Preparation And Dielectric Properties Of Polyimide/silica Composites

Aromatic polyimide is a kind of high performance polymer material. It has high thermal stability, good mechanical strength and low dielectric constant. The dielectric constant of silicon dioxide is lower than most of other inorganic materials. The aim of this Ph.D thesis work is to improve the dielectric properties of polyimide/SiO₂ composites through combining the good properties of polyimide and silicon dioxide. The preparations, the relationship between structure and properties (especially the dielectric properties) were investigated.First, the polyimide/SiO₂ hybrid films which were prepared following an improved sol-gel process, were investigated in preparation, structure and properties. As TEOS (Tetraethoxysilane) and APTEOS (aminopropyl-triethoxysilane) were under hydrolysised and condensed , the phase of silicon dioxide bonded with PMDA-ODA polyamic acid. After the obtained precursor solution was formed as films and thermally imidizated, the polyimide/SiO₂ hybrid film was obtained. FTIR spectra confirmed the formation of Si-O-Si network and the conversion from PAA into polyimide in hybrid films. AFM and TEM observations showed that SiO₂ was very small and mainly dispersed in fine network state. As SiC>2 content was increased, relative small portion of SiO₂ could form into obviously separated domains in polyimide matrices. The size of these domains increased to micron while the content of SiO₂ content was 30 wt%. TG analysis indicated that the thermal and oxidization stability of PIs decreased with the increase of SiO₂ content. The onset decomposing temperature of hybrids were higher than 520℃, which could meet the request to dielectric materials. The tensile strength and elongation at break of the PI/SiO₂ hybrid films containing 5 wt% silica increased to 99.93 MPa and 26.8% in comparing with 87.5MPa and 17.5% of the pure polyimide. As polyimide chains strongly interacted with silica and the bristle nature of SiO₂, the tensile strength and elongation at break of the Pl/SiO₂ hybrid films containing 20wt% silica decreased 68.31 MPa and 4.3%. With SiC>2 content increasing, the dielectric constants of the hybrid films increased from 3.4 to 5.4, which showed a controllable possibility in certain scope. The dielectric loss increased from 0.01 to 0.02, which showed good stability.Polyimide/SiO₂ nanocomposite films were prepared by blending nano-SiO₂particles with polyamic acid and imidization with heat treatment. FTIR spectra confirm the chemical structure of polyimide/SiCh nanocomposite. TG illustrated that the thermal and oxidization stability of Pis increased with the increase of SiC>2 content. The onset decomposition temperature of PI composites are above 500 °C. TEM showed that the nano-SiO2 particles modified with silicon coupling reagent (aminopropyl-triethoxysilane) could be dispersed more homogenously in polyimide matrix than the un-modified nano-SiC>2 particles. The dielectric constants of polyimide nanocomposite films rose from 3.4 to 4.9 and dielectric loss of polyimide nanocomposite films rose from 0.01 to 0.03 slightly with SiC>2 content increasing from 0 to 50wt%.The relationship between the dielectric constant and dielectric loss for hybrid films and nanocomposite films and temperature, frequency were investigated. As silica gel prepared by sol-gel had stronger interaction with polyimide chain than interaction between nano-silica particles, the variable scope of dielectric constant of hybrid films is smaller than the dielectric constant of nanocomposite films. The dielectric properties of hybrid films showed better stability in temperature and frequency.Several models were discussed to predict the dielectric constant of polyimide/SiO2 hybrid films and polyimide/SiO2 nanocomposite films. The results showed that these theories, such as Logrithmic law, Maxwell-Garnett model, Bruggenman model, etc., were not suitable for these polyimide/SiO2 films. The experience model was used to predict dielectric properties of hybrid films and obtained the experimental factor n. The values of are between -1 and +1. When n is 1, it implies the structure of composite is Parallel connection. When n is -1, it implies the structure of composite is series connection. When n is zero, it implies the structure of composite is unordered composite. As the difference in the silica gel shape, the experimental n from experience models is 0.923 as BTDA-ODA hybrid films, 0.953 as BTDA-MDA hybrid films, 0.63 as BTDA-PDA hybrid films, 0.983 as PMDA-MDA hybrid films, 0.981 as PMDA-ODA hybrid films respectively. The results showed that common theories were not suitable for these nanocomposite films. The EMT (Effective-Medium Theory) model had been used to fit the dielectric constant of nanocomposite films and obtained correction factor, n. A small value of n indicated filler particles having a near-spherical shape, while a high value of nindicated a largely non-spherically shaped particle. As the difference of the scatter degree, the correction factor n from experience models is 0.0220 as PMDA-ODA nano-composite films, 0.0318 as BTDA-ODA nanocomposite films respectively.