| Conductive polymer composites have been extensively used in many fields,such as electronic,energy and chemical engineering.To ensure a high conductivity, conducting fillers are requested more loading,namely,a high content of the fillers in the composites.A typical example is that it is used to add more than 20%conductive carbon black(CB) in the composite to form conducting network.However,high concentration of fillers in some disadvantages for the composites,such as high melting viscosity,poor processability and mechanical properties.Hence,lowering the content of conducting filler in a composite is of importance for improving the properties of the composites.A series of conductive polymer composites of carbon black(CB),carbon nanotubes and polybutylene terephthalate were prepared by melt-mixing method. The conductivity,morphology,mechanical behavior,positive temperature coefficient (PTC),rheology and crystallization were studied.The results are following:.1.The electrical percolation threshold of CB/PBT is 4.5 wt%of CB loading, with the addition of 0.9 wt%of MWNTs,the percolation threshold was reduced to 3.9 wt%,while the conductivity near percolation threshold were increased by an order of magnitude from 10-5 S/cm to 10-4 S/cm.A synergy effect was found between MWNTs and CB in the polymer matrix,and they are easy to form conducting network,which leads to lower percolation threshold and better processibility.2.Scanning electron microscope(SEM) were employed to observe the morphology of the conductive composites and the distribution of the fillers in the composites.MWNTs with high aspect ratio were distributed in the spherical CB particles,a cross-linked conductivity network was forrned,leading to reduced percolation threshold.3.The measurement of PTC of the composites showed that their PTC intensity was very weak.The coincidence of the temperature-conductivity curve and the DSC temperature rising curve indicated the correctness of the theory of thermal expansion.4.The measurement of dynamic rheology showed that with the introduction of conductive filler,the storage modulus(G'),loss modulus(G") and complex viscosity of the composites increased,and the loss tangent decreased.The existence of "second platuea" of G'～ω,G"～ωwith lowωsuggested the change of the structure of the composites.When filled PBT with CB,the filler particle aggregates wrapping network formed,while filled with MWNTs,rheological percolation threshold of the composites was lower than the former due to the high aspect ratio of MWNTs.5.DSC was used to analyze the crystallization and to study crystalline kinetics of the ternary composites.The spectra and data indicated that the non-isothermal crystallization of the composites was mainly dependent on the content of CB/MWNT and cooling rate.When the content of CB and/or MWNTs was fixed to a constant rate,the crystallization temperature decreased with the increase of cooling rate. When cooling at the same rate,crystallization temperature increased with increasing content of CB and/or MWNTs in the composites,suggesting their roles of nucleating agent in the composites.6.XRD diffraction results showed that the diffraction peak of the composite system evidently became narrow with an increase of the CB or MWNT loading, respectively.This phenomenon indicated that the crystal size of PBT became uniform and the distribution became narrow.7.The mechanical properties of the composites were investigated.With the addition of appropriate amount CB and/or MWNT in the PBT matrix,the mechanical performance of the composites increased.As filling excessive conductive fillers,the fillers tended to aggregate,which lead to poor distribution and stress concentration points were formed.
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