Investigation On Mechanical Properties And Electric Properties Of Polyimide/Silica Nano-Hybrid Films

As a type of high-performance polymers, PI is already widely used in aerospace and microelectronic industries because of their outstanding thermal stability, mechanical and electrical properties. However, with the rapid technological improvement of these industries, higher performance PI films are required. In recent years, a number of studies have been conducted by different workers of the word on modification of PI with inorganic nano-particles. But unfortunately, there still remain disputing results on the mechanical and electrical property of hybrid film, and theoretic analysis is not deeply enough.In this paper, PI/SiO₂ nano-hybrid films were prepared through the Sol-Gel process using triethoxysilane (TEOS) and methyl-triethoxysilane (MTEOS) as inorganic precursor respectively. In the process of preparation, polymer matrix was based on the polycondensation of 4, 4′-diaminodiphenylether (ODA) and pyromellitic dianhydride (PMDA), and inorganic phase was obtained via the hydrolysis and condensation of inorganic precursor.The chemical structures of hybrid films have been analyzed by Fourier Transform Infrared Spectroscope (FT-IR). The result showed that the silica could be introduced into PI matrix by schematic route of this experiment. The micrographs of Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM) indicated that silica particles were finely dispersed in polymer matrix. In certain range of silica content, the silica particles dispersed in nano-size. The shapes of these silica particles were largely depended on the type of organic precursor employed in the synthesis. For example, the shape in the TEOS based hybrid systems was sphericity-like, but it was rod-like in the MTEOS based hybrid system. The curve of Wide-angle X-Ray diffraction (WAXD) confirmed that the introduction of silica destroy the ordering of polyimide molecular chains.The mechanical property of PI /SiO₂ hybrid films was tested by Electron Universal Testing Machine. The result showed that the tensile strength and elongation at the break of series A films reached to maximum at silica content of 3wt%, and to series B films the silica content was 1wt%. The introduction of silica was good for mechanical property of hybrid films, but with some disadvantages simultaneously. The silica nano-particles were similar to crosslinking agent. And they were good for improving the tensile strength and elongation at the break of hybrid films. Mechanical properties of hybrid films were reduced mainly by destroying the ordering of PI molecular chains.In this paper, the electrical property of hybrid film and its influence factors have been discussed mainly. The different parameters are respectively depended on varied influence factors. The number of polar group and the combination between organic and inorganic phases are main factors influencing dielectric coefficient and dielectric loss of hybrid films. For series A and series B films, the dielectric coefficients of them are higher than that of pure PI film, and change little with the frequencies increasing. But the dielectric loss of them increased obviously with frequencies. Volume electrical conductivity of series A films increased with the silica content. For series B films, it reached to maximum at silica content of 12.5wt%, and decreased with increasing silica content. The introduction of silica improved the thermal conductivity of hybrid films. Simultaneously, nano-particles possessed high energy of surface absorption, and consumed more energy. To some extent, thermal breakdown and electric breakdown of hybrid films were restrained, and breakdown strength of hybrid films were improved. The silica content influenced corona-resistance property of hybrid film directly. The corona-resistance of series A and series B films reach to maximum when silica content was 30wt% and 25wt%, respectively, and it was 47 times of pure PI film.