Intrinsic low-dielectric Cardo polyimide with high heat resistance, high transparency, and low birefringence

Low-dielectric, transparent, and high-heat-resistant polyimide (PI) films are vital for flexible electronics and high-frequency communication devices. However, balancing these properties within intrinsic polyimides remains a significant challenge. In this work, a nonplanar diamine monomer, 2,7-bis(3,5-bis(trifluoromethyl)phenyl)-9,9-bis(4-nitrophenyl)-9H-fluorene (BTFDA), containing a Cardo structure and four trifluoromethyl groups, was synthesized and copolymerized with commercial monomers to produce a series of polyimides (CPI-X). These CPI-X exhibit exceptional heat resistance, with glass transition temperatures (T_g) exceeding 350 °C, decomposition temperatures (T_{d5 %}) above 530 °C, and low thermal expansion coefficients (CTE < 20.8 ppm/K). Additionally, they have excellent optical properties with over 92 % transparency at 450 nm and low birefringence (Δn = 0.00042). They also demonstrate good hydrophobicity with low water absorption (M_a% = 0.04 %) and high-water contact angles (C_a = 97.6°). Most importantly, the CPI-X films show excellent dielectric properties (D_k = 2.25, D_f = 0.00365 at 1 MHz, D_k = 2.13, D_f = 0.0065 at 10 GHz). Such intrinsic PI films with low D_k values may be ideal candidates for next-generation interlayer media.