Study On The Design, Preparation And Properties Of Polyimide-based Composites With High Dielectric Constant

Electronic, informational and other industries have continuously provided the impetus for the development of easy-processable materials with high dielectric constant. Polyimide, as a kind of polymer with superior properties, can blend with high dielectric constant inorganic particles to obtained new composite materials with the combined advantages of polyimide and fillers. These new composites are expected to have the merits of easy- processing, high thermal stability, light weight, and high dielectric constant.Polyimide/BaTiO₃, polyimide/LTNO (Li and Ti doped NiO), polyimide/MWNTs (multi- walled nanotubes), and polyimide/BaTiO₃/MWNTs composites were prepared through a colloidal process. In this process, the preparing suspension of different filler particles in poly(amic acid) solution, the film casting of the suspension, and the imidization with heat treatment are involved in sequence. The relationship between the structure of the above composites and the properties, especially dielectric properties, of the composites were investigated.Polyimide/BaTiO3 composites were prepared with adding coupling modified BaTiO₃ into polyimide matrix. The influence of processing factors, such as volume fraction, particle size, distribution modal, amount of coupling agent, polymerization method, temperature, and frequency, on the dielectric properties were studied. Results revealed that the dielectric constant (s) and the dielectric loss (tanδ) of the composites increased by using modified particles, bigger fillers or employing in situ polymerization and two-modal distribution. The e and tanδ of these composites increased with the increase of the volume fraction of BaTiO₃ particles, ε, tanδ was 35 and 0.0082 respectively at 10 kHz as the composite containing 50 vol% BaTiO₃. Also, the dielectric properties of the composites displayed good stability within a wide range of temperature or frequency. The coupling agent modified BaTiO₃ particles could homogenously dispersed in polyimide matrix because of improved compatibility between two phases, which led to the better thermal stability of the composites.LTNO particles with very high dielectric constant could be prepared by a simplesol-gel process. Its high dielectric constant originated from the bi-layer structure, which had boundary-layer capacitors (BLC) effect. Polyimide/LTNO composite films were obtained by mixing LTNO particles with the poly(amic acid) precursor solution, casting the solution into film, and then thermal curing. The results revealed that the obtained composites had worse thermal resistance and morphology stability. With the content of LTNO particles in the polyimide matrix increasing, the dielectric constant, loss and the conductivity of composite films increased greatly. For the sample with the Li-03 content at 0.4 in volume fraction, the dielectric constant and loss was 570 and 1.2 respectively. By changing the content of LTNO and the content of Li and Ti in the LTNO particles, the dielectric properties of the composite could be modulated efficiently.Polyimide/MWNTs nanocomposites and polyimide/BaTiO3/MWNTs three-phase composites were also prepared by mixing MWNTs or BaTiO3-MWNTs with the poly(amic acid) precursor solution, casting the solution into film, and then thermal curing. To increase the compatibility between polyimide matrix and MWNTs, MWNTs were modified with mixed acid (H2SO4:HNO3 = 3:1). The results showed that the modified MWNTs were dispersed homogeneously in the polyimide matrix. With the incorporation of MWNTs, the mechanical properties of the resultant nanocomposites were greatly improved due to the strong interfacial interaction between the modified MWNTs and the polyimide matrix. The thermal stability of the nanocomposites and three-phase composites was slightly lower than pure polyimide because of the drop of thermostability of MWNTs through acid-treatment. Moreover, the nanocomposites and the three-phase composites had obvious percolation transition;Pc was 0.07 and 0.12 respectively.The polymide/BaTiO3 was used to explore the valid of some mathematic models for predicting the dielectric constant of two-phase composite. The results showed that some theories, such as Logrithmic law, Maxwell-Garnett model, Bruggenman model, Beer model etc., were not fit for this two-phase composite. But the EMT model and Yamada equation were proved to be a good tool to predict the dielectric properties of this composite. The morphology factor from two models was 0.1 and 10.2 respectively.