Study On Preparation And Properties Of Polymer-based Composites With High Thermal Conductivities

Ever increasing demand for higher density circuits in electronics has greatly accelerated theminiaturization and integration of chip electronic components with high performance andmulti-function, smaller size, high efficiency and low cost. The improvement in size andperformance should result in the generation of a greater amount of heat in a smaller volume ofspace. To ensure proper device operation, the unwanted heat must be removed. Thermalconductive polymer composites offer new possibilities to solve this problem due to the polymeradvantages such as light weight and ease of processing. In this study, our interest to improve thethermal conductivity of polymer is focused on the selective addition of high thermal conductivefillers, such as alumina (Al₂O₃), graphene sheets (GSs) and boron nitride (BN) nanoplatelets.Firstly, we carried out hyperbranched aromatic polyamide (HBP) grafting Al₂O₃nanoparticles.In the grafting procedure, the Al₂O₃nanoparticles were firstly treated with a silane coupling agentto generate amine groups on their surface, and then grafting of the HBP started from the modifiedsurface by solution polymerization. The interface structure of the nanoparticles was characterizedby XRD, FT-IR,1H-NMR and TGA. Then the Al₂O₃nanoparticles with three different interfacestructures have been selected as reinforcement fillers for epoxy composite, i.e. surface untreatedAl₂O₃nanoparticles, γ-aminopropyl-triethoxysilane modified Al₂O₃nanoparticles (Al₂O₃-APS)and hyperbranched aromatic polyamide grafted Al₂O₃nanoparticles (Al₂O₃-HBP). Our studyreported the influence of the interface structure of Al₂O₃nanoparticles on the morphology,thermalproperties and dielectric properties of epoxy composites. It was found that the incorporation of theAl₂O₃-APS and Al₂O₃-HBP nanoparticles not only improved the dispersion of the nanoparticles inepoxy matrix, but also enhanced the thermal properties and influenced largely the dielectricproperties of epoxy composites as compared with the composites filled with surface untreatedAl₂O₃nanoparticles. In addition, the epoxy composites containing Al₂O₃-HBP nanoparticlesexhibited higher thermal conductivity than those containing as received Al₂O₃or Al₂O₃-APS nanoparticles.Secondly, A composite of poly(vinylidene fluride)(PVDF) was prepared with GSs which are anovel filler by a solution method. Our results revealed that the addition of GSs could increase thepermittivity and thermal properties of PVDF. The composites showed a transition from electricalinsulator to semiconductor at GS concentration of4.5wt%. The thermal conductivity of thecomposite with0.5%GSs was increased by approximately a factor of2when compared with thepure PVDF. The thermal conductivity of the composites is up to0.58W/m K when theconcentration of GSs was10wt%. The structure-properties relationships of PVDF/GSscomposites were also studied. The results of differential scanning calorimetry (DSC) and X-raydiffraction (XRD) show that low content addition of GSs to the PVDF matrix promotes an phaseto β phase transformation of the polymer crystal. The composites exhibit significant increases indynamic mechanical properties and thermal stability compared to the neat PVDF. In addition, theincorportation of GSs in PVDF indicated excellent optical transparency at the low weight fractionsof GSs and modified wettability of PVDF.Thirdly, we reported a simple method to fabricate boron nitride (BN) nanoplatelets using asonication-centrifugation technique and investigated the effects of functionalization BNnanoplatelets on thermal properties of epoxy composites. Two methods have been used forfunctionalizing BN nanoplatelets: non-covalent functionalization by octadecylamine (ODA) andcovalent functionalization by hyperbranched aromatic polyamide (HBP). Epoxy composites werefabricated by incorporating three kinds of fillers: BN nanoplatelets, BN nanoplateletsfunctionalized by ODA (BN-ODA), and BN nanoplatelets functionalized by HBP (BN-HBP). Ourresults show that the BN-HBP results in a strong interface and thus the composites exhibitsignificantly increased glass transition temperature, thermal decomposition temperature, thermalconductivity, dynamic thermal mechanical modulus and dielectric properties. BN-ODA producedintermediate interface interaction, resulting in a moderate improvement of thermal properties anddielectric properties. The composites with BN nanoplatelets show the least improvements ofthermal properties and dielectric properties.