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 cg = 4 wt% and the critical relaxation exponent n = 0.56. The scaling relations such as zero-shear viscosity η0 and equilibrium modulus Ge against relative distance from the gel point ε, that is η0∝ε−γ and Ge∝εz, were examined in the vicinity of cg, where a fractal dimension df = 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.
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