Copolyimides with Adamantyl Side Chains: Achieving Enhanced Optical Transparency and Low Dielectric Properties

Colorless polyimide (CPI) films are critical substrate materials in the fields of flexible displays and wearable devices and have attracted significant research interest in recent years. In this work, a diamine monomer bearing pendent adamantyl groups, 3,5-diamino- N -(adamantyl)benzamide (ADDM), was synthesized. The diamine was copolymerized with 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) to prepare copolyimide films with different ADDM contents. PI films demonstrate outstanding thermal behavior and excellent mechanical strength, characterized by a glass transition temperature ( T g ) over 370 °C and tensile strength above 98 MPa, as well as favorable solubility and low water absorption. The introduction of ADDM structural units incorporates bulky adamantyl groups into the polymer side chains, which restricts compact packing of polymer backbones and diminishes molecular polarizability, ultimately optimizing their dielectric properties and enhancing the optical transparency of the material. Among them, API-100 (6FDA-ADDM) shows the optimal optical transparency, with its transmittance at 450 nm reaching up to 83.6%, yellowness index (YI) as low as 3.29, and haze merely 1.85. The results of high-frequency dielectric tests indicate that the dielectric constant of API-100 is 2.74 at 10 GHz, with a dielectric loss factor of 0.0056. In summary, the PI films exhibit superior comprehensive characteristics and are highly promising for use in microelectronic components and flexible display technologies.