Molecular design, synthesis and characterization of intrinsically black polyimide films with high thermal stability and good electrical properties

A series of intrinsically black polyimide (PI) films have been designed and synthesized by the copolymerization of pyromellitic dianhydride (PMDA), 4,4′-oxydianiline (ODA) and 4,4′-diaminodiphenylamine (NDA) via a two-step polycondensation procedure, followed by the high temperature imidization reaction of the poly(amic acid) (PAA) intermediates. Incorporation of the electron-rich NDA diamine apparently enhanced the formation of charge transfer complex (CTC) between the electron-donating diamine units and the electron-accepting dianhydride moiety; thus greatly absorbing the visible light and endowing the derived PI films deep colors. PI-5 with the 100% molar ratio of NDA in the diamines exhibited a cutoff wavelength value of 555 nm, which was 127 nm higher than that of PI-1 without NDA component. The PI films were nearly totally opaque at the wavelength of 500 nm when the molar ratio of NDA exceeded 80% in the diamines. PI-5 showed International Commission on Illumination (CIE) lab color parameters of 23.36 for L*, 46.43 for a*, and 40.26 for b*, indicating the essentially black plus red color instead of the yellow plus blue color for PI-1 film (L* = 89.93, a* = −11.09, and b* = 84.23). In addition, introduction of NDA into the copolymer films increased the high temperature dimensional stability of the films. PI-5 exhibited a glass transition temperature (T_g) and coefficient of thermal expansion (CTE) of 431.6 °C and 18.8 × 10⁻⁶ /K, respectively, which were superior to those of the PI-1 film (T_g = 418.8 °C, CTE = 29.5 × 10⁻⁶ /K). At last, incorporation of NDA components somewhat decreased the tensile properties and thermal decomposition temperatures of the PI films.