Preparation and properties of high temperature colorless transparent polyimide containing semi-alicyclic and bisbenzoxazole structure

In recent years, with the development of the flexible display field, there has been an increasing demand for colorless polyimide (CPI) that can be used for transparent displays. In this work, two cis- and trans-isomers of diamines 2,2'-((1r, 4r)-cyclohexane-1,4-diyl)bis (benzoxazol-5-amine) (trans-CHDB) and 2,2'-((1s, 4s)-cyclohexane-1,4-diyl)bis (benzoxazol-5-amine) (cis-CHDB) containing benzoxazole and cyclohexane structures were synthesized, and two series of polyimides CPI were prepared with three commercial dianhydrides: 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) and cyclobutane-1,2,3,4-tetracarboxylic dianhydride (CBDA). Presence of bisbenzoxazole structure enhanced rigidity and linearity of CPI molecular chain. CPI trans-CHDB/CBDA exhibited excellent comprehensive performance with a high T_g of 404 °C, coefficient of thermal expansion of 27.4 ppm K⁻¹ Outstanding tensile modulus of 4.0 GPa. By introducing cyclohexane structure to suppress charge transfer interaction, optical transparency of CPI was increased. The transmittance of trans-CHDB/CBDA at 450 nm reached to 83.7 %, which was supposed to a promising application prospect for flexible display substrates.