Reducing Intermolecular Friction Work: Preparation of Polyimide Films with Ultralow Dielectric Loss from MHz to THz Frequency

Polyimide (PI) films with simultaneously low dielectric loss (Df) values at high frequencies and excellent general properties are urgently needed with the rapid development of high frequency communication technology. Herein, we proposed that dielectric loss can be decreased by reducing intermolecular friction work. Inspired by liquid crystals with low intermolecular frictions, novel liquid-crystal-like PI films were designed by simultaneously introducing mesogen units and promoting groups (-CF3) and then being thermally imidized via a unique solvent-assisted continuous ramping process without any stretching or shearing process. The liquid-crystal-like PI films exhibited ultralow Df values such as 0.00365 at 1 MHz, 0.00184 at 10 GHz, and 0.0027 at 0.75THz, which were 28.7%, 45.4%, and 97.0% lower, respectively, than those of PI films without liquid-crystal-like structures. The results of molecule simulations further demonstrated that liquid-crystal-like structures can reduce intermolecular friction work by slowing dipole rotation, leading to ultralow Df values of PI films. The liquid-crystal-like PI films also exhibited thermal expansion coefficients of 11.4 ppm/K, tensile strengths of 228.7 MPa, Young's moduli of 6.15 GPa, and elongations at break of 16.4%.