Effects of the side groups of the spirobichroman-based diamines on the chain packing and gas separation properties of the polyimides

Three spirobichroman-based diamines, FSBC, SBC and MSBC, with different substituted groups (-CF3, -H, and -CH3) were synthesized. The diamines were reacted with 4,4'-hexafluoroisopropylidene bisphthalic dianhydride (6FDA) to form 6FDA-FSBC, 6FDA-SBC, and 6FDA-MSBC polyimides, respectively. The organosolubilities, molecular weights (M-n), densities, glass transition temperature (T-g), thermal stabilities, fractional free volume (FFV), d-spacing and gas separation properties of the spirobichroman-based polyimides were systematically studied. The three spirobichroman-based polyimides had high M-n, and T-g, excellent thermal stabilities, and good solubilities in a wide range of organic solvents. 6FDA-FSBC exhibited the highest gas permeability which could be attributed to its insufficient chain packing conditions rooting from the incorporation of spriobichroman backbone with the Spiro and non-coplaner nature, and the bulky CF3 substituting group that increased the polymer FFV and decreased the polymer chain flexibility. 6FDA-MSBC had the highest H-2/CH4 selectivity due to the present of the CH3 group in the diamine moiety which had the turnstile-like rotary thermal motion that inhibited the diffusion of large molecules like CH4 but had little effect on the diffusion of small molecules like H-2.