Investigation On Structure,Thermal Conductivity And Dielectric Performances Of 2D-Nanosheets Filled PVDF-Based Composites

With the development of morden industry,the usage of polymers on flexible electrostatic capacitors and thermal conductive devices have became a research hotspot.Owing to the special structural property of high-aspect-ratio,two-dimontional?2D?nanomaterials possess significant applications in enhancing dielectric and thermal conductive properties of polymer-based materials.In this subject,poly?vinylidene fluoride-co-hexafluoropropene??PVDF-HFP?is chosen as polymer matrix,using 2D hexagonal boron nitride nanopellets?BN?,micro-sized silver sheets?Ag MSs?,exfoliated perovskite structural Ca₂Nb₃O₁₀ nanosheets?CNO?and Al₂O₃ coated CNO?CNO@Al₂O₃?as fillers to fabricate some composite materials with high thermal conductivity???or high dielectric constant??'?.The composites are prepared via simple physical blending,and subsequently,hot-molding technologies.According to structural characterization and performance testing results,the effect of introducing 2D nanosheets on the structure,thermal conductivity and dielectric property enhancement of PVDF-HFP is investigated.Results show that BN nanopellets dispersed in amorphous regions of PVDF-HFP matrix.The bridging effect between BN and PVDF-HFP spherulites contribute to enhance?of the PVDF-HFP/BN composites at relative low BN loading?5 wt%?,the out-of-plane and in-plane?of PVDF-HFP/BN5 are 0.17 Wm^-1K^-1 and 2.35 Wm^-1K^-1,respectively.The co-filling of BN nanopellets and Ag MSs?2 wt%?was used to further improve the thermal conductivity???of PVDF-HFP-based composites.The nano-micro cofilled composites simultaneously possess high?and excellent insulation properties attribute to the construction of BN-Ag MSs locally-connected regions in polymer matrix.The locally-connected structure is conductive to heat but non-conductive to electric field.Additionally,the introduction of Ag MSs could induce denser overall structure of the composites,which also help enhance?of materials.As BN contents is 20 wt%,out-of-plane and in-plane?of the co-filled composite is 44.7%and 61.0%higher than that of PVDF-HFP/BN.Simultaneously,dielectric constant and breakdown strength of the co-filled composites are also enhanced.Exfoliated perovskite structural Ca₂Nb₃O₁₀ nanosheets?CNO?were synthesized and the PVDF-HFP/CNO composites were prepared.Results indicate that the self-polarization field around CNO surface could induce the polymer chains to crystallize into?-phase PVDF,enhancing interfacial polarization?Maxwell-Wagner-Sillars,MWS effect?,finally increase dielectric constant of the composite.Inevitably,dielectric loss of the PVDF-HFP/CNO composite is also high.Coating Al₂O₃ shell-layer onto CNO through hydrothermal method to fabricate the core-shell structured CNO@Al₂O₃ 2D nanosheets.The addition of Al₂O₃ shell-layer could naturally reduce the dielectric mismatch between CNO and PVDF-HFP matrix.Meanwhile,abundant Al-OH groups on CNO@Al₂O₃ surface contribute to bonding PVDF-HFP chains with nanosheets via strong hydrogen bond.As consequence,the PVDF-HFP/CNO composites possess enhanced dielectric constant,suppressed dielectrc loss as well as high breakdown strength.In summary,2D nanosheets located in amorphous regions of semi-crystalline polymer matrix,resulting bridging effect between nanosheets and spherulites.According to SAXS results,the fluctuation of the composite long period?L_W?could reflect some microstructure and performance variation of the 2D nanosheets filled composites.Obviously,reduced L_W of the composites suggests that low separation and deformation degree of the crystalline lamellae,simultaneously,more crystalline lamellae exist per unit volume.The composites with high dielectric constant or thermal conductivity always possess low L_W.On the other frount,increasing the crystallinity of the composite could reduce free volume in material,subsequently benefit to the construction of thermal conductive network,finally contribute to enhance thermal conductivity of the composites.Importantly,2D nanosheets with high surface energy or high dielectric constant could induce the crystallization behavior of surrounding PVDF-HFP chains,which could easily crystallize into?-phase PVDF around the surface of nanosheets.Consequently,dielectric constant of the composites could be enhanced.