Study On Structure And Properties Of Carbon Fillers Filled Polymer Composites

Using Poly（vinylidene fluoride）（PVDF） as a matrix,carbon black （CB）,vapor grown carbon fibers（VGCF） and multi-wall carbon nanotubes（MWNT） as fillers,a series of conductive polymer composites are prepared,and their electrical,dielectric,dynamic mechanical properties and crystallization behavior are studied systemically in this thesis.Particular attention has been paid on the mechanism of a sudden increase in conductivity and dielectric constant when percolation happens.The volume conductivities of carbon filler filled PVDF composites present percolation phenomena clearly,and the filler characteristics,such as filler size,shape,structure and interfacial force between the filler and matrix play significant roles in percolation threshold.The percolation thresholds of the VGCF/PVDF、MWNT/PVDF and MWNT-COOH/PVDF composites are 3,1.53 and 2.38 vol%,respectively,indicating that the conductive network formation of the MWNT filled system is easier than that of the VGCF filled system.The aspect ratio of fillers and the interfacial force between the filler and matrix lead to the difference in percolation threshold of the composites.The interfacial energies of VGCF/PVDF and MWNT/PVDF are 1.4526 and 2.7935 KmN/m, respectively.The interface force between MWNT-COOH and PVDF is larger than that between MWNT and PVDF.Thus,MWNT-COOH is more difficult to form a conductive network compared to MWNT.The dielectric constants of carbon fillers filled PVDF composites also show percolation phenomena.When percolation happens,the dielectric constants of VGCF/PVDF,MWNT/PVDF and MWNT-COOH/PVDF composites are 77,243 and 459,respectively,which are about 10,32 and 60 times higher than that of pure PVDF.In an electrical field, Maxwell-Wagner interfacial polarization effect causes conductive fillers to form micro-capacitors in the composites.With the increase of filler concentration,the number of micro-capacitors increases and the distance between filler particles decreases,which lead to the increase of dielectric constant.However,the different features of VGCF、MWNT and MWNT-COOH result in the differences in dielectric properties of composites.For example,the dielectric constant of MWNT/PVDF composites is larger than that of VGCF/PVDF and MWNT-COOH/PVDF composites when they have the same volume fraction of fillers,and when the percolation happens,the dielectric constant of MWNT/PVDF composites is smaller than that of MWNT-COOH/PVDF composites. The non-isothermal crystallization behaviors of PVDF and carbon fillers filled PVDF composites were studied.The results show that,when the cooling rate increases,it becomes more difficult for all the systems to crystallize perfectly.The addition of CB,VGCF,MWNT promotes the nucleation of PVDF,and affects the parameters of crystallization,such as T_p,t_1/2 andΔH_c.The Ozawa equation and the method developed by Mo are successful in describing the non-isothermal crystallization process of PVDF、CB/PVDF、VGCF/PVDF and MWNT/PVDF composites.