Study On Electromagnetic Interference Shielding And Microcellular Foaming Properties Of Flexible PVDF-Based Polymer

With the intelligent communication equipment increasing rapidly,electromagnetic wave pollution has become one of the non-negligible pollutions,the research and application of high-performance electromagnetic interference（EMI）shielding materials have aroused great concern in the academic and engineering circles.Compared with traditional metal-based EMI shielding materials,conductive polymer composites（CPCs）have become one of the research hotspots in the field of EMI shielding in recent years,due to the advantages of flexibility,light weight,corrosion resistance,and low processing costs.However,the conductivity of CPCs is much lower than that of metallic materials.Therefore,it has become a key research issue that how to reduce the density and thickness of CPCs and increase the electromagnetic wave absorption loss of the CPCs as much as possible while achieving efficient electromagnetic shielding performance.In this thesis,the effects of carbon nanotubes（CNTs）and metallic nickel（Ni）on the electrically conductive and EMI shielding properties of polyvinylidene fluoride（PVDF）materials were mainly studied.Then the supercritical CO₂ foaming properties of PVDF-based composites were discussed,and the influence mechanism of the cell structure on the electrically conductive and EMI shielding properties of PVDF-based composites was revealed.The main contents are as follows:1.A novel carbon nanotube-coated nickel（Ni@CNTs）chain was in-situ synthesized by solvothermal method,and then a series of flexible PVDF/CNTs/Ni@CNTs composites were prepared by solution mixing and compression molding.Subsequently,the effects of carbon nanotubes（CNTs）and Ni@CNTs on the electrically conductive and EMI shielding properties of flexible PVDF/CNTs/Ni@CNTs composites were studied.By adjusting the ratio of CNTs and Ni@CNTs,a flexible material with excellent electromagnetic shielding performance was obtained,and its maximum shielding effectiveness（SE）/thickness（d）was 102.8 dB·mm-1.In addition,the heat treatment is used to improve the crystallinity of the composites,which made the EMI shielding performance of the composites improve by about 10%.The mechanical testing proved that the composites exhibited good flexibility and tensile properties,with an average tensile strength of 70 MPa and an average elongation at break of 16.9%.2.Flexible PVDF/10wt%CNTs composites were prepared by solution mixing and injection molding.Then,the composite foams with different cell structure were obtained by supercritical CO₂ foaming at different saturation temperatures.The effects of cell structure on the electrical conductivity and EMI shielding properties of composite foams were studied.It is found that due to the different effects of different cell structures on the distribution of conductive fillers in composites,the electrical conductivity and EMI shielding properties of composite foams changed accordingly.The micro-open-cell structure has greater improvement on the electrical conductivity and electromagnetic shielding performance of the composite than the micro-closed and nano-pore structure.The electrical conductivity of the composite foam with the best cell structure can reach 0.121S/cm,and the maximum EMI SE of the composite foam is 36.3dB in the meanwhile.3.The urchin-like nickel particles prepared by the liquid phase reduction method were added to the PVDF/CNTs composites to improve the EMI shielding performance of the composites.Next,the foaming properties of PVDF/CNTs/Ni composites at different saturation temperatures were studied,and the influence of Ni on the foaming properties of PVDF/CNTs composites was obtained.The variation of the electrically conductive properties and EMI shielding properties of composites with different cell structures were discussed.It could be found that the electrical conductivity and EMI shielding properties of the PVDF/CNTs/Ni composite foams have been improved significantly.Compared with solid composites,the electrical conductivity of the composite foam is nearly doubled,up to0.214 S/cm.In the meanwhile,the EMI SE of the composite foam reached 43.4 dB,and the specific shielding effectiveness（SSE）/thickness is 402.0 dB/（g/cm²）.In addition,the composite foam also exhibited high compressive properties,and its compressive strength is higher than 3.5 MPa.