Structure Tuning And Dielectric Properties Of PVDF-based Composites

Percolative polymer composites have attracted much attention as dielectricmaterials due to their excellent electrical properties and facile processing. However, itis difficult to synchronously increase high dielectric constant, low dielectric loss andgood flexibility. The structure-property relationship in the composite system has beenthoroughly studied experimentally and theoretically. In this work, we systematicallystudied the effects of structures of conductive particles and interfacial structures ofthe composites on the dielectric properties of PVDF-based composites. Through thesurface modification and coating of various conductive particles, the interfacialcompatibility between the particles and PVDF matrix was thoroughly improved. Thedielectric properties of the composites were significantly enhanced owing to theincreased interfacial polarization induced by optimized interfacial structures anddistribution pattern of the particles. Furthermore, the structure model and strategytowards high dielectric constant, low loss and high thermal conductivity werepresented for potential high integration system applications. The main content andresult of this thesis were listed as follow:1. The PVDF/m-and n-Ag composites were prepared by mechanical mixing anda hot press technique. The size effects of Ag particle on dielectric properties of thecomposites were studied. The PVDF/n-Ag composites exhibited remarkableimprovements of dielectric constant when compared with the PVDF/m-Agcomposites over the whole frequency range. The magnitudes of the dielectric constantand conductivity for the PVDF/n-Ag composite were much higher than that of thePVDF/m-Ag composite at the same Ag volume loading. The results suggest that thedielectric properties depended on the synergistic effects of the interfacial area,interparticle distance, and interfacial adhesion. The significant increase in dielectricproperties of the PVDF/n-Ag composites was attributed to increase in interfacial area,decrease in interparticle distance, and improved interfacial adhesion due to decreasedparticle size.2. Ag nanoparticles were coated by PDOPA through the oxidativeself-polymerization of dopamine and the Ag@PDOPA/PVDF nanocompositemembranes were prepared via a simple solution processing. The successful coating of Ag nanoparticles with PDOPA was demonstrated by TEM, SEM and TG. In addition,the Ag@PDOPA/PVDF nanocomposite has higher dielectric constant and lowerdielectric loss than that of Ag/PVDF nanocomposite due to the barrier effect ofPDOPA to reduce the leakage current. PDOPA is rich in hydroxyl groups, aminegroups and imine groups and can interact strongly with PVDF, contributing to thestrong interfacial bonding. The percolation threshold of Ag@PDOPA/PVDFnanocomposite is significantly higher than that of Ag/PVDF nanocomposite,implying electric barrier effect of PDOPA.3. PVP-coated Ag nanoparticles were in situ synthetized and compounded withPVDF via a simple solution processing. The structure and dielectric properties of theAg@PVP/PVDF composites were characterized and studied systematically. Thedielectric constant of the17vol%Ag@PVP composite is12.6times higher than thatof pure PVDF. The composites have very low dielectric loss in the frequency range100Hz to106MHz and the loss is even lower than that of neat PVDF above105MHz.Amide groups in PVP can solvate with halogen groups in PVDF, which contributes tothe good interfacial compatibility and good dispersion of Ag nanoparticles in thePVDF matrix. The reduced dielectric loss is partially attributed to the enhancedinterfacial adhesion. The strong polarity of PVP and PVDF induced strong interfacialpolarization and thus the dielectric properties of the composite are promising.4. Ag/EVER particles were prepared by compounding EVER and Ag particles.The Ag/EVER particles were introduced in to the PVDF matrix to prepare thethree-phase Ag/EVER/PVDF composite, and the dielectric behavior of the compositewas investigated. The results showed that the typical “conductor/polymer”percolation effect was not observed in the composite as a result of the EVER layer,which prevented Ag particles from contacting with each other directly and restrictedthe movement of electrons under external field. The high dielectric constant of112and a relatively low loss of0.1were achieved at100Hz with15.3vol%Ag. Theduplex interfacial polarization in the Ag/EVER/PVDF composite contributes to thehigh dielectric constant, good frequency stability and low loss.5. GO nanosheet was first prepared by the oxidation of graphite using a modifiedHummers method. GO nanosheets were chemically converted to rGO during PVPcoating. rGO-PVP/PVDF nanocomposite films were prepared with solution mixing and a hot press technique. Due to the presence of the PVP surface-functionalizedlayers on graphene nanosheets, the resulting composites have much lowerconductivity in comparison to the counterparts containing bare rGO nanosheets.Consequently, the composites not only exhibit a significantly suppressed dielectricloss, especially at a relatively high frequency, but also present an extremely highpercolation threshold for graphene-based composite, implying large number ofmicro/nanocapacitor structures in the composite and outstanding dielectric properties.Moreover, different from the typical percolative composites whose loss increasesabruptly beyond the percolation threshold, the loss of the prepared compositesremains little changed with the variation of the filler concentration. These resultsenable the preparation of the percolative dielectric composites with a very widewindow of the filler concentration to exhibit desirable dielectric and thermalperformances for the highly integrated electronic devices applications under highelectric fields.