Low Dielectric Fluorene-Based and Fluorinated Copolyimide Films with Simultaneously Improved Properties via Thermal Cross-Linking Reaction

Fluorinated polyimide (PI) films have garnered significant attention for low dielectric applications. However, most existing fluorinated PI films exhibit relatively poor thermal stability, mechanical properties, and chemical stability. It is urgent to develop high-performance fluorinated PI films that simultaneously exhibit high heat resistance, superior thermal stability and mechanical strength, decent chemical stability, and low dielectric properties to fulfill the requirements of 5G communication technology. In this study, a hydroxyl-containing fluorene-based and fluorinated copolyimide ( PI–C 0 ) has been designed and synthesized by a facile one-step solution polymerization. The hydroxyl-containing fluorene-based cardo structure in PI–C 0 chains can provide thermal cross-linkable sites to prepare cross-linked PI ( PI–C X ) films. After the thermal cross-linking reaction of PI–C 0 film, the cross-linked PI–C X films are insoluble in most common organic solvents, indicating that their solvent resistance is improved. Moreover, the cross-linked structure in PI–C X films can effectively improve their thermal and mechanical properties as well as maintain low dielectric properties. Among PI–C X films, PI–C 72 film shows outstanding comprehensive properties including a high glass transition temperature ( T g ) of 413 °C, a high 5% degradation temperature ( T d5 ) of 510 °C, a tensile strength ( T S ) of 121.76 MPa, a low dielectric constant ( D k ) of 2.63 at 5 GHz, and a low dissipation factor ( D f ) of 0.0090 at 5 GHz. Therefore, the combined advantages of a convenient synthesis method and solution processability of the precursor PI–C 0 film and balanced properties in thermal, mechanical, and dielectric properties of these cross-linked PI–C X films make them promising favorable potential as a low dielectric material for 5G high-frequency interlayer dielectric applications.