Colorless Polyamide–Imide films with tunable coefficient of thermal expansion and their application in flexible display devices

Abstract

In a previous study, our group reported the fabrication of a series of colorless polyamide–imide (PAI) films (named TFDB-DABPO-TPC-6FDA) with good thermal stability. To further improve their properties, herein, the effect of adding blocks in molecules on the performance of polyimide (PI) films was studied. By changing the order and amount of the four reactants added each time, seven PAIs with different structures and block lengths were obtained. Their glass transition temperatures (T_g) ranged from 359 °C to 368 °C, and their transmittance at 430 nm (T₄₃₀) was 82.7%–85.1 %. Importantly, the coefficient of thermal expansion (CTE) of these seven PAIs decreased almost linearly from 31.1 to 18.5 ppm/K, making it possible to fabricate PI films with a desired CTE value for specific applications. Furthermore, the PAI-6-T film, which had the longest polyamide block and the best overall performance (T_g = 362 °C, CTE = 18.5 ppm/K, T₄₃₀ = 83.1 %, tensile strength = 179.4 MPa, dielectric constant (D_k) = 2.53, and dielectric loss (D_f) = 0.0088 at 10 GHz) was used as a substrate to fabricate a flexible printed circuit board (FPCB) and an organic light-emitting diode (OLED). Mechanical–electrical test shows that Ag was firmly bonded with the PAI-6-T film, and the resistance of FPCB@PAI-6-T only increased by 14.8 % after 150,000 folding tests. The flexible OLED@PAI-6-T exhibited pure green emission, a turn-on voltage of 3.0 V, a luminance of 1000 cd/m² at 6 V, a current efficiency of 1.31 cd/A, and an external quantum efficiency of 0.44 %.