Preparation And Multifunctional Regulation Of Thermal Conductive PVDF Composites Based On Hybrid Network Of Nanofillers

Adding thermal conductive fillers into polymer matrix to preparation filled type thermal conductive polymer composites has attracted the extensive attention due to that the composites exhibit many advantages,such as easy preparation,low cost and wide application in electrical products and microelectronics packaging.Common thermal conductive fillers include conductive fillers,such as:carbon nanotubes(CNTs),graphene nanoplatelets(GNPs),graphite,etc.and insulating fillers,such as:boron nitride(BN),aluminum nitride(AlN),aluminum oxide(Al2O3).Utilizing two fillers with different size and different dimensions to constitute hybrid fillers and filling the polymer composites is one of the most effective,most simple methods to prepare filled type thermal conductive polymer.However,with the development of electronic information fields,composites with single thermal conductive performance have been unable to meet the market demand.Thus,the preparation of highly thermal conductive polymer composites is of great significance.The research of this paper is focus on the hybrid network density impact on the thermal conductivity of poly(vinylidene fluoride)(PVDF).Study the influence of adding hybrid conductive fillers(CNTs/GNPs)on thermal and electric conductivity of PVDF.In order to explore the influence of different hybrid network on thermal and electric conductivity of PVDF,fixing a kind of filler content and changing another kind of filler content were investigated.In addition,through regulating the content and size of insulating filler BN and controlling the dispersion or network structure of CNTs,the optimum process of fabricating composites with high thermal conductivity and high dielectric constant was researched.The main results obtained are listed as follows:(1)GNPs and CNTs were introduced into PVDF matrix to prepare ternary nanocomposites via a two-step processing strategy(solution compounding and melt compounding).GNPs and CNTs had obvious nucleation effect on PVDF,but the hybrid network hardly affected crystallinity of PVDF,through the analysis on crystallization behavior of composites.Through the characterization of microstructures,rheological behavior and the measurement of thermal conductivity,adding a few GNPs into PVDF/CNT-x,the CNTs aggregated more serious due to the volume exclusion of GNPs.The EMT theoretical simulation results showed that owing to aggregating of CNTs,the contact thermal resistance between CNTs was slightly increased.rheological results showed adding a few GNPs into PVDF/CNTs-x composites had little influence on network density which leaded to an slight increasing of thermal conductivity;however,adding a few CNTs into PVDF/GNP-x,three dimensional hybrid network was formed,meanwhile,modulus was increased significantly and larger platform was appeared which explained that denser network was formed in the composites.So,thermal conductivity of composites was significantly enhanced and synergistic efficiency was also improved.Through conductivity test,CNTs played a dominant role in the conductive network,but compared with adding a few GNPs into PVDF/CNTs-x,introducing a few CNTs into PVDF/GNP-x could obviously improve conductivity of composites,and simultaneously have higher synergistic efficiency.(2)Ternary nanocomposites with high thermal conductivity was successfully prepared through introducing conductive filler CNTs and graphene-like insulating fillers BN into PVDF.Crystallization behavior showed that BN and CNTs had obvious nucleation effect on PVDF,and simultaneously adding CNTs and BN into PVDF,crystallinity would increase with the increasing of BN content.The characterization of microstructure and the test of dielectric properties and conductivity illustrated that a few BN could disperse the CNTs,and conductivity of composites was enhanced obviously,meanwhile,more interfacial polarization between PVDF and CNTs and more micro-capacitance structure were formed which leaded to improvement of dielectric constant.However,owing to space steric hindrance effect of a lot of BN particles,CNTs network was destroyed and conductivity,dielectric constant was decreased dramatically.Through analysis of rheological behavior,dense network structure formed when the BN content was more than 10 wt%.However,adding CNTs into PVDF/BN-x,even if the BN content was 1 wt%,the dense three dimensional network also formed,and enhanced network density was the main factor on improving the thermal conductivity.(3)Tunning the size of BN and introducing hybrid fillers of CNTs and three different sizes BN into PVDF,three kind of ternary nanocomposites were fabricated.Through analysis of rheological behavior,microstructure and the test of conductivity to changing of filler network,thermal conductivity and dielectric properties were measured.The main discovery was as followed:For PVDF/aBN-x/CNT,a few CNTs were inserted in aBN of nanoscale.On one hand,CNTs was dispersed by aBN,therefore,the modulus of PVDF/aBN-x/CNT had larger increase than PVDF/aBN-x.At the same time,owing to the minimal dependence on frequency in the low frequency,there was denser network.On the other hand,due to adsorption of aBN on CNTs surface,it was difficult for CNTs to directly joint,so the composites had lower conductivity and worse dielectric property.For PVDF/bBN-x,it was the easiest to form thermal conductive network owing to the largest aspect ratio,so the samples had larger thermal conductivity.For PVDF/bBN-x/CNT,on one hand,CNTs had will dispersion through volume exclusion of bBN.On the other hand,CNTs brought increase of melt viscosity of material,and in this condition,relatively larger shear stress possibly brought to the BN particles,which facilitated bBN to form network.Therefore,more thermal conductive paths were built and at the same filler content,this group sample had the largest thermal conductivity.For PVDF/cBN-x/CNT,owing to the largest size of cBN which was hardly formed network,cBN only had volume exclusion to CNTs which made CNTs forming denser network.For these reasons,PVDF/cBN-x/CNT samples existed the largest conductivity and dielectric constant,and modulus was also enhanced obviously.However,the thermal conductivity was lower than the sample containing bBN owing to cBN of contribution to thermal conductivity hardly forming network.