Soluble polyimides with ultralow dielectric constant and dielectric loss and high colorless transparency based on spirobisindane-bis (aryl ester) diamines

With the emergence of 5G technology, the traditional polyimide with a relatively high dielectric constant (D_k) and dielectric loss (D_f) are not suitable the requirements of ultra-high transmission and ultra-low loss. In particular, reducing the D_f value of polyimide has become a significant difficulty in science and engineering. Herein, an elaborate design was proposed for the preparation of polyimide with both low D_k and low D_f, based on a pair of diamine isomer containing two esters and a spiro-bis-indene structure (p-SBI2EA and m-SBI2EA), and their polymerization with 4,4′-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA). The resultant polyimide films showed an exceptional dielectric property, with D_k values of 2.83 and 2.88, and D_f values of 0.0043 and 0.0048 for p-SBI2EA-6FDA and m-SBI2EA-6FDA, respectively, at high frequencies (40 GHz). Interestingly, introducing the spirobisindane structure significantly displayed preeminent solubility in both high-boiling-point and low-boiling-point solvents. Furthermore, the PI films exhibited perfect optical performance, with cutoff wavelengths (λ_cut-off) of 315 and 321 nm, transmittance exceeding 86 % and 89 % at 450 nm, and yellowing indices (YI) as low as 1.72 and 1.63, respectively. The features of the PIs render them auspicious candidate materials for applications in 5G high frequency communication and optical fields.