Effect of Polar Groups in Polyimide Main Chain on Its Dielectric Properties

In this paper, three polyimide films (PI-I, PI-II, PI-III) with different main chain structures were synthesized from three types of dianhydride monomers and one type of diamine monomer. Polar groups (-C(=O)-, -SO₂-) were introduced into PI-II and PI-III, respectively, while PI-I was used as a control to study the effect on dielectric properties of introducing polar groups into the dianhydride main chain. Firstly, infrared spectra and thermogravimetric characterization were performed on the three prepared PIs, followed by measurements of their dielectric permittivity (εʹ), breakdown strength (E_b) and conduction current density. It is found that introducing polar groups into the main chain of dianhydride can increase the dielectric permittivity. The dielectric permittivity of PI-III is 4.096 (50 Hz, 25 °C), an increase of 28.3% compared to PI-I. Furthermore, introducing polar groups can increase the breakdown strength and reduce the conduction current density. PI-II achieves the highest breakdown strength (E_b = 544.24 MV/m) and the lowest conducted current density. Therefore, introducing polar groups can simultaneously increase the dielectric permittivity and breakdown strength of polyimide, then improving its dielectric properties. This work provides a reference for better designing polyimide films with performance requirements.