Microstructure Design Of PVDF Based Composites And Their Electromagnetic Shielding Performance

Electromagnetic pollution has become a more and more serious problem with the widespread practical applications for modern computer and telecommunication technologies.Thus much attention has been paid to develop high-performance electromagnetic interference(EMI)shielding materials.Compared with metals and metallic composites,conductive polymer composites(CPCs)have been extensively investigated for EMI shielding nowadays because of their outstanding advantages,such as light weight,resistance to corrosion,good processability,tunable conductivity,and broad absorption bandwidth.Over the past decades,graphene sheets,carbon nanotubes,and carbon nanofibers have been considered as popular conductive filler options for their remarkable structural,mechanical,and electrical properties,such as smaller diameters,larger aspect ratios,much higher conductivities and strengths.Moreover,they can promote electrical and EMI shielding properties without significantly damaging other admirable properties of the polymer matrix.Being lightweight is a key technological requirement for the development of practical EMI shielding systems especially in the areas of aircraft,spacecraft,and automobiles,because it means material and energy savings.Therefore,large researchers offer lightweight EMI shielding materials using various approaches.For commercial applications,the EMI SE should generally reach at least 20 dB which typically requires the CPCs with a volume electrical conductivity not less than 0.01 S/cm.In general,conductive networks formed by infusing the conductive fillers were CPCs' only source of electrical performance for most polymer matrixes are essentially insulating.Therefore,superior conductive interconnected networks are often needed to achieve such high conductivity,0.01 S/cm.And owning to the homogenous dispersion structure,filler loadings much higher than the electrical percolation threshold are generally required to construct superior conductive interconnected networks.Thus,it seems fascinating to promote the EMI shielding and electrical performances with as possible as lower filler contents on the grounds that a high loading often means high production costs and poor processability.Though a mass of polymer matrixes have been studied for EMI shielding there are few reports about polyvinylidene fluoride(PVDF).PVDF shows advantages of high chemical resistance,high temperature sustainability and high strength.It is often used as a piping material and also has wide applications in the fields of piezoelectric,pyroelectric,etc.Moreover,some former investigations have shown that the polymer nanocomposites possess excellent wave absorption properties,especially when PVDF is used as a matrix.1.In the first part,we have fabricated a composites based on PVDF,multiwalled carbon nanotube(MWCNTs)and polystyrene(PS)via mechanical mixing and hot compaction.Then the sample was submerged with enough dimethylbenzene to dissolve the PS phase resulted in the porous structure.After drying we got the lightweight porous PVDF/MWCNTs composites.This approach is also suitable for the fabrication of almost all thermoplastic polymers filled with various fillers.The unique 3D construction attenuated the power effectively by reflecting and scattering it many times in the interior of the materials,which makes the porous composites potential to be used as lightweight EMI shielding or electromagnetic absorption materials.Through computation and analysis,we can infer that such 3D network porous composites are more absorptive to electromagnetic radiation in X-band frequencies.That's to say the dominant contribution to EMI SE of such composites is absorption rather than reflection in the X-band frequency region.2.Three-phase composites based on carbon conductive fillers and nanostructure particles embedded into a polymer matrix were also studied as functional excellent electromagnetic shielding materials.Embedded particles typically create synergies with the conductive fillers to form superior conductive networks,thereby improving the EMI SE.Here,based on the previous work porous composites with MWCNTs and hollow glass microspheres(HGMs)are fabricated by the same method mechanical mixing,hot-pressing,and selective etching.Such inorganic hollow micro-spheres are shown to have a good chemical stability,high temperature resistance,low weight,and low cost.They are ideal raw materials for preparing absorbent materials and their micron size promotes facile combination with the polymer matrix.We show that by embedding HGMs into the system,the continuity of the conducting pathways can be promoted and electrical conductivity of the materials is enhanced.We also show that HGMs increase the scattering and reflection of electromagnetic incident waves.Thus,it demonstrates that the creation of a synergistic effect between HGMs and the 3D conductive polymer network greatly enhances the EMI SE of the composite materials.Additionally,HGMs have a very low thermal conductivity,and its addition into the porous samples is shown to increase the porosity of the system and decrease the efficiency of heat transfer in the composites.The process reported in this work allows the fabrication of EMI shielding materials with lower thermal conductivities that meet the operational requirements for different application fields.3.Till now,constructing a segregated CPC(s-CPC)system has become a promising technique to realize lower filler contents and it has also been extensively studied in recent years.In such a segregated system,conductive fillers basically lie on the polymer particle interfaces rather than scattering over the whole matrix.As far as we know,there is little information on PVDF based composites with a segregated structure.In the present work,a water vapor induced phase separation process was carried out to fabricate PVDF particles.Then we prepared segregated PVDF/MWCNTs composites by hot pressing.The compacted MWCNTs layers acting as a conductive network increased both the electrical conductivity and the EMI SE even at a low filler loading.The advantage of the segregated structure was confirmed again by comparing the performance of the samples with different hot-pressing temperatures.We also studied the effect of sample thickness on the shielding property of the s-CPCs and found that the higher thickness,the better.4.It is reported that magnetic materials such as magnetic metals,magnetic oxides and metal alloys have strong microwave attenuation ability.And according to some previous literatures,EMI shielding performance of polymer composites depended largely on the dielectric and magnetic properties of the fillers.So some magnetic particles combining with conductive fillers have also been widely investigated to develop superior conductive networks for high EMI shielding.Here,we introduced carbonyl iron(CI)magnetic particles and distributed both CI and conductive fillers(carbon black(CB))on the polymer particle interfaces,fabricating segregated PVDF/CB/CI composites with both magnetic and electrical properties.The segregated channels composed of CB and Cl particles acting as a conducting network provided the electrical conductivity required for the EMI shielding materials.While CI had the composites moderate magnetic performance.With the aid of the synergistic effect of the two conductive and magnetic fillers EMI shielding properties of the sample was greatly improved.