Short-side-chain regulation of colorless and transparent polyamide-imides for flexible transparent displays

Abstract

Improving the heat resistance, dimensional stability and mechanical properties of colorless and transparent polyimides remains a popular and promising topic as it is considered to be the basis for their application in next-generation flexible displays. However, enhancing these properties without sacrificing optical transparency is challenging. Although recent research has revealed that hydrogen bonding is a potential route to balance optical and other physical properties, no systematic studies or comparisons have been performed. Here, we synthesized five amide-containing diamines with different substituents and prepared the corresponding polyamide-imides (CPAIs). Short-chain substituents were introduced to regulate the conformation between benzene and imide rings, as well as the ability of amide hydrogen bond formation. The effects of the position and number of –CH3 and/or -CF3 substituents on blocking the formation of charge-transfer complexes and on the strength of intermolecular hydrogen bonds, which affect CPAI transparency and other properties, were investigated. The results demonstrated that the combination of the twisted conformation from ortho-methyl groups and hydrogen bonds from the amide promoted the optical, thermal and mechanical properties of the CPAIs. In particular, CPAI-7 exhibited a high optical transmittance at 400 nm of up to 86 %, a Tg as high as 402 °C, a low thermal expansion coefficient of 17 ppm K−1 and an extremely high tensile modulus of 5.2 GPa. The excellent and comprehensive performance of the CPAI films illustrates their potential application in the field of flexible displays.