Soluble and Transparent Polyimides from Unsymmetrical Diamine Containing Two Trifluoromethyl Groups

A new unsymmetrical diamine monomer containing two trifluoromethyl (CF3) groups is prepared from 2-bromo-5-nitro-1,3-bis(trifluoromethyl)benzene. The monomer is polymerized with typical dianhydrides, in this case PMDA, BPDA, BTDA, ODPA, and 6-FDA, using a one-pot synthetic method to obtain corresponding polyimides. All of the prepared polyimides are readily soluble in many organic solvents and can be solution-cast into transparent, flexible, and tough films. These films have a UV-vis absorption cut-off wavelength at 340-375 nm and light transparencies of 87-91% at a wavelength of 550 nm. Incorporation of two CF3 groups unsymmetrically into rigid polyimides improves their solubility and transparencies without decreasing their physical properties. The polymers exhibit high thermal stability with 5% weight loss at temperatures ranging from 534 to 593 degrees C in nitrogen and from 519 to 568 degrees C in air, and high glass transition temperatures (T-g) above > 300 degrees C depending on their molecular structures. They also have a coefficient of thermal expansion (CTE) value of 46-69 ppm/degrees C. In addition, they show low refractive indices in the range of 1.535-1.602 at a wavelength of 633 nm due to the unsymmetrical incorporation effect of the two CF3 groups. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4413-4422