that 6FDA-F had the highest gas permeability among the four tested gases. Importantly, the dihedral angle from molecular simulation was used to clarify the phenomenon wherein the 6FDA-P gas permeability lower than 6FDA-F. A detailed analysis indicates that -CH 3 increased the gas permeability of the polyimides, while the turnstile-like rotary thermal motion of -OCH 3 formed channel obstacles and improved the gas selectivity. The polymers containing pyridine ring and those containing benzene ring follows different gas separation mechanisms, based on the gas solution-diffusion behaviour. Unlabelled Image • Polyimides with substituent of benzene ring shown the best overall separation performance for CO 2 /CH 4 and O 2 /N 2. • Dihedral angle between adjacent plane used to insights into the mechanisms of gas permeability enhancement for polymer. • The -CH 3 increases gas permeability, while the -OCH3 forms channel obstacles and improves gas selectivity. • Polyimides containing pyridine ring and those containing benzene ring follows different gas separation mechanisms. [ABSTRACT FROM AUTHOR]