Study On The PI/Nano-Al₂O₃ Three-layer Composite Films

In recent years, increasing attention had been paid to the polyimide/inorganic nanocomposite materials. It has been proved that the thermal and electrical properties of PI films can be improved, however, mechanical properties of PI films decreased by incorporation of fillers such as silica, alumina and titania into the pristine polyimide matrix. It put forward the requirements for preparation of PI films with good comprehensive properties.In this paper, a series of polyimide(PI)/nano-Al₂O₃ three-layer composite films were prepared with thermal imiditation after pure poly(amic acid) or Al₂O₃/poly(amic acid) solution was cast on glass plate step by step, poly(amic acid) were synthesized by step-polycondensation of 4, 4′-oxydianiline(ODA) and pyromellitic dianhydride(PMDA) in the solution of N, N-dimethylacetamide (DMAc), Al₂O₃/poly(amic acid) solution was prepared by the addition of the nano-Al₂O₃ which had been treated by coupling agent into the poly(amic acid) solution by ultrasonic wave through the ultrasonic-mechanical method. The influence of film forming temperature and time of the first and second layer and thickness ratio of pure layer and single hybrid layer on the mechanical properties of PI composite films was analyzed. On this basis, a scheme of orthogonal test, which included three factors and three levels, was used and analyzed. The result showed that the optimal preparation condition of PI composite film with good mechanical properties was conculuded as that film forming temperature 180℃, film forming time 90min, the thickness ratio of pure layer and hybrid layer 2:1.The micro-morphology and structure of PI films were studied by scanning electronic microscope(SEM) and fourier transform infrared spectrum(FT-IR). SEM analysis showed that there was clear three-layer structure in the PI composite films, and the silane coupling agents varieties and the nano-Al₂O₃ content had much effect on the dispersion of nano-Al₂O₃ particles in the PI composite films.The transmittance of UV-Vis, thermal stability, mechanical properties and electric breakdown strength of PI films were tested. UV-Vis transmittance analysis revealed that the transmittance of PI composite films was 7.2% higher than hybrid film at the wavelength of 600nm, and the silane coupling agents varieties and the nano-Al₂O₃ content had obvious effect on the transmittance of the PI composite films. In these four kinds of composite films, the transmittance of PI/nano-Al₂O₃ composite film with KH560 was the best, which was higher 8.8% than unmodified film at the wavelength of 600nm. TGA analysis showed that the decomposition temperature of PI composite film was 28.1℃higher than pure PI film and 17.4℃higher than hybrid film at 10% weight loss respectively. The decomposition temperature of PI composite films increased with increase of nano-Al₂O₃ content, but the silane coupling agents varieties had less effect on the thermal stability of PI composite films. The tensile strength of PI composite film was 8.4% higher than pure PI film and 11.9% higher than hybrid film respectively, the elongation at break of PI composite film was 43.6% higher than hybrid film, but was slightly higher than pure PI film. The mechanical properties of PI/nano-Al₂O₃ composite film with KH550 were the best. The tensile strength and elongation at break of PI composite film were 119.1Mpa and 19.1%, which were 14.2% and 78.5% higher than unmodified PI composite film respectively. PI composite films had improved electric breakdown strength in comparison with pure PI film and hybrid film. The silane coupling agents varieties had less effect on the electric breakdown strength of PI composite films. When the nano-Al₂O₃ content was low, electric breakdown strength of PI composite films rised with increase of nano-Al₂O₃ content. However, further increase in the nano-Al₂O₃ content lead to a gradual decrease in the electric breakdown strength. The electric breakdown strength of PI composite films with nano-Al₂O₃ content 6wt% was 284kV/mm, which was the best in the PI composite films with different nano-Al₂O₃ content.