Preparation And Properties Investigation Of PVDF-Based Dielectric Composites

Polymer-based dielectric nanocomposites have exhibited important application values in electronic devices and electrical engineering due to the advantages of good mechanical properties,easy processing,light weight,and low cost,which have attracted widespread attention over the world.Although researchers have done plenty of research on improving the dielectric properties of polymer-based nanocomposites and a series of developments have been obtained,there are still challenges need to be solved,such as how to improve the dispersion of fillers in polymer matrix,increase the interface interaction between filler and matrix,solve high dielectric loss originated from high filling content and high dielectric constant.On this basis,this article focuses on modifying or coating fillers to improve the dispersion of fillers in polymer matrix and enhance the interface interaction between fillers and matrix,thereby reducing the dielectric loss in composites and improving the dielectric properties of polymer-based nanocomposites.The main research contents and results are as follows:1.In order to improve the dispersion of OH-MWCNTs in polymer matrix and enhance the interface interaction between filler and matrix,poly(2-(Perfluoroalkyl)ethyl methacrylate)(PFEMA)coated MWCNTs(PFEMA@MWCNTs)were fabricated via surface-initiated atom transfer radical polymerization(ATRP)method.Subsequently,the MWCNTs and PFEMA@MWCNTs were blended with PVDF through solution blending method to prepare naocomposites.SEM results showed that the fluoro-polymer on MWCNTs surface improved the dispersion of MWCNTs and enhanced the interface interaction between MWCNTs and PVDF.In addition,the fluoro-polymer insulating layer impeded the contact of MWCNTs,as a result,the dielectric loss originated from interface polarization and leakage current was significantly suppressed.When the filler content was1.7 wt%,the dielectric constant of PFEMA@MWCNTs/PVDF composite was 23 at 1 KHz,which was 1.7 times that of pure PVDF.The dielectric loss of this ncomposite was merely0.045,which was much lower than the dielectric loss of the MWCNTs/PVDF composite at the same content.In addition,the percolation behavior indicated that the percolation threshold of this composite was about 6.5 times as high as that with unmodified MWCNTs,and it exhibited better insulation property.2.Poly(methyl methacrylate)grafted carbon nanotubes(PMMA@MWCNTs)were nondestructively prepared via the integration of mussel-inspired polydopamine(PDA)chemistry and surface-initiated atom transfer radical polymerization(ATRP)method.The MWCNTs/PVDF and PMMA@MWCNTs/PVDF composites were prepared through solution blending process.The results showed that the encapsulation of PMMA on the MWCNTs surface not only improved the dispersibility of MWCNTs in PVDF matrix but also enhanced the interfacial interaction between MWCNTs and PVDF.FTIR and XRD results found that PMMA@MWCNTs nanofillers could effectively induce the crystal structure of PVDF to transform from the?-phase to the?/?-phase,and nearly 100%?/?-phase PVDF formed when the nanofiller loading was higher than 5 wt%.Compared with the MWCNTs/PVDF composite,the PMMA@MWCNTs/PVDF composite exhibited obvious improvement in the percolation threshold because the PMMA shells hindered the direct contact of the MWCNTs and inhibited the formation of leakage cuttent.When the filler content of PMMA@MWCNTs was 1.7 wt%,the dielectric constant of the composite at 1 KHz was 78.9,which was 5.4 times that of pure PVDF.The dielectric loss of this composites was only 0.21.Therefore,the PMMA@MWCNTs/PVDF composite exhibited high dielectric constant and suppressed dielectric loss.3.In the above research,the experimental processes were complicated due to the MWCNTs need to be pretreated and coated,and the solution blending process cannot prepare dielectric composites in large quantity.In this part,we first synthesized methyl methacrylate-glycidyl methacrylate copolymer(MG)by continuous solution polymerization,and then the compatibilizer MG,c-MWCNTs and PVDF were blended through melt blending method.The results found that MG copolymer could improve the dispersion of c-MWCNTs and enhance the interface interaction between c-MWCNTs and PVDF.Meanwhile,the MG copolymer impeded the direct contact of c-MWCNTs and made it diffecult to form conductive networks in the composites,thus the precolation threshold of this composite increased.The dielectric constant and dielectric loss of the c-MWCNTs/MG/PVDF composite filled with 2.4 vol%c-MWCNTs were 23.34 and 0.049at 1 KHz,respectively.Compared with the composite without MG,the c-MWCNTs/MG/PVDF composite exhibited high dielectric cnstant and low dielectric loss.On the other hand,the mechanical properties of the composite with MG copolymer were also improved.4.Simultaneously achieving excellent electroactive,high dielectric performance in a transparent dielectric composite was prepared by compounding nano-montmorillonite(Na~+MMT)and ionic liquid(IL)with poly(vinylidene fluoride)(PVDF)through one-step melt blending technique.The synergistic effect of IL and Na~+MMT on the structure and properties of PVDF were systematically studied.TEM and SEM results found that the existence of IL could contribute to the exfoliation of Na~+MMT in PVDF matrix.Moreover,the best exfoliation of MMT was achieved when the long chain IL was employed,on this basis,the exfoliated MMT layers were oriented in parallel in the PVDF matrix.The synergy between IL and Na~+MMT significantly increased the polar crystals content and the transmittance of PVDF regardless of the short-chain or long-chain IL was used.The polar crystal content in the PVDF/IL/MMT composite was 100%,and the transmittance at the wavelength of 800 nm was more than 80%.Dielectric results indicated that the PVDF/IL exhibited lowest dielectric constant and dielectric loss when long-chain IL was applied.When IL,Na~+MMT and PVDF were ternary blended,the exfoliated MMT layers could inhibit the migration of IL under electric field and reduce the dielectric loss in the composites.For example,the dielectric constant of P-IL1.0-MMT0.5 nanocomposite at 1KHz was 15.7,while the dielectric loss was merely 0.05.Therefore,the PVDF/IL/MMT composites exhibited excellent dielectric and energy storage properties.