Study On Multi Phases Matrix Composies With High Dielectric Constant And Low Dielectric Loss

Among the conductive fillers, MWCNTs are often used to prepare Hik-PMCs due to their excellent electric properties and large ratio of the length to diameter, which can bring the insulation-conductor transition, and thus resulting in exceptionally high permittivity. However, due to the formation of electronic conduction paths, conductive filler/polymer composites generally have high dielectric loss. On the other hand, to keep the processing adventages of polymers, the lower percolation therold become more attractive. The composite with double percolation structure haven’t effectively improved the dielectric constant and decreased the dielectric loss so far. Therefore, how to adjust the structure of the novel matrix of composite and to obtain the excellent combination of dielectric properties still face enormous challenges. And this thesis is focusing on the researches of combining the two themes.Herein, a fixed loading(0.4 wt%) of multi-walled carbon nanotubes(MWCNTs) was embodied into incompatible polyetherimide(PEI)/bismaleimide(BD) system to prepare a series of 0.4MWCNT/PEI/BD composites. As the PEI content increases, the structure of composite successively changes from sea-island to co-continuous phase and phase inversion. More interestingly, MWCNTs prefer to selectively distribute in BD phase, tend to enrich around the PEI dense zone and arrange normally to the radius of the PEI sphere zone, so the morphology of MWCNTs and thus dielectric properties of composites can be facilely controlled. The dielectric constant and loss of 0.4MWCNT/PEI/BD composite with 10 wt% PEI are about 4.5 and 0.1 times the values of 0.4MWCNT/BD composite, respectively. Different equivalent circuits were built for these composites, revealing the origin behind the method developed herein for controlling unique dielectric properties of 0.4MWCNT/PEI/BD composites.Secondly, the PEI20/BD shows the reverse phase when the weight ratio was fixed at 20:100, which was selected as the matrix and the distribution of MWCNTs in different phases(h-MWCNT/20PEI/BD and MWCNT/20PEI/BD composites) were prepared through the Haake instrument and solution processing, repectively. And the results of the study show that the dielectric properties are significantly different between the two kinds of materials. At the same content of MWCNT, compared with MWCNT/20PEI/BD composite with MWCNT distributed in the BD phase, the percolation threshold of h-MWCNT/20PEI/BD composite with MWCNT distributed in the PEI phase has a higher dielectric constant and lower dielectric loss. At 100 Hz, the dielectric constant of h-0.45MWCNT/20PEI/BD reached as high as 1742, which is 25 and 562 times the values of 0.5MWCNT/20PEI/BD and PEI/BD. At the same time, the percolation threshold is as low as 0.35 wt%, 0.6 times the value of MWCNT/20PEI/BD composites. The percolation threshold reduction is based on MWCNTs distributed in PEI phase to form a conductive network structure, and the increase of dielectric constant is attributed to the enhanced interfacial polarization and increased micro capacitance, rather than a simple additivity rule. These interesting results show that by adjusting the location and dispersion state of MWCNTs in incompatible polymer blend can provide a simple method for the preparation of composite with excellent performance of comprehensive dielectric properties.