Solution‐processable colorless polyimides with ultralow coefficients of thermal expansion for optoelectronic applications

Abstract This work reports colorless polyimides ( PIs ) that are applicable as plastic substrates in image display devices, which produce an ultralow coefficient of thermal expansion ( CTE ) by solution casting without thermal imidization and mechanical stretching. An effective monomer molecular design is proposed for this purpose. Chemical imidization ( CI ) process compatibility is the key factor in attaining the target properties. We focused on a PI system derived from 1,2,3,4‐cyclobutanetetracarboxylic dianhydride ( CBDA ) and a novel para ‐amide‐linked diamine ( AB‐TFMB ) with CF 3 groups as it has great potential as an ultralow CTE material, although it offers no CI process compatibility because of its poor solubility. The CBDA / AB‐TFMB system was modified by copolymerizing with 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride. This approach drastically improved CI process compatibility. The CTE of the PI films linearly decreased with increasing CBDA content. At a CBDA content of 70 mol%, the PI achieved an ultralow CTE of 7.3 ppm K −1 , non‐coloration/non‐turbidity, a very high glass transition temperature of 329 °C and sufficient ductility. The ultralow CTE results from the highly oriented main chains along the X–Y direction during the casting process as supported by the very high birefringence exceeding 0.1. Thus, our materials almost achieved the target properties required for novel coating‐type high‐temperature plastic substrate materials. © 2016 Society of Chemical Industry