Development of Fluorinated Colorless Polyimides of Restricted Dihedral Rotation toward Flexible Substrates with Thermal Robustness

With the rapid expansion of product forms of flexible displays and electronics, the replacement of brittle glass-based substrates with transparent polymer materials is strongly demanded. Colorless polyimides (CPIs) are reckoned as the most promising transparent polymer substrate materials due to their inherent thermal stability, electrical insulation, high transparency, excellent foldability, and proven roll-to-roll processing capability. However, CPIs prepared based on current common strategies have obvious weakness of either transparency or/and thermal/mechanical stability, all of which cannot fulfill the harsh requirements of high-temperature device fabrication. Through great effort, we have developed a series of CPIs based on newly designed highly fluorinated aromatic diamines. The strong electron-withdrawing nature and high bond energy of Ar–F and Ar–CF3 enable a high level of optical transparency while retaining high thermal decomposition stability. This is realized importantly due to the restricted dihedral rotation effect in specific diamines, where multiple –F or –CF3 substitutions effectively restrict the free torsion of benzene rings and create the unique geometrical isomerism. This effect brings the advantages of high glass transition temperatures via increasing the chain rigidity and improved optical transparency due to the breaking of electron conjugation along the chain. Upon further copolymerization optimizations, an excellent synergy ascension in CPI films among high optical transparency, high thermal robustness, and mechanical strength, as well as low thermal expansion, has been achieved, making them promising candidates for new flexible electronics.