Construction Of Poly (Vinylidene Fluoride) Based Composite Films With High Dielectric Constant And Low Loss

Poly(vinylidene fluoride)(PVDF)has attracted increasing attention in various applications due to its excellent piezoelectric,pyroelectric and ferroelectric properties.The semi-crystalline polymer exhibits five different crystalline structures(?,?,?,?and ?),corresponding to different conformations of PVDF chains.Among these different phases,the ?-phase shows the highest dipolar moment,piezoelectric coefficient and dielectric constant as well,rendering ?-phase as the most important phase in fabricating advanced modern electronic devices,such as sensors,actuators and transducers.To make up the disadvantage(low dielectric constant)of PVDF,several methods were poposed,such as addition of ceramics or conductive nanofillers into PVDF matrix for building binary system.The proposed methods are improving all the time to aquire composite materials with optimized dielectric properties.However,with the increasement of dielectric constant,some bad impacts appears as well,such as the increased loss,weakened mechanical strength and breakdown strength.To improve the dielectric properties of PVDF-based composites,namely possessing high dielectric constant,comparatively lower loss,higher mechanical strength and breakdown strength,we chose ceramics with high dielectric constant and carbon materials with high conductivity as nanofillers.Moreover,by surface-modified treatment,multiphase composition and film structure design,a series of PVDF based composites with high dielectric constant and low loss were constructed.The datailed research information is as follows:1.A three-phase nanocomposites,including poly(vinylidene fluoride)(PVDF)and two nanofillers,namely,surface-modified multi-wall carbon nanotubes(mCNTs)and barium titanate nanoparticles(mBTs),are constructed.By the surface-modified treatment,the dispersity of nanofillers was increasd and the compatibility between PVDF and naofillers was enhanced as well.Meanwhile,the interfacial interactions between PVDF and nanofillers was also strengthened.The ternary nanocomposites were fabricated by a phase-separation and hot-pressing process.The composite films shows higher dielectric constant,lower loss and increased mechanical tensile property.2.PVDF-based ternary composite,including electrochemical exfoliated graphene(EEG)with high ?-? conjugation and good dispersity and surface-modified boron nitride(DBN),was fabricated by solution casting method.Due to its high ?-?conjugation,EEG was given excellent conductivity.Meanwhile,amounts of oxygen-containing functional groups anchored onto edges of EEG nanoplates,it could enhance the interfacial interactions between PVDF and EEG by forming hydrogen bond.Because of that,EEG(2.0%)/PVDF shows the higher dielectric constant of 591 and loss of 1.21.By additon of DBN into EEG/PVDF,its good dispersty could efficently decrease the contact chances of EEG nanoplates,which would provide DBN(3.1%)/EEG(2.0%)/PVDF with high dielectric constant(6655)and lower loss(0.83)due to the synergistic effect of DBN and EEG.Moreover,the thermal conductivity of the ternary composite film can reach up to 1.33 W m-1K-1,which is 5.68 times more than that of pure PVDF(0.234 W m-1K-1).3.In order to further decrease the loss of EG/PVDF,carboxyl groups were introduced by non-covalent modification based the high ?-? conjugation degree of graphene,which could decrease the chances of forming conductive path between EEG nanoplates.Compared with graphene oxide and chemical reduced graphene oxide,carboxyl groups-induced graphene(PyEG)and PVDF composite showes high dielectric constant(480)and low dielectric loss(0.27)obtained at a low filler content of 0.74 vol%.The enhanced dielectric properties could be reasonably explained by a proposed model.4.To eliminate the forming conductive path by fillers and decrease the loss of PVDF based composite,a multilayer-structured MXene(Ti3C2Tx)/PVDF film was fabricated via spin coating,spray coating and hot-press methods.Due to the good insulativity of PVDF,4MXene/5PVDF(namely four layers of MXene and five layers of PVDF)exhibits a high dielectric constant(41)and an ultralow dielectric loss(0.028,smaller than that of pure PVDF)at 1 kHz.Surprisingly,the MXene/PVDF films show good broadband dielectric behaviors and the dielectric constant of 4MXene/5PVDF can reach up to 32.2 at 1 MHz,which can retain as high as 78.4%of that at 1 kHz.Moreover,the multilayered structure could maintain comparatively high breakdown strength,higher discharged energy density and efficiency.This work provides a promising design paradigm to construct polymer films with a high dielectric constant and low dielectric loss.