In this study, KMnO4 was applied as an etchant to partially decompose the silica microspheres into mesoporous structures. 1H,1H,2H,2H-perfluorodecyltriethoxysilane was grafted onto the surface of the hollow silica. A fluorinated silica/polyimide hybrid material was prepared. Meanwhile, the influence of fluorinated silica on the dielectric property and moisture absorption property of the hybrid material was discussed. It is found that the fluorinated silica can lower the dielectric constant and hygroscopicity of the hybrid film. The fluorinated silica is characterized by strong hydrophobicity, good dispersibility, and good thermal stability. Even at a very low filling level of only 2 wt%, the dielectric constant of the polyimide hybrid is significantly reduced to 2.61 without sacrificing thermal stability. The water absorption rate of the polyimide hybrid film is reduced from approximately 3.1% to 2.1 wt%. Moreover, this provides a new idea for reducing the dielectric and water absorption properties of materials.

A series of three-phase composite films with different filler contents were prepared by in-situ polymerization. The composite films comprise polyimide (PI), poly (vinylidene fluoride) (PVDF), and titanium dioxide (TiO2). Compared with PI/TiO2 composite films, the PI/TiO2-PVDF composite films not only get a significant increase in dielectric constant, but also own better mechanical properties. Our results show that with the loading of 50wt% PVDF particles, the dielectric constant of PI/TiO2-PVDF composite films increased from 6.5 to 18.14 at 1 MHz and room temperature, while the tensile strength of PI/TiO2-PVDF composite films increased from 45 to 72 MPa. In addition, the films maintain a low loss tangent of about 0.02. PI/PVDF composite films were also prepared. It was found that dielectric constant of PI/PVDF composite was significantly lower than that of PI/TiO2-PVDF composite films when the loading of PVDF is 50wt%.

Flake graphite @polyimide (FG@PI) particles with core @shell like structure were successfully synthesized and subsequently incorporated into PI matrix to prepare pI/FG@PI composite films. The scanning electron microscope (SEM) and transmission electron microscope (TEM) images showed that flake graphite (FG) was covered by PI coating and showed a fluffy structure. Thermogravimetric analysis (TGA), FT-IR, and XRD were also used to characterize FG@PI particles. The PI intermediate layer increases interaction between particles and films, which is very beneficial for increasing dielectric constant and inhibiting dielectric loss of pI/FG@PI composite. The experimental result shows that the pI/FG@PI composite film with 50wt% filler loading exhibits a high dielectric constant of 83.5 (about 24 times that of pure PI) and low dielectric loss of 0.11 at 1 MHz. Furthermore, its tensile strength is maintained at 78 MPa. This shows that the structure of PI intermediate layer is very beneficial for preparation of high dielectric constant and low dielectric loss composite.

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In this work, we prepared polyimide (PI) composite films by directly filling the matrix with self-derived carbon particles. Without any surface modification layer, these specially made particles were compatible with their parent matrix quite well. For the composite film with 25 wt% filler content, the dielectric constant was 72.5 at 1 MHz at room temperature, and the dielectric loss was only 0.069. The conductivity of the corresponding composite was measured to be below 10−6 Sm−1, and the breakdown strength was 165 MVm−1. In addition, the tensile strength of the composite film was measured to be 73 MPa. These results indicate that carbonized PI can be used as an excellent filler to prepare PI-based composite films with high dielectric constant.