High-Strength Transparent Films with Enhanced UV Stability and Folding Reliability via Molecular Control of Chain Rigidity and Localized Interactions

A high-strength, transparent, and flexible optical film was developed by incorporating biphenyl-based oligoamide segments into a conventional colorless polyimide backbone. The biphenyl segments increased effective chain rigidity, while the localized amide-rich sequences provided potential sites for multiple interchain hydrogen-bonding interactions that may contribute to intermolecular cohesion. The optimized film exhibited excellent optical properties, including a total transmittance of nearly 90%, a haze below 1%, and a yellowness index below 3. However, a further increase in the biphenyl-based component content resulted in pronounced intrinsic coloration. The film also exhibited high resistance to ultraviolet-induced yellowing, with a change in yellowness index of less than 1.0 after 72 h of UV irradiation. It achieved a tensile modulus of 6.1 GPa and a tensile strength of 174.5 MPa while retaining comparable elongation at break. Importantly, the resulting film withstood 200,000 folding cycles under severe cyclic deformation at a folding radius of only 1 mm without visible cracking, creasing, or crazing. These results demonstrate that the combined regulation of chain rigidity and localized interchain cohesion provides an effective molecular design strategy for high-strength, transparent, and flexible aromatic films with enhanced folding reliability for cover-window and display-related optical applications.