Molecular necklace strategy for enhancing modulus and toughness of colorless transparent polyimides for cover window application

The burgeoning of flexible displays has spawned an urgent need for next-generation cover window materials, which need to combine high modulus, toughness, and ultra-high transmittance. Here, inspired by Velcro® closure, a colorless transparent supramolecular polyimide composite (CPI–C) has been prepared by self-assembly of a crown ether, 18CE6, with a diamine monomer, followed by in situ polymerization with a dianhydride monomer. In the supramolecular composite, the 18CE6 macrocycles were threaded by the CPI chain and fixed on the amide structural unit by hydrogen bonds. Numerous necklace-like molecular chains were mutually entangled to form a Velcro®-like aggregate. Due to strong interactions between the chains imparted by the interlocking rings, they were prevented from sliding under an external force. Our experimental results demonstrated that the unique supramolecular composite improved both the mechanical strength and toughness of the PI film without sacrificing its optical properties. The obtained CPIs showed good optical transmittance (86% at 450 nm), an extremely high Young's modulus (8.3 GPa), and a high elongation at break (23.8%). The described approach may provide guidance for fabricating high strength and high toughness colorless transparent polymer films for flexible cover windows. • Colorless transparent supramolecular composite film of 18CE6/polyimide was prepared. • Composite mechanism of the polyimide and 18CE6 was discussed based on H 1 NMR spectra. • Interlock chain structure improved the mechanical properties of the polyimide without sacrificing its transmittance. • Colorless transparent supramolecular composite film is potential as cover window of flexible displays.