Construction Of Aramid Nanofibers From Bottom To Top And Its Application In Thermal Conductive Composites

With the development of modern aerospace,national defense and electronic power equipment,it is urgent to develop thermal management materials with high thermal conductivity,high temperature resistance and excellent mechanical properties.Twodimensional(2D)fillers with high thermal conductivity,such as graphene(GNP),boron nitride nanosheets(BNNS)and carbon nanotubes(CNTs),are usually combined with one-dimensional(1D)polymer materials to improve the inherent thermal conductivity of polymer-based composites,but unreasonable filler/substrate structure design will seriously damage the thermal conductivity and mechanical properties of composites.Aramid nanofibers(ANF)have the characteristics of large aspect ratio,high strength,high temperature resistance and chemical solvent resistance.The method of introducing graphene and boron nitride into ANF to prepare high thermal conductivity composites with excellent mechanical properties and high temperature resistance has been widely reported.However,the difficulties in preparation and low yield of ANF seriously hinder its application in thermal management materials.Therefore,based on the bottom-up low-temperature polymerization method,this paper efficiently prepared ANF materials with large aspect ratio,excellent mechanical properties and high-temperature thermal stability.Subsequently,reducing graphene oxide(r GO)with water dispersibility was introduced into ANF substrate,and r GO/ANF high thermal conductivity composite membrane with excellent mechanical properties and high temperature thermal stability was prepared by vacuum filtration.In order to further expand the application of ANF-based thermal conductivity materials in the field of thermal conductivity and insulation,r GO was replaced by BNNS with high thermal conductivity and insulation,and an ANF-based high thermal conductivity composite with excellent mechanical properties,flexibility and electrical insulation was constructed by sol-gel method.This paper mainly carried out the following work:(1)Based on the dimethylacetamide-lithium chloride(DMAC-Li Cl)solvent system,in the process of radical polymerization of p-phenylenediamine(PPD)and terephthaloyl chloride(TPC),non-reactive polyethylene glycol dimethyl ether(DME)was introduced to regulate the stacking of PPTA macromolecules in the lateral direction,thus inducing the formation of ANF with large aspect ratio.The results show that the length of ANF is more than 5 μm and the average diameter is 22.78 nm when the amount of DME is 12%.By studying the mechanical properties of ANF film,it is found that the maximum tensile strength at break of ANF film is as high as 74 MPa,and the maximum elongation is as high as 4.6%.In addition,the prepared ANF film has thermal stability comparable to that of PPTA fiber.(2)To prepare high thermal conductivity composite materials,r GO with water dispersibility was introduced into ANF matrix,and the pearl-like layered thermal conductivity composite film was prepared by simple vacuum filtration and self-assembly.During this process,large and thin r GO nanosheets overlap each other laterally,and the r GO nanosheets are arranged layer by layer(LBL)parallel to the in-plane direction of the composite membrane,and these overlapping layers form a continuous heat conduction path.In particular,the dense structure of the composite film after hot pressing drying can effectively reduce the interfacial thermal resistance and phonon scattering.The results show that the thermal conductivity of r GO-50/ANF composite film is as high as 18.28 W/(m k)and its tensile strength is as high as60.2 MPa.Besides,the composite film can still maintain good thermal conductivity in the high temperature environment of 200℃,and can be used to reduce the working temperature of light emitting diodes(LED).(3)In order to expand the application of aramid nanofiber-based thermal conductivity composite materials in the fields of electronic packaging,thermal conductivity and insulation,BNNS was introduced into ANF matrix,and a high-strength and high-thermal conductivity flexible composite film with pearl-like layered structure was prepared by sol-gel method.Firstly,the peeling of ultra-thin BNNS nanosheets with large transverse size was realized by hydrothermal-ball milling method.Subsequently,BNNS was modified by siloxane to improve the dispersibility of boron nitride nanosheets in the matrix and enhance the interfacial force between the thermal conductive filler and the matrix.The results show that,based on the ANF three-dimensional network framework,BNNS thermal conductive fillers are overlapped on its surface layer by layer under the action of strong interface,and the reasonable assembly of the two makes the thermal conductivity of the trimethoxysilane boron nitride nanosheet/aramid nanofiber composite membrane(BNNS@APTES-50/ANF)as high as 15.4 w/(m k),which is641.66% higher than that of pure ANF.The thermal conductivity of boron nitride nanosheets/aramid nanofibers(BNNS@APTES-50/ANF)composite film modified by silane is as high as 15.4 w/(m k),which is 641.66% higher than that of pure ANF.After 8 cooling and heating cycles,the thermal conductivity of BNNS@APTES-50/ANF shows only slight fluctuation,which indicates that it has good cyclic thermal stability.In addition,BNNS@APTES/ANF composite film still has excellent tensile properties and high volume resistivity under high load,which indicates that it has potential application prospects in the field of thermal management of high temperature insulation.