Effect Of The Liquid-Crystalline Polymers Modifier On The Performance Of BaTiO₃/P（VDF-TrFE-CTFE） Nanocomposites For Energy Storage Applications

Dielectric nanocomposites with high dielectric constant,low dielectric loss and high enengy density have been widely used in power electronics industry and energy storage.Traditional ferroelectric ceramics(such as BaTiO3,BaSrTiO3,PbZrTiO3,etc.)possess the high dielectric constant,however,there are some disadvantages,such as high dielectric losses,high processing temperature and low breakdown strength.Polymer dielectric materials usually have high breakdown strength and good processability,while suffer from the low dielectric constant.Ceramic/polymer dielectric materials have been widely studied because they can combine the advantages of ceramics with high dielectric constant and polymers with well processity and high breakdown strength.However,due to the large contracts of surface energy and electrical properties of nanofillers and matrix materials,they lead to the aggregation and phase separation of nanofillers in polymer matrix,the uneven distribution of electrical field near the nanofiller-matrix interfaces and the complicated interfacial polarization,resulting in the decrement of discharged energy density."Interfacial modifier engineering" is an effective way to overcome above issues.Mesogen-jacketed liquid crystalline polymer(MJLCP)formed the columnar nematic phase and exhibited a semirigid rod feature own to the strong steric effect of side-chain.In addition,the rod length and surface chemistry of the rod could be controlled,which provided the varied choices for the interfacial modifier.Combined the properties of MJLCP,we choosed the MJLCP as the interfacial modifier of barium titanate(BT)particles.On hand,the problems of nanofiller’ dispersity and compatibility can be resloved in the poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene)(P(VDF-TrFE-CTFE))polymer matrix.On the other hand,the relationship between interfacial chemical structure and thickness and energy storage of nanocomposites will be investigated in detail.Based on this research,the ceramic/polymer dielectric nanocomposites with optimal energy storage property will be confirmed.The detail content was as followed:In the first part,four kinds of liquid crystalline fluoro-polymer poly{2,5-bis[(4-trifluoromethoxyphenyl)oxycarbonyl]styrene}(PM3F),poly{2,5-bis[(2,3,4,5,6-pentafluoro)oxycarbonyl]styrene}(PM5F),poly{2,5-bis{[3,5-bis(trifluoromethyl)]oxycarbonyl}styrene}(PM6F)and poly{2,5-bis[(2,3,5,6-tetrafluoro-4-trifluoromethyl)oxycarbonyl]styrene}(PM7F)were designed as the modifier.The active organic functional groups were introduced onto the surface of BT nanoparticles by using hydrogen peroxide,y-aminopropyltriethoxysilane(y-APS),and 4-cyanopentanoic acid dithiobenzoate(CPDB).After then,four kinds of core-shell structured BT@liquid crystalline fluoric-polymer nanoparticles were synthesized by reversible addition fragmentation chain transfer(RAFT)polymerization.The FT-IR、TGA and TEM results revealed that the liquid crystalline fluoric-polymer was successfully introduced into the surface of BT nanoparticles.The polymer nanocomposites were prepared by ball-milling and solution blending process.The SEM experiment results showed that liquid crystalline fluoro-polymer modifier can improve the dispersion and compatibility of BT nanoparticles in the P(VDF-TrFE-CTFE)matrix.The dielectric properties and energy storage performance of the nanocomposites were depended on the molecular structure of modifier,especially the number of electron-rich fluoric groups.Compared with the pure P(VDF-TrFE-CTFE),the BT@liquid crystalline fluoro-polymer/P(VDF-TrFE-CTFE)polymer nanocomposite films exhibited the higher dieletric constant and lower dieletric loss.In all the nanocomposites,the BT@PM3F/P(VDF-TrFE-CTFE)and BT@PM5F/P(VDF-TrFE-CTFE)nanocomposites presented the higher dieletric constant.Moreover,BT@PM3 F/P(VDF-TrFE-CTFE)nanocomposites produced the highest breakdown strength and reached to 542 kV/mm-1 and the corresponding discharged energy density is 14.5 J cm-3,which is about 1100%over the biaxially oriented polypropylenes(BOPP)(1.2 J cm-3 at 640 MV/m),indicating the PM3F was fit for the interfacial modifier.In the second part,the length of PM3F polymer molecular chain is proportional to the polymerization degree own to the property of simi-rigid polymer.By designing polymerization degree,three different thickness of core-shell structured BT-3F nanoparticles were prepared by using RAFT polymerization.The frequency dependent dielectric properties and energy storage capability of the P(VDF-TrFE-CTFE)nanocomposites with modified BaTiO3 as the filler were investigated.The results showed the permittivity,breakdown strength and energy density of the polymer nanocomposites were significantly affected by the thicknesses of rigid-fluoro-polymer.Moreover,a high discharge energy density of 16.18 J cm-3 was achieved in 5 vol%BT nanocomposites when the shell thickness was approximately 11 nm.The findings provide an innovative approach to prepare dielectric composites with high energy density,and promote deep understanding on the influence of interfacial regions thickness on the dielectric performance.