Study On Particle Selective Dispersion Mechanism And Construction Of Efficient Particle Networks For Electromagnetic Shielding Polymer Composites

With the development of society into the information age,high frequency electromagnetic wave is widely used for information transmission.Electromagnetic wave brings great convenience to our production and life,but causes a lot of harms.Electromagnetic radiation,for example,can cause cumulative damage to human health.Meanwhile,electromagnetic waves from different sources may interfere,thus affecting the normal use of instruments and equipment.Therefore,electromagnetic interference(EMI)shielding materials are becoming a hot topic in both academic and industrial fields.Conductive polymer composites(CPCs)is an excellent EMI shielding material,with light weight,high strength,easy processing,corrosion resistance and other advantages,can meet the application of multiple scenarios.Typically,in the preparation of CPCs,polymer is used as a matrix and conductive fillers are added to matrix.Generally,conductive fillers are carbon black,carbon nanofibers,carbon nanotubes,graphite,and metal particles and so on.Carbon nanotubes(CNTs)are outstanding reinforcement materials due to the advantages of good toughness,high strength,strong conductivity and large length-diameter ratio.Recent studies have found that the conductivity of CPCs is directly related to phase morphology of polymer and dispersion and distribution of conductive fillers.Although researcher on construction of conductive networks have made good progress,the influence of particle networks on EMI shielding performance has been less reported.It is still unclear that how to improve the EMI shielding performance by adjusting polymer phase morphology by processing methods,the influence of selective dispersion of conductive filler on the EMI shielding performance of composites,and the influence mechanism of the surface conductivity of composites on the EMI shielding performance.In order to solve the above problems,the selective dispersion mechanism of EMI shielding polymer composites and construction of efficient particle networks were studied in this thesis.(1)The selective dispersion mechanism of conducting particles was studied.Using polylactic acid(PLA)/polyε-caprolactone(PCL)/carbon nanotubes(CNTs)composites as templates,and the phase morphology of the polymer was regulated by processing methods to study the effect of phase morphology on the EMI shielding performance of the composites.It was found that CNTs almost migrated to PCL phase in a very short time in PLA/PCL/CNT composites prepared by different processing methods.The migration mechanism of CNTs was analyzed from dynamic and thermodynamic theory,and from dynamic and static diffusion models.PCL had low viscosity and low cohesion energy,and had a strong affinity for CNTs thermodynamically.In addition,a new interface of PCL was continuously generated in the mixing process.It is easy for PCL to wrap,pull,and migrate CNTs into its interior quickly during the melt mixing.The PLA/PCL/CNT composites with the same contents of PLA:PCL(5:5)and CNTs were prepared under four different mixing sequences and three different mixing rotor rate.The composites prepared under different processing conditions all showed a co-continuous morphology,but the sizes of the co-continuous morphology were different.The different mixing sequence of PLA,PCL and CNTs resulted in the differences in viscosity and elastic modulus of polymer.When the viscosity and elastic modulus of polymer differred greatly,it was easy to form a state of low viscosity phase covering high viscosity phase.The phase with low viscosity and elastic modulus was very hard to divide the phase with high viscosity and modulus during the mixing process.Therefore,the resulting co-continuous structure was large in size and uneven.Instead,the two phases with close viscosity and elastic modulus,i.e.matching viscosity,it was easy to form a state of two-phase mosaic and divide each other easily during the mixing process.Thus,the resulting co-continuous structure was small in size and even.Furthermore,the phase morphology of polymers was also different at different mixing rotor rate.The size of the two continuous phases was larger at high shear rate and smaller at low shear rate.The EMI shielding property of composite decreased with the increase of two continuous phase sizes.Because the smaller size of two continuous phases created more interfaces.The more interfaces with different conductivity in the composites,the stronger the loss of electromagnetic energy.(2)Because of strong affinity between PCL and CNTs,it was feasible to add a small amount of PCL to induce CNTs aggregation to optimize the conductive network in PLA/PCL/CNT composites.The content of PCL gradually increased with the CNT contents of 5 wt% or 10 wt%.The effect of PCL content on EMI shielding networks was studied for the PLA/PCL/CNT composites.When the CNTs content is 5 wt%,the electrical conductivity and EMI shielding performance of the composite reach high values with 5 wt% PCL.At this point,the most efficient EMI shielding network,called CNT/CNT-cluster conductive network,is constructed inside the composites.Specifically,the average SE value and conductivity of the sample containing 5 wt%PCL are 77% and 136% higher than those of the sample containing 28.5 wt% PCL,and10% and 15% higher than those composites without PCL,respectively.Similar results are also found in the samples with 10 wt% CNTs.The results show that the CNT/CNT-cluster EMI shielding network has conspicuous advantages over the traditional EMI shielding network.To explore the formation conditions of CNT/CNT cluster EMI shielding network,the content of CNTs gradually increased maintained the contents of PLA and PCL.It was found that only when CNTs content was sufficient,CNTs dispersed in PLA could connect CNT clusters to construct a unique and efficient CNT/CNT-cluster EMI shielding network.In addition,the formation conditions of the CNT/CNT-cluster conductive network are further discussed in PP/PCL/CNT and PS/PCL/CNT composites.The formation conditions of CNT/CNT-cluster EMI shielding network are summarized as: First,one polymer with low viscosity,low cohesion energy and strong thermodynamic affinity for CNTs can induce CNTs to aggregate into CNT clusters.Second,CNTs in another polymer have better dispersion,so that CNTs dispersed in another phase polymer can connect CNT clusters to form CNT/CNT-cluster conductive network.(3)In order to study the effect of surface conductivity on EMI shielding properties,sandwiched composites with different surface conductivity were prepared by a melt blending and layer by layer hot pressing.First,the influence of conductive surface and insulating surface on EMI shielding properties of composite materials was discussed.For the sandwiched composites with 10 wt% CNTs in the middle layer,the SE value of the composite decreased by about 10 d B when one side of the surface was an insulating layer.The SE value of the composite decreased by about 25 d B when the two surfaces were insulated.The results indicated the conductive surface significantly improved the EMI shielding performance of the composites.The higher the surface conductivity,the better the EMI shielding performance.When the surface conductivity of the composite increased unilaterally,the EMI shielding performance of the composite was improved slightly.When the surface conductivity of the composite increased on both sides,the EMI shielding performance was improved greatly.When the surface conductivity was small,the absorption loss increased to enhance the EMI shielding performance.When the surface conductivity was high,the reflection loss increased to enhance the EMI shielding performance.Finally,the electromagnetic simulation results indicated that surface conductivity of the composites had an important effect on the electric and magnetic field attenuation of electromagnetic wave.When the upper and lower surfaces of composite materials had high conductivity,the intensity of electric field and magnetic field decreased obviously after electromagnetic wave passes through the material due to the Salisbury screen effect of electromagnetic waves.