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.