Graphene:Liquid-phase Exfoliation,Functionalization,and Application In PDMS Modification

Thermal dissipation in microelectronic devices is an important issue which influences their performance.The thermal interface material is applied between the heat source and the heat sink,which can effectively help the electronic device to transfer heat to the outside,and is an important component of thermal management.Polymer-based thermal interface materials have received extensive attention,and the preparation of thermally conductive composites with high thermal conductivity has been extensively studied.Graphene,as a two-dimensional material with excellent thermal conductivity,is often used to prepare thermal conductive composites.However,the efficient preparation of graphene is the key to restrict the commercial application of graphene.Liquid phase exfoliation is a method for large-scale production of high-quality graphene,and the surface of graphene can be modified to greatly expand the application range of graphene.In this paper,a hyperbranched polyethylene homopolymer（HBPE）and a hyperbranched polyethylene copolymer（HBPE@POSS）consisting of a HBPE backbone and multiple POSS terminal groups were first synthesized.Further,the graphite is exfoliated into graphene nanoplates（GNPs）under ultrasonic conditions with HBPE and HBPE@POSS as stabilizers,respectively,and the stable graphene dispersion is achieved by the non-covalent interaction between hyperbranched polymer and graphene surface.Based on this,the thermally conductive composites were prepared by solution blending of graphene nanoplates and polydimethylsiloxane（PDMS）.The thermal conductivity,heat resistance,and mechanical properties of the composites were investigated.The thermal conductivity of the composite material is further improved by designing the orientation structure.The detailed research contents are summarized as follows:（1）Two hyperbranched polymers were synthesized by Pd-diimine catalyst through a unique"chain walking"polymerization mechanism under the conditions of ethylene pressure of 1 atm and 25°C.One is the HBPE that is synthesized via homopolymerization of ethylene,and the other is the HBPE@POSS that is synthesized via copolymerization of ethylene and acryloylisobutylsilsesquioxane（POSS）.The structures and properties of the two hyperbranched polymers were characterized by hydrogen nuclear magnetic resonance（¹H-NMR）spectroscopy,gel permeation chromatography（GPC）,Fourier transformed infrared（FTIR）spectroscopy,X-ray diffraction（XRD）,differential scanning calorimetry（DSC）,dynamic light scattering（DLS）,Thermogravimetric（TG）and melt rheological analysis.The results show that the branch densities of the HBPE and HBPE@POSS are 100/1000 C and99/1000 C,respectively,and the content of POSS group is 1.9 mol%.The relative molecular weight and hydrodynamic diameter of HBPE@POSS copolymer are smaller than that of HBPE homopolymer due to the grafting of POSS monomer.The presence of POSS microcrystalline region in HBPE@POSS copolymer was also confirmed,resulting in its thermal and rheological properties different from those of HBPE homopolymer.（2）Graphite was exfoliated into graphene dispersions by ultrasound in low boiling point organic solvents with the HBPE and HBPE@POSS as stabilizers.The effects of various conditions such as solvent,initial graphite feed concentration and initial polymer feed concentration on the graphene concentration and exfoliation efficiency of graphene were systematically studied.The concentration of graphene in resulting dispersions was characterized by ultraviolet-visible light（UV-Vis）,the lateral dimensions and morphology of graphene were characterized by transmission electron microscopy（TEM）,and the thickness of graphene was characterized by atomic force microscopy（AFM）.The structure of graphene was characterized by X-ray diffraction（XRD）,Raman spectroscopy（Raman）and X-ray photon spectroscopy（XPS）.The interaction between graphene and hyperbranched polymer was characterized by thermogravimetry（TG）,scanning electron microscopy（SEM）and Fourier transform infrared（FTIR）spectroscopy.The results show that the concentration of graphene achieved with HBPE and HBPE@POSS can reach 0.12mg·mL^-1 and 0.13 mg·mL^-1,respectively.The lateral dimensions of resulting graphene nanoplates is between 100 nm and 1000 nm,with the thickness being between 1.2 nm and 2.6 nm,and the defects of graphene nanoplates are less.It has been confirmed that some polymer irreversibly adsorbs on the surface of graphene.After wash for three times,18.2 wt%HBPE and 14.8 wt%HBPE@POSS were still adsorbed on the surface of graphene.（3）The thermally conductive composites were prepared via solution mixing process with PDMS as matrix and the resulting graphene as nanofiller,and the thermally conductive composites with orientation structure were designed.Scanning electron microscopy（SEM）,laser flash,dynamic thermomechanical analysis（DMA）and thermogravimetric（TG）were used to characterize the filler distribution,thermal conductivity,physical properties and heat resistance of the composites.The results show that the graphene dispersion with HBPE@POSS as stabilizer has good compatibility with the PDMS matrix.When the content of graphene filler is 4.0 wt%,the thermal conductivity at room temperature is as high as 0.927 W·m^-1·K^-1.At the temperature of–139°C,the addition of graphene increased the storage modulus of the composite by up to 63.0%.Simulation of thermal conductivity shows that the interface thermal resistance has a great influence on the thermal conductivity of the composite.At the same time,the heat resistance of composites have also been improved compared to pure PDMS.In the vertically oriented graphene structure,the content of graphene filler is 16.6 wt%,and the thermal conductivity of composites at room temperature is as high as 2.182 W·m^-1·K^-1.The various structures broaden the application fields of thermal conductive composites.