High Tg, Low Dielectric, and Colorless Transparent Polyimides Based on Rigid 9,9-Spirobixanthene Diamines

The formation of intramolecular charge transfer complexes leads to poor optical transparency and imparts characteristic colors, such as yellow or even brown, to conventional aromatic polyimide films, greatly limiting their applications in the optical field. To address this issue, a strategy for designing diamine monomers used in the preparation of highly transparent polyimide films with high heat resistance, a low dielectric constant, and a fully aromatic structure is proposed. We introduced units containing rigidly twisted quinoxaline and strongly polar sulfone groups into the diamine monomers through chemical modification. This strategy retains the excellent thermal performance of traditional aromatic polyimides while increasing the free volume within the molecular chains, and the introduction of sulfone groups also weakens the occurrence of intramolecular charge transfer. We employed the designed diamine in conjunction with the fluorine-containing dianhydride 6FDA to prepare high-temperature-resistant, low-dielectric-constant polyimide films with a fully aromatic structure in a one-step progress. The obtained SPTXXO/6FDA (PI-1) film exhibits high optical transparency (λcutoff = 318 nm, T450nm = 95%) and high heat resistance (T5% at 520 °C, Tg at 346 °C). Meanwhile, the dielectric constant of the film at 10 kHz is as low as 2.27 and the dielectric loss is as low as 0.01. Importantly, this low dielectric property can be further maintained at higher frequencies. In addition, the film is soluble in common organic solvents, which facilitates the low-cost progress of the material using conventional methods.