Preparation of soluble colorless poly(amide‐imide) films with high glass transition temperature and low coefficient of thermal expansion by coordination interaction of Li cations

Abstract To meet the needs of high heat‐resistant and colorless polyimide (CPI) films, a series of poly(amide‐imide) (PAI) films were synthesized by copolymerization of 1,2,3,4‐cyclobutanetetracarboxylic dianhydride (CBDA), terephthaloyl chloride (TPC), 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,2′‐bis(trifluoromethyl)benzidine (TFDB) with the addition of lithium chloride (LiCl) for ionic coordination. The coordination of Li cations with carbonyl groups significantly reduced the coefficient of thermal expansion (CTE) and improved the mechanical properties. Moreover, by adjusting the ratio of CBDA and TPC, all PAI films exhibited high glass transition temperature (Tg), low CTE and high transparency. Among these PAI films, PAI‐6‐2Li exhibited excellent comprehensive performance with a high Tg of 363 °C, low CTE of 6.4 ppm/K, high transmittance of 84% at 400 nm and tensile modulus of 7.2 GPa, which was expected to be used in the field of flexible displays.