Study On Preparation And Performance Of Multi-walled Carbon Nanotube/High Performance Polymer Composite Electromagnetic Shielding Materials

With the rapid development of electronic equipment,the problems of electromagnetic radiation and electromagnetic interference are becoming more and more prominent,which seriously affects the normal operation of electronic equipment and the safe transmission of information,and endangers human health.Electromagnetic shielding material can effectively protect electronic equipment and its environment,prevent the leakage of electromagnetic information,cut off the propagation path of electromagnetic waves,and suppress the radiation and interference of electromagnetic waves,which is one of the most important technical means to solve the problem of electromagnetic radiation and electromagnetic interference.The application of traditional metal-based electromagnetic shielding materials is limited by the disadvantages of heavy weight,easy corrosion and difficult processing.Therefore,polymer-based electromagnetic shielding materials with the advantages of light weight,corrosion resistance and easy processing are favored.In order to prepare polymer-based electromagnetic shielding materials suitable for extreme service environments,the development of multi-walled carbon nanotubes and high-performance polymers(such as PEEK)composite electromagnetic shielding materials has increasingly become a significant topic,due to it can satisfy the special needs of aerospace,weapons,information communication and other fields.To prepare PEEK-based composite electromagnetic shielding materials with integrated structure and function,multi-wall carbon nanotubes(MWCNTs)were selected as conductive filler.In order to make multi-walled carbon nanotubes uniformly dispersed in PEEK matrix and compatible with PEEK matrix interface,a soluble Poly(aryl ether ketone)(az-PAEK)with azide group was synthesized.Poly(aryl ether ketone)was successfully grafted onto the surface of carbon nanotubes(PAEK-g-MWCNTs)by cycloaddition reaction between azide group in polymer and carbon-like double bond in carbon nanotubes.The grafting content of the polymer was regulated by controlling the feed ratio of az-PAEK to MWCNTs.When the feed ratio is 1:100,Poly(aryl ether ketone)grafted carbon nanotubes are the most suitable as conductive fillers.The conductivity percolation threshold and electromagnetic shielding percolation threshold of the composites can be reduced to 0.372vol%and 0.496vol%,respectively,which is far lower than the existing similar PEEK-based composites.adding a small amount of PAEK-g-MWCNTs can toughen PEEK.The results show that the conductivity,storage modulus,tensile strength and 5%weight loss temperature of 7.66 vol%PAEK-g-MWCNTs/PEEK composites reach 1.123 S/cm,2309 MPa,118.2 MPa and 561?,respectively.Meanwhile,the electromagnetic shielding value of the composite with a thickness of 2 mm is always maintained between 43.3 and 53.6d B in the broadband range of 8.2?40 GHz.With the development of electromagnetic shielding materials,higher requirements are put forward for their lightweight.Therefore,in order to reduce the density of composite and optimize the electromagnetic shielding performance of composites,PAEK-g-MWCNTs/PEEK foam was prepared by near melting point supercritical CO2technology.The reason why the near melting point saturated supercritical technology is selected is conducive to solve the problems that the matrix peek has crystal region,hinders the dissolution and diffusion of CO2,and the MWCNTs and the increase of melt strength lead to the difficulty of foaming.The composites with different filler content can obtain the maximum volume expansion under the optimal foaming conditions by adjusting the foaming process parameters.For instance,the highest volume expansion ratio of 0.0217 vol%PAEK-g-MWCNTs/PEEK foam reaches 11.42,while the cell size reaches 29.10?m and the cell density reaches 3.52×10⁸ cells/cm³,respectively.The conductivity percolation threshold and electromagnetic shielding percolation threshold of the material are reduced to 0.0457 vol%and 0.123 vol%through the introduction of porous structure,which are 87.72%and 75.20%lower than that of PAEK-g-MWCNTs/PEEK composites,respectively.The conductivity and electromagnetic shielding value of the foam increased with the increase of filler content,and showed a trend of initially increase and subsequently decrease compared with the increase of filler content.The 1.525 vol%PAEK-g-MWCNTs/PEEK foam has the highest specific electromagnetic shielding value in the frequency range of 8.2 to 40GHz,and the specific electromagnetic shielding of the foam can reach 287.6 d B·cm²·g^-1 at 40 GHz,which is 44.5 d B·cm²·g^-1 higher than that of the composite materials with the same mass fraction.Moreover,due to the existence of crystalline regions,PAEK-g-MWCNTs/PEEK foam can withstand high temperatures at 315?,and also has good solvent resistance,which is suitable for extreme service environments.In order to further reduce the density of composite and optimize the electromagnetic shielding performance of composites,high melt index PEEK resin was selected and thermoplastic polyimide(TPI)was added to the MWCNTs/PEEK system to increase the dissolution of CO2 in the composites and the melt strength of the blend system,and the multi-phase high-performance polymer-based electromagnetic shielding foam was prepared.MWCNTs/PEEK/TPI foam with different phase structures can be obtained by adjusting the ratio of PEEK/TPI.When the ratio of PEEK/TPI is 1:1,the maximum expansion ratio of MWCNTs/PEEK/TPI foam initially increases and subsequently decreases with the increase of MWCNTs content.When the MWCNTs content is 0.0115 vol%,the density of the foam is as low as 0.0604 g/cm3and the highest volume expansion rate reaches 22.14,while the maximum cell size is50.38?m and the cell density reached 1.28×10⁸ cells/cm³,respectively.The conductivity and electromagnetic shielding value of MWCNTs/PEEK/TPI foam increased with the increase of MWCNTs content.When the filler content is 3.083vol%,the conductivity of the foam reaches 0.1438 S/cm.The conductive percolation threshold and electromagnetic shielding percolation threshold of MWCNTs/PEEK/TPI foam are0.03989 vol%and 0.05552 vol%,respectively,which are 89.58%and 91.33%lower than those of MWCNTs/PEEK/TPI composite.The MWCNTs/PEEK/TPI foam reached the highest value when the filler content was 0.6313 vol%compared to the electromagnetic shielding value.The foam reached 349.5 d B·cm²·g^-1 at 40 GHz,which was 208.6 d B·cm²·g^-1 higher than the same mass fraction,and increased 61.9 d B.Moreover,MWCNTs/PEEK/TPI foam can withstand temperatures up to 300?for a long time and has good solvent resistance in most solvents or solutions,which is also suitable for extreme service environments.