Colorless poly(amide‐imide) copolymers for flexible display applications

Abstract Colorless polyimide (CPI) films showed great potential for flexible display applications due to their unique combination of excellent optical transparency, and outstanding thermal and mechanical properties. Herein, a series of colorless poly(amide‐imide)s were prepared via a conventional two‐step method, using 1,2,3,4‐cyclobutanetetracarboxylic dianhydride (CBDA), 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 2,2′‐bis(trifluoromethyl)benzidine (TFMB), and 4,4′‐diaminobenzanilide (DABA) as the monomers. The thermal, mechanical, and optical properties of these copolymers were optimized by adjusting the molar ratios of these monomers. The amide linkages significantly improved the mechanical properties and dimensional stability of the resultant CPI films due to strong hydrogen bonding interactions. In particular, CPI‐f films with monomer compositions of TFMB (50 mol%), DABA (50 mol%), 6FDA (50 mol%), and CBDA (50 mol%) exhibited tensile strength of 177 MPa, modulus of 5.5 GPa, coefficient of thermal expansion value of 18 ppm K −1 , and glass transition temperature of 394°C. Furthermore, the optical properties of these copolymers were well balanced due to the adoption of CBDA, which can suppress the formation of charge transfer complexes. Specifically, these CPI films displayed transmittance at 550 nm of >88%, b * values of <5.5, haze values of <1%. The aforementioned results revealed that copolymerization is an effective strategy to achieve CPIs with excellent comprehensive properties.