Synthesis of colorless transparent poly(amide-imide) with high modulus and low thermal expansion coefficient and its performance study

To meet the needs of high dimensional stability and high transparency of colorless polyimide (CPI) films, a series of poly(amide-imide) (PAI) films were synthesized by copolymerization of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), terephthaloyl chloride (TPC) and 2,2′-bis(trifluoromethyl)benzidine (TFDB). By adjusting the ratio of 6FDA and TPC, the resulting PAI films exhibited high glass transition temperature (Tg) ranging from 347 to 366 °C, low coefficient of thermal expansion (CTE) values of 4–48 ppm/K, excellent optical performance with transmittances at 400 nm (T400) of 79–83 %, and sufficient mechanical performance. The rigid rod-like amide bonds in the molecular chains increased with the introduction of TPC, which increased the hydrogen bonding between the molecular chains, significantly reduced the CTE and improved the tensile strength and modulus. In particular, PAI-7 exhibited excellent comprehensive performance with a high Tg of 366 °C, a relatively low CTE value of 13 ppm/K, excellent transmittance of 81 % at 400 nm and high tensile modulus of 6.5 GPa, which was expected to be used in the field of flexible displays.