Studies On The Preparation And Electrical/Thermal Conductivities Of Polymer-based Composites

With the rapid development of science and technology, polymer-basedfunctional nanocomposites have been investigated for many potentialapplications. Therefore, the method to prepare high-performancenanocomposites is becoming more and more important. Polymer-basedelectrically/thermally conductive composites, which are prepared bycompounding polymer matrix and functional fillers, can be used in manyelectronic fields. Usually, high loading of electrically/thermally conductivefillers, which is needed to meet the requirements, not only impairs themechanical properties and processability but also increases the cost anddensity of the composites. The incorporation of rigid particles would reducethe toughness of the composites and make the composites brittle. Therefore,we are focusing on the reduction of filler content while maintaining theexcellent electrical/thermal properties. The main work is as follows:1. Mltiwalled carbon nanotubes (MWNTs) were incorporated intopolyamide12(PA12) by melt compounding, we studied the electrical,dynamic mechanical and thermal properties of the binary nanocomposites.The results indicate that the electrical conductivity increases rapidly withMWNTs. The PA12/MWNTs binary composites showed an electricalpercolation threshold of1.45wt%. Also, the storage modulus and degradationtemperature increase with the increase of MWNTs. In order to further enhancethe electrical conductivity,20wt%polyethylene-octene elastomer grafted with maleic anhydride (POE-g-MA) was introduced to prepare PA12/POE-g-MA/MWNT ternary nanocomposites and the electrical percolation thresholddecreased to0.63wt%. Variation of the compounding sequence of the threecomponents results in large difference in electrical conductivity of the ternarynanocomposites at the low loading of2wt%MWNTs, while the differencebecomes slight at high MWNT loading of4wt%.2. Graphene was prepared by thermal reduction. According to the IR,TEM and XRD results, it is confirmed that the graphene was completelyexfoliated. PA12/graphene binary nanocomposites were by meltcompounding and the graphene can be dispersed homogeneously in PA12matrix. Electrically conductive PA12/graphene binary composites exhibited alow percolation threshold of0.3vol%, and the dynamic mechanical andthermal properties increase with the content of graphene. After theincorporation of POE-g-MA, the electrical conductivity can be furtherenhanced with the variation of compounding sequence. According to the SEMand TEM results, the localization of graphene varied with the compoundingsequence. Both highest electrical conductivity and storage modulus wereobtained when most graphene were localized in PA12matrix rather than inPOE-g-MA phase.3. The high thermally conductive composites were prepared by simplesolution mixing method. First, we studied the effect of graphite size on thethermal conductivity and electrical conductivity. The results indicated that thethermal and electrical conductivities increased with the graphite size.Meanwhile, we compared the through-plane and in-plane thermalconductivity. With the increase of graphite content, the in-plane thermalconductivity became much higher than through-plane thermal conductivity,indicating that the anisotropic behavior of graphite flakes. Then small amountof carbon fibers or multiwalled carbon nanotubes (MWNTs) were introduced into the composites. With the partial substitution of graphite with carbonfibers or MWNTs, a positive synergistic effect between graphite and the smallamount of carbon fillers was observed on the thermal conductivity andflexural property. However, the thermal conductivity and flexural propertywere impaired due to the agglomerations with high content of MWNTs.4. We prepared epoxy (EP)/aluminum nitride (AlN) thermallyconductive insulating composites. First, three different sizes of AlN wereadopted to investigate the influence of size on the thermal conductivity. Thecomposite containing10m AlN exhibited the lowest thermal conductivity,but the composites containing45m AlN and106m AlN showed thesimilar thermal conductivity. The experimental results were consistent withthe theoretical calculation results. Second, we studied the synergistic effectbetween different aspect ratio fillers on the thermal conductivity. Thecomposite with aluminum oxide fiber and10m AlN did not show positivesynergistic effect on the thermal conductivity, but the positive synergisticeffect was exhibited between45m and10m AlN or106m and10mAlN. Lastly, we investigated the influence of coupling agent on the thermalconductivity of the composites. The results demonstrated that themodification not only improved the thermal conductivity but also improvedthe insulation performance of the composites.