Super-heat resistant, transparent and low dielectric polyimides based on spirocyclic bisbenzoxazole diamines with Tg > 450 °C

Maintaining ultra-high heat resistance and sufficient colorless transparency at the same time is a challenge for polymer materials because of conflicting design principles, but such materials are urgently needed for some optoelectronic devices. In this study, four new isomeric diamines with a spirocyclic bisbenzoxazole structure were synthesized, and their polyimides were prepared by condensation with commercial diamines, 4,4 '-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and cyclobutane-1,2,3,4-tetracarboxylic acid dianhydride (CBDA) followed by chemical imidization. The resultant polyimide (PI) films have excellent optical transparency with cut-off wavelengths in the range of 346 nm-380 nm, and 85%-90% transmittance at 550 nm. Also, these PIs possessed excellent solubility in common organic solvents, even in low-boiling-point solvents, such as chloroform, dichloromethane and tetrahydrofuran. Most importantly, the colorless polyimides displayed a super-high glass transition temperature (T-g), high thermal stability, and favorable mechanical properties, and their T-g reached 491 degrees C. The polyimides possessed low moisture absorptions of 0.52%-0.61% and low dielectric constants (D-k) of 2.32-2.93. The excellent comprehensive properties of the spirocyclic bisbenzoxazole-containing colorless transparent polyimides (CPIs) render these materials promising candidates for optical and optoelectronic applications.