Viscoelastic behavior of high molecular weight polyimide/cyclohexanone solution during sol-gel transition

Abstract It is very important to optimize rheological properties of polyimide (PI) solution for film casting or fiber spinning. Herein, a soluble 2,2-bis(3,4-di-carboxyphenyl) hexafluoro propane dianhydride-2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl PI sample with high absolute molecular weight of 390,000 g/mol was obtained by precipitation fractionation. The viscoelastic behavior of PI in cyclohexanone solution was investigated at different concentrations (0.1–12 wt%). PI gel can form at a suitable concentration. Analysis of viscoelastic storage and loss moduli, G ' and G ' ' , based on the Winter and Chambon model gives the gel concentration c g  = 4 wt% and the critical relaxation exponent n = 0.56. The scaling relations such as zero-shear viscosity η 0 and equilibrium modulus G e against relative distance from the gel point e , that is η 0 ∝ e − γ and G e ∝ e z , were examined in the vicinity of c g , where a fractal dimension d f  = 2.54 indicates formation of a compact gel structure. Addition of N,N-dimethylformamide (DMF) strongly decrosslinks the PI solution, suggesting a screening of the interchain dipole–dipole interactions by the highly polar DMF solvent.