Abstract: A series of soluble polyimides (3a-3c) containing side substituent and fluorene structures were prepared by one-step high-temperature solution condensation polymerization using self-made diamine monomer 9,9-bis(3,5-dimethoxy-4-(4-amino-2-methylphenoxy) phenyl) fluorene and three commercial available aromatic dianhydride monomers: diphenyl ether tetracarboxylic dianhydride, biphenyl tetracarboxylic dianhydride and benzophenone tetracarboxylic dianhydride. The structure, solubility, thermal stability and mechanical properties of the polyimide were characterized by ¹H NMR, GPC, DSC and other equipment. The results showed that the polyimides could be well dissolved in N,N-dimethylacetamide, N-methylpyrrolidone, dimethylformamide, oxolane and other solvents. The glass transition temperature (T_g) of the polyimide was higher than 244 ℃, and the 10% mass loss temperatures in air and N₂ were higher than 460 ℃ and 500 ℃,respectively, which proved that the thermal resistance of the polyimide was excellent. The tensile strength of the film was above 78.7 MPa, and the elongation at break was above 7.9%, which indicated good mechanical properties. The permeability coefficients of the film to CO₂ and O₂ were higher than 15.9 and 4.95 barrer, respectively. The separation selectivity coefficients for CO₂/CH₄ and O₂/N₂ were as high as 46.42 and 4.93, which proved excellent gas separation performance. Therefore, this soluble polyimide with substituted side groups and fluorene structures exhibited excellent comprehensive performance.

Key words: polyimide, film, solubility, gas separation performance