Polyimide Modified with Different Types and Contents of Polar/Nonpolar Groups: Synthesis, Structure, and Dielectric Properties

Polyimide-based dielectric materials, as excellent high-temperature-resistant polymers, play a crucial role in advanced electronic devices and power systems. However, given the limitations of composite PI materials, it is a significant challenge to simultaneously improve the dielectric constant and breakdown strength of intrinsic polyimide dielectric materials to achieve high energy density. In this study, an indiscriminate copolymerization method was proposed, which utilizes two different diamine monomers with bulky side groups (-CF3) and high polarity (C-O-C), respectively, to copolymerize with the same dianhydride monomer and prepare a series of intrinsic PI films. Remarkably, PI films with a highly dipolar rigid backbone maintain excellent thermal and mechanical properties while enhancing dipole polarization. Meanwhile, a high breakdown strength of PI is shown, because the bulky side groups act as deep traps to capture and disperse charges during the charge transfer process. Under the optimal copolymer ratio, the dielectric constant and dielectric loss are 4.2 and 0.008, respectively. At room temperature, the highest breakdown strength reaches 493MV/m, and the energy storage density and charge-discharge efficiency are 5.07 J/cm³ and 82%, respectively. Finally, based on density functional theory calculations, the copolymerization tendencies of the three monomers are verified, and it is speculated that the copolymerization ratio of PI-60% is the most stable and exhibits the best overall performance, which perfectly aligns with the experimental results. These experimental results demonstrate the exciting potential of intrinsic polyimide in thin film capacitors.