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A series of aromatic polyimides were synthesized from three Cardo-based diamines, BAF, DDA, BAP, with different building blocks (ether linkage, dioxane linkage and C–C linkage), and commercially available dianhydride 6FDA. All polyimides were readily soluble in conventional aprotic and protic solvents, which guaranted the fabrication of self-standing membranes for gas separation applications. The Cardo-based polyimides also exhibited excellent thermal stability, with weight loss temperatures (T 5 %) of over 538 °C, and good mechanical properties, with tensile strength in the range of 41–64 MPa. Compared to 6FDA-BAF, the substitution of dioxane linkage for ether linkage resulted in increased chain rigidity, looser polymer chain packing, and larger fractional free volume. Consequently, as displayed by single gas permeation tests, 6FDA-DDA showed higher permeability coefficients without significantly sacrificing selectivity. In addition, the CO 2 permeability of 6FDA-BAP was around 2 times as high as that of 6FDA-BAF, while the CO 2 /CH 4 selectivity increased by 11%. This can be explained by the lower rotational mobility and the relatively higher imide content caused by the absence of the flexible ether linkage and dioxane linkage in polymer backbone. In short, fundamental insights into structure-property relationships of the Cardo-based polyimides observed in this study provided guidance for the molecular structure design of the gas separation membranes. [Display omitted] • Three Cardo-based polyimides containing different building blocks were successfully synthesized. • The effect of rigidity and microstructure on gas separation performance of the Cardo-based polyimides was elucidate. • The substitution of dioxane for ether linkage led to the increased chain rigidity and the looser chain packing. • The polyimide containing C–C linkage showed better combination of permeability and gas-pair selectivity. [ABSTRACT FROM AUTHOR]

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]