A novel aromatic polyimide with rigid biphenyl side-groups: Formation and evolution of structures in thermoreversible gel

A new thermoreversible polyimide gel was prepared and characterized. This polyimide was synthesized from 4,4′-oxydiphthalicanhydride (ODPA) and 3,5-diamino-benzonic-4′-diphenyl ester (DABBE) via a one-step polymerization procedure in m-cresol. With lowering temperature, a gel from the polyimide (PI) solution was formed due to the solvation power decrease. On heating, a gel/sol transition could be observed by an endothermic peak of DSC curve, and the gel/sol transition enthalpy and temperature went up with the increase of the solution concentration. The original aggregation structure of the gel is amorphous. While the gel was kept at 15 °C, two kinds of order structure developed, which were determined by X-ray. One was layer structure which formed from aggregations of main chains with d-spacing of 15.7 Å (2θ=5.64°) or from side chains with d-spacing of 36.6 Å (2θ=2.41°); The other was crystallosolvate region formed within aggregation of main chains (2θ=10-35°), and the kinetics of isothermal crystallosolvate was analyzed by means of the Avirami equation. At the same time, a possible chain-packing model was suggested. Based on X-ray, DSC and FTIR experiments, the evolution of the ordered structure was also investigated in detail. The results imply that the hydrogen bonding interactions exist between solvent molecule and macromolecule, which is a possible mechanism of the formation of the above gels and order structures.