Preparation of Adamantyl-Based High-Performance Polyimides: Synergistic Enhancement of Thermal Stability, Dielectric Properties, and Transparency

Colorless polyimide (CPI) films combine low dielectric loss at high frequencies, remarkable thermal properties, and mechanical performance, playing a crucial role in flexible electronic products and high-frequency communication devices. However, achieving a synergistic balance of the aforementioned properties in polyimide (PI) materials remains a core challenge in the current research field. In this work, various semialicyclic PIs containing adamantane structures were developed via rational molecular design. The introduction of alicyclic adamantane units effectively suppressed benzene ring stacking and disrupted intermolecular π-conjugation, and further suppressed the formation of charge transfer complexes (CTC). The prepared PIs possessed outstanding transparency and appeared colorless. Specifically, the PI films showed a transmittance exceeding 82% at a wavelength of 450 nm, with a yellowness index (YI) below 4.7 and a haze lower than 2.5. Meanwhile, these PIs demonstrated exceptional thermal properties, with a glass transition temperature ( T g ) exceeding 360 °C and a 5% weight loss temperature (T d5% ) reaching 540 °C. Furthermore, benefiting from the highly twisted structure of the PI molecular backbone, the materials also possessed low refractive indices, low birefringence (ranging from 0.0044 to 0.0111), and outstanding mechanical performance (with a tensile strength of 94.6–122.3 MPa). Notably, PIs also exhibited prominent dielectric properties. The dielectric constant ( D k ) of ADDA-BAFL and ADDA-TFDB was 2.44 and 2.54, while their dielectric loss factor ( D f ) was as low as 0.0052 and 0.0061 at 10 GHz, respectively. In conclusion, due to their excellent comprehensive properties, the colorless and transparent PIs with adamantane structures are expected to serve as ideal candidate materials for applications in the optical and optoelectronic fields.