Highly transparent and soluble polyimides with synergistic effects of pyridine and cyclohexane

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In order to prepare novel polyimides (PI) with high transparency and solubility in common solvents, three diamine monomers containing pyridine or methyl-substituted pyridine and cyclohexane, 1,1′-bis[4-(5-amino-2-pyridinoxy)phenyl]cyclohexane (IIIa), 1,1′-bis[4-(5-amino-3-methyl-2-pyridinoxy)phenyl]cyclohexane (IIIb), and 1,1′-bis[4-(5-amino-2-methyl-6-pyridinoxy)phenyl]cyclohexane (IIIc), were synthesized and characterized. The diamine monomer (IIIa) was subsequently polymerized with 3,3′,4,4′-oxydiphthalic anhydride, 4,4′-(hexafluoroisopropylidene)-diphthalic anhydride (6FDA) to obtain PIs, PI-1 and PI-2. Furthermore, PI-3 (IIIb/6FDA) and PI-4 (IIIc/6FDA) were prepared for structure–property comparison studies. The structures of the resulting PIs were characterized by fourier transform infrared spectroscopy (FT-IR), X-ray diffraction, and elemental analysis. The physical properties of the PI films were investigated by thermogravimetric analysis, differential scanning calorimetric analysis, dynamic mechanical analysis, and ultraviolet–visible spectroscopy. In general, the PIs showed excellent thermal properties and good mechanical properties. Most had good solubility in polar organic solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone (NMP), m-cresol, and even soluble in low boiling solvents such as tetrahydrofuran (THF) and CHCl3. The PI films also exhibited high optical transmittance of 94–97% at 500 nm and cut-off wavelength at 345–359 nm. In summary, the introduction of methyl, cyclohexane, and pyridine groups into the backbone was shown to improve the solubility and transparency of PI films.

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