| Polymers, due to their corrosion resistance, light weight, good mechanical properties and processability, applied more and more widely in the life of people. Recently, with the rapid development of electronics, energy and other fields, there is an increasing demand in polymer materials. However,except for few conductive polymers, most polymers have been unable to meet the fuctional requirements due to their insulation in electricity and heat.Therefore the development on polymer composites with electrical, thermal and electromagnetic interference (EMI) shielding properties is one of the important subjects in front of the scientific resear-ch workers. By filling the functional nano-fillers in polymers to prepare functional polymer composites which have advantages of low cost, short development cycle, and easy popularization, which attracted much attention at home and abroad.Functional nano-fillers can not only provide functional properties for polymers, but also enhance the strengths of polymer composites. However,high loading of fillers, which is needed to meet the requirements, may impair the processability and toughness of composites due to the intrinsic rigidities of most nano-fillers. Therefore, we are forcusing on how to construct a specific structure to form networks inside the composite at a lower filler loading and improve electrical and thermal properties while maintaining the mechanical properties at a lower filler loading.Based on the above analysis, we used thermosetting epoxy resin and thermoplastic polypropylene (PP) as matrix in this thesis, and anisotropic graphene aerogels (AGAs), carbon nanotubes (CNTs), calcium carbonate(CaC03), boron nitride (BN), graphene nanoplatelets (GNPs) and graphitized carbon fibers (GCFs) were used as nano-fillers to prepare certain composites with specific sturctures inside. We have systematically researched the influence of fillers and paking structures on electrical, thermal and EMI shielding performances of polymer composites, while mechanical properties have also been measured.The main work of this thesis points four parts as follows:1. Study on the EMI shielding properties of Epoxy/AGA composites.During the study on polymer/graphene composites, dispersion and spatial distribution of graphene sheets are crucial factors to the properties of their polymer composites. AGA with highly aligned graphene networks are prepared by a directional-freezing followed by freeze-drying process and exhibit different microstructures and performances along the axial (freezing direction) and radial (perpendicular to the axial direction) directions. Thermal annealing at 1300 ? significantly enhances the quality of both AGA and conventional graphene aerogels (GA), meanwhile the electrical conductivities and the EMI shielding effectivenesses (SE) of the composites filled with thermally annealed AGA (TAGA) and thermally annealed GA (TGA).Epoxy/TAGA composites show highly anisotropic mechanical and electrical properties and excellent EMI SE at very low graphene loadings. Compared to the epoxy composite with 0.8 wt% TGA with an EMI SE of 27 dB, the composite with 0.8 wt% TAGA has an enhanced EMI SE of 32 dB along the radial direction with a slightly decreased SE of 25 dB along the axial direction.With 0.2 wt% of TAGA, its epoxy composite exhibits a SE of 25 dB along the radial direction, which meets the requirement of ?20 dB for practical EMI shielding applications.2. Study on the electrical and thermal properties of Epoxy/TAGA composites. 3D structure has drawn a great interest in enhancing thermal properties of polymers. AGAs with highly aligned structures were prepared by a directional-freezing method, and were subsequently thermal annealed to prepare TAGAs. Epoxy composites were prepared by vacuum impregnation method to impregnate the epoxy resin into the graphene conducting network,and we studied on the electrical, thermal and mechanical properties measured along the aligned graphene sheets (axial direction) and perpendicular to the axial direction (radial direction). AGA and TAGA have highly ordered structures and the resultant materials showed anisotropy in architectures and properties Experiments was carried out with increasing of TAGA content of 0.5, 0.8, 1.2, 1.5 wt% and annealing temperature of 1000, 1600, 2200 and 2800 ?, in which high TAGA content and high temperature significantly benefited to the electrical and thermal conductivity of epoxy composite. With 1.5 wt% of TAGA-2800, the epoxy composite exhibited an excellent thermal conductivity of 6.57 W·m-1·K-1 in the axial direction. By further study of the influence of directional-freezing rate, we concluded that faster freezing rate could make smaller TAGA pore size and thus obtained higher thermal conductivities and mechanical properties of epoxy/TAGA composites, but the pore size had little effects on electrical conductivities. The electrical and thermal conductivities of the as obtained epoxy/TAGA composites are simply outstanding.3. Study on the electrical property and toughness of PP/CNT/CaCO3 composites. We prepared PP/CNT/CaCO3 nanocomposites by melt-compounding method, and studied on their electrical and mechanical properties. Although the presence of CNTs makes PP electrically conductive,the resulting PP/CNT binary nanocomposites become brittle. To toughen PP/CNT nanocomposites, CaC03 inorganic nanoparticles are added to fabricate PP/CNT/CaCO3 ternary nanocomposites. The CaCO3 particles not only benifited to the toughness and Young's modulus of the composites, but also played an important role in volume-exclusion by reducing the percolation threshold and improving the electrical conductivity. By the addition of 30 wt%of pristine CaC03, the electrical conductivities improved, the percolation threshold of the nanocomposites reduced from 6.2 wt% to 5.6 wt%, and the impact strength of PP/9 wt% CNT composite increased from 16.0 KJ·m-2to 24.4 KJ·m-2. In order to improve the interreaction at the interfaces of polymer and fillers, a specific aluminate coupling agent was used and thus improved the electrical and impact properties. The percolation threshold of ternary nanocomposites reduced to 3.6 wt% and the impact strength of PP/9 wt%CNT composite increased to 33.1 KJ·m-2 by the addition of 30 wt% of modified CaC03. The dual roles of CaC03 in volume-exclusion and toughening are well demonstrated, and the PP nanocomposites with simultaneous toughness and electrical properties will have wider application prospect.4. Study on the thermal properties of PP/BN/GNP (GCF) composites.Thermally conductive ternary composites were prepared in a Hakke mixer by melt-compounding PP matrix with BN and carbonaceous fillers. The inorganic fillers BN, AIN and SiC were compared to chose BN as thermally conductive filler, and then GNPs or GCFs was selected as carbonaceous filler. We changed the content of GNPs or GCFs (1-5 wt%) in a total filler content of 60 wt%, finding both GCFs and GNPs were benefit to the thermal conductivities of the composites, and GNPs showed a better enhancement. In addition, the absence of UP-101 further enhanced the thermal conductivities, which is ascribed to the good dispersion of fillers, the interaction between fillers and matrix and the thermostability of the composites. In a total filler content of 60 wt%, ternary composites with 5 wt% modified GNPs and GCFs have thermal conductivities of 1.55 W·m-1·K-1 and 1.36 W·m-1·K-1, respectively, which are 86 % and 112 % higher than that of PP/60 wt% modified BN binary composite(0.73 W·m-1·K-1), and also higher than PP/80 wt% modified BN composite(1.35 W·m-1·K-1). The partial replacements of the carbonaceous fillers also improve the flexural strength without sacrificing the flexural modulus of the ternary composites. Therefore, the introduction of carbonaceous fillers decreased the total loading of conductive fillers, reduced the difficulty of processing and endowed composites with certain mechanical properties. |
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