Preparation And Properties Of Thermal Conductive And Electrical Insulating Polymer Composites Based On Polydopamine-coated Nanofillers

The global fifth-generation(5G)communication technology has opened a new stage of mobile Internet.The intelligent electronic devices in the age of 5G have greater data transmission capacities,faster transmission speeds and computing speeds.However,these will increase the power consumption of chips and make the heat productivity of the core processor increase significantly.In addition,the electronic devices in the age of 5G have been more functional and diverse.So their internal structures are more complex and the sealability of the overall mechanism is higher.In such a compact and enclosed space,the heat dissipation of electronic devices has become a serious challenge.Therefore,the development of thermal conductive and electrical insulating polymer composites is becoming a global research hotspot in the field of electronic insulation.In order to meet the new requirements of the rapid development of science and technology for thermal conductive and electrical insulating polymer composites,and solve the problems faced by traditional thermal conductive and electrical insulating polymer composites.Four kinds of highly thermal conductive and electrical insulating polymer composites with good comprehensive performance were prepared by constructing different thermal conductive networks based on polydopamine-coated nanofillers.The effects of surface coating of thermal conductive fillers by polydopamine on the microstructure,thermal conductivity,electrical insulation and mechanical properties of the composites were investigated,and the relationship between the structures and properties of the composites was clarified.The specific results were summarized as follows:(1)Copper nanowire(Cu NW)with high aspect ratio and inherent continuity was selected and coated it with polydopamine(PDA)to enhance its electric insulation properties.Highly thermally conductive but electrically insulating epoxy nanocomposites were prepared by incorporating polydopamine-coated Cu NWs.The results showed that the thermal conductive,mechanical properties and electrical insulation properties of the epoxy composites could be improved by coating copper nanowires with polydopamine.At the volume fraction of 3.1%of Cu NW-PDA,the thermal conductivity of the epoxy composites reached 2.87 W m^-1 K^-1,which was 15.1 times of pristine epoxy and 2.5 times of epoxy/Cu NW composite(1.23 W m^-1 K^-1).The tensile strength of epoxy/Cu NW-PDA composite was 74.1 MPa,which was higher than that of epoxy/Cu NW composite(25.6 MPa).What's more,the volume resistivity of epoxy/Cu NW-PDA composite remained to be greater than 10¹⁴?·cm,which proved that the composite exhibited good electrical insulation properties.As a result,the polydopamine on the surface of copper nanowires,on one hand,solves the problem of easy agglomeration of copper nanowires and improves the dispersion of copper nanowires in epoxy matrix.On the other hand,promotes the interfacial interaction between copper nanowires and epoxy matrix,which is beneficial to reduce the interfacial thermal resistance between fillers and matrix and improve the thermal conductivity of the composites.This method solves the problem of heat conduction and non-conductivity of metal filler,not only reduces the contents of thermal conductive filler,but also maintains the good mechanical properties of the composite.(2)Polydopamine-coated multiwall carbon nanotube(MCNT-PDA)was prepared by surface modification of MCNT by PDA.In this chapter,PP/MCNT-PDA composites were prepared by traditional melt blending method,in which MCNT-PDA formed a continuous segregated structure.Firstly,MCNT-PDA were dispersed in PS phase of the bicontinuous structure formed by incompatible PP and PS.Secondly,the selective extraction of PS phase lead to the uniform deposition of MCNT-PDA on the porous PP pore walls.Finally,compression molding made the porous skeleton tightly compacted to form uniform and dense thermal conductive networks.When the content of MCNT-PDA was 2 wt%,the in-plane thermal conductivity of PP/MCNT-PDA composite increased from 0.14 W m^-1 K^-1 to 1.59 W m^-1 K^-1,which is 11.3 times of that of pure PP,and 74.5%higher than that of PP/PS/MCNT-PDA composites(0.91 W m^-1 K^-1).In addition,the surface modification of MCNT by PDA blocked the free movement of electrons in the heat conduction network,and PP/MCNT-PDA composite exhibited excellent electrical insulation properties(3.8×10¹⁴?·cm).Based on spatial confining forced network method,the bicontinuous phase formed by incompatible polymers was prepared to realize the selective distribution of thermal conductive fillers and construct the thermal conductive network.The surface coating of MCNT by PDA could not only improve the dispersion of MCNT in PS,but also hinder the formation of conductive path,thus achieving the goal of thermal conductive and electrically insulating properties of composites.(3)Firstly,BNNS was prepared by liquid phase exfoliation and coated it with polydopamine.Secondly,BNNS-PDA-Ag was prepared by metal coordination and reduction ability of PDA to Ag⁺,silver nanoparticles(Ag)were in-situ reduced and deposited on the surface of BNNS by the metal ion chelating and redox activity of PDA.Finally,a small amount of GO was introduced to prepare PVA/BNNS-PDA-Ag composite films with high orientation,high thermal conductivity and good flexibility by adopting a simple doctor blading method.A small amount of GO and surface modification of BNNS by polydopamine can further improve the thermal conductivity and mechanical properties of PVA composite films.When the contents of GO and BNNS-PDA-Ag were 0.5 wt%and 10 wt%respectively,the in-plane and through-plane thermal conductivity of PVA/BNNS-PDA-Ag/GO composite film was 6.54 and 1.03 W m^-1 K^-1,respectively.Compared with PVA/BNNS composite film,the thermal conductivity was increased by 103%and 134%respectively.At the same time,the tensile strength of PVA/BNNS-PDA-Ag/GO composite film was still 97.1 MPa,which was higher than that of PVA/BNNS composite film(68.6 MPa)due to the hydrogen bonding between BNNS-PDA-Ag and PVA matrix and the nano reinforcement of GO.These results showed that Ag nanoparticles deposited on the surface of BNNS and a small amount of GO play the role of"heat conduction bridge",which increases the heat transfer contact area,reduces the interface thermal resistance between BNNS and improved the continuity of heat conduction network.The surface coating of BNNS by PDA could improve the compatibility and enhance the interfacial interaction between BNNS and PVA matrix,thus promoting the mechanical properties of PVA/BNNS-PDA-Ag/GO composite films.(4)Herein,graphene oxide(GO)was used as thermally conductive fillers due to its excellent thermal conductivity and coated it with polydopamine to enhance its electric insulation performance.A unique“particle-constructing”method was adopted for fabricating highly ordered three-dimensional GO-based polymer composites,throughout which the GO-PDA formed intact,uniform and well-defined network structure.The results showed that the thermal conductivity and mechanical properties of PS/GO-PDA composites were improved by the construction of three-dimensional thermal conductive network.The composite,even with a very low GO-PDA loading of 0.96 vol%,exhibited both high in-plane(4.13 W/m K)and through-plane(4.56 W m^-1 K^-1)thermal conductivity,which were 1777%and 1959%higher than that of pure PS due to the three-dimensional thermal network.At the same time,the tensile strength of PS/GO-PDA composite increased to 21.9 MPa,72.4%higher than that of pure PS(12.7 MPa),and the storage modulus increased to 2.6 GPa,1.7 times higher than that of pure PS(1.5 GPa).What's more,PS/GO-PDA composites also presented excellent electrical insulating properties,and the volume resistivity of the composites was still higher than 10¹⁴?·cm.The PDA layer on the surface of GO not only blocks the conductive path in the three-dimensional thermal conduction network,but also improves the interfacial force between GO and PS matrix through?-?interaction,which improves the comprehensive properties of PS/GO-PDA composites.