Ultra-high Tg colorless polyimide film with balanced optical retardation and coefficient of thermal expansion for flexible display

Traditional colorless polyimide (CPI) films often have high coefficient of thermal expansion (CTE), high optical retardation (R_th) or low glass transition temperature (T_g) values. However, it remains challengeable to establish a balance between low R_th and low CTE without sacrificing the high T_g of CPI film for flexible display applications, especially for flexible thin-film-transistor liquid-crystal display (TFT-LCD) substrate. A set of colorless polyimides films are prepared from copolymerization of alicyclic dianhydrides hydrogenated pyromellitic dianhydride (HPMDA) and cyclopentanone bis-spironorbornane tetracarboxylic dianhydride (CpODA) with aromatic diamines 9,9′-bis (4-aminophenyl) fluorene (FDA) and 4,4′-diaminobenzanilide (DABA). The optical transparency, anisotropy and dimension stability of CPI films are initially optimized by adjusting the DABA-to-FDA molar ratios, and later by HMPDA-to-CpODA ratios. T_g trend analysis, d-spacing calculated from WAXD patterns, and the FTIR wavenumber shifts of the functional groups implied that the strength of hydrogen bonding is relatively higher in CpODA-based CPIs than the HPMDA systems, which may resulted in improved thermal properties of CPI films. It is shown that the polyimide PI-7-5 film displayed excellent comprehensive properties with an ultra-high T_g of 458 °C, low CTE of 29.0 ppm/K, and also a low R_th of 187 nm. Notably, it has a 90 % transmittance in the visible region, along with robust mechanical properties such as a tensile strength of 111 MPa, and an elastic modulus of 3.8 GPa, and good film folding reliability against extensive folding/unfolding cycles, making it an excellent choice for flexible TFT-LCD substrate.