| High-k (dielectric constant) conductor/polymer composites have received increasingattentions in recent years owing to their great potential in many cutting-edge fieldsincluding microelectronic, mechanical engineering, biomedicine, energy, etc. However,according to the percolation theory, the dielectric loss is usually quite high owing to theinsulator-conductor transition near the percolation threshold (fc), although high dielectricconstant will also be obtained. Therefore, how to prepare high k conductor/polymercomposites with low dielectric loss is a key problem. At present, forming a coating on thesurface of the conductor is an effective method to reduce the dielectric loss. While theabove scheme can not meet the requirements of modern industrial materials, because of thelargely increased percolation threshold (fc≥10wt%) and processing difficulty, extremelydamaged heat resistant and mechanical property, sharply decreased charge storage capacityof a single capacitance. So, how to greatly reduce the dielectric loss and ensure a lowpercolation threshold based on high dielectric constant is an important research topic in thefield of high dielectric constant material. The aim of this study reported herein focuses onthese problems.First, we have synthesized hyperbranched polyaniline (HSiPA) with thecharacteristics of both the polyaniline and hyperbranched polysiloxane, which is excellentelectrical conductivity, high thermal stability and improved solubility. The MWCNTs werecovered with HSiPA though the π-π interaction between the SP2structure of HSiPA and πbanded surface of MWCNTs, and then the hybrids were added into the EP resin to explorethe dielectric properties as the changes of the different content of the coating layer. And theresults show that the dielectric properties of the composites are closely related to theamounts of HSiPA. Essentially, as the content of HSiPA increases, the dispersion ofMWCNTs in the matrix is improved, while the charge storage capability gradually reduces.The dielectric constant and dielectric loss (100Hz) of HSiPA4/MWCNT0.5/EP ternarycomposites were7.1and4.3×10-3times the corresponding value of MWCNT0.5/EPcomposite, and the percolation threshold is only1.26wt%. The substantially increaseddielectric constant is due to the improved dispersion of MWCNTs because of the coating of HSiPA, and then the number of capacitance per volume increases and the interfacialpolarization effect enhances. The greatly reduced dielectric loss is due to the presence ofcladding layer, which cuts down the leakage current by preventing the connection ofMWCNTs.Second, in order to further investigate the origin of the dielectric properties,end-capped hyperbranched polyaniline (EHSiPA) was synthesized, and the hybrids wereprepared through π-π interaction, then the effect of the incorporation of the hybrid to EPresin on the dielectric properties were evaluated. Results disclose that the dielectricconstant and dielectric loss (100Hz) of EHSiPA3/MWCNT0.4/EP ternary composites were1.4and6.8×10-3times the corresponding value of MWCNT0.4/EP composite. Spacecharge distribution shows that the greatly improved dielectric constant is mainly due to thelargely increased micro-capacitance, but not the interfacial polarization.Finally, to decrease the fcof the composites, and keep high dielectric constant and lowdielectric loss, based on the molecular design, a novel multi-branched polyaniline (MSiPA)was synthesized by the hydrolysis of organic silicone modified polyaniline. And MSiPAwas used as the coating layer of MWCNTs, then ternary composites were prepared. Theresults show that the composites not only obtain good dielectric properties, but also haslow fc(0.48wt%). The decreased fcis attributed to the unique conjugated multi-branchedstructure and a low space steric of MSiPA, and the polyaniline chain could cover theoutside of the MWCNTs through π-π interaction, and then MWCNTs could be effectivelydispersed by the less amount of MSiPA. |
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