Conductive And Rheological Behaviors Of Multiwalled Carbon Nanotubes Filled Polymer Composites

Carbon nanotubes (CNT) have excellent electrical conductive, thermal conductive and mechanical properties. Currently, CNT is considered to be the most ideal fibrous filler for the preparation of high-performance, multi-functional polymer composites. In this dissertation, a variety of multi-walled carbon nanotubes (MWCNTs) filled polymer composites were prepared by melt blending. The effects of the content and dispersion of MWCNTs, polymer matrix type on the conductive and rheological behaviors of composites were studied systematically. The relations between conductive and rheological behaviors were discussed, with research interest having been focused on finding the influence of particle content, particle dispersion state and particle-polymer interaction on the conductive and melt rheological behaviors. The following aspects have been dealt mainly:1) At first, the conductivity percolation (CP) and dynamic rheological behaviors of MWCNTs filled polystyrene (PS) and high-density polyethylene (HDPE) composites were investigated. In case of low volume fraction (φ), MWCNTs dispersed in PS matrix were quite homogeneous, while they formed cluster aggregates at φ>0.02. The CP transition of MWCNTs/PS and MWCNTs/HDPE composites appeared in φ range from 0.015 to 0.021 and 0.010 to 0.027, respectively, with the volume resistivity (ρv) of the former being slightly lower than the latter at the same φ. Moreover, the critical strain values (γc) of the Payne effect for MWCNTs/PS composites were greater than MWCNTs/HDPE at the same φ. The dependence of ρv and γc on φ indicated the stronger particle-polymer interactions and weaker particle-particle interactions in the MWCNTs/PS composites than those in MWCNTs/HDPE. The addition of MWCNTs caused the increase of the linear dynamic moduli and viscosity of two composites, with the dependence of the storage modulus (G’) and the loss modulus (G") on frequency (ω) having weakened gradually with increasing φ. Furthermore, the solid-like viscoelastic behavior of G’≈G" exhibited at φ=0.035 and 0.025 for MWCNTs/PS and MWCNTs/HDPE composites, respectively, caused by the differences of particle-polymer and particle-particle interactions in two composites. The linear dynamic viscoelastic functions of two composite systems were well fitted with a two-phase model, meaning that the global complex modulus of the composites can be divided into that of the bulky polymer phase far away from the filler inclusions with a certain strain amplification effect and that of the filler phase. The strain amplification factors (Af) of MWCNTs/PS were lager than those of MWCNTs/HDPE, and the filler phase dynamic moduli of the former were smaller than the latter. Moreover, Af of the former increased significantly with increasing φ and decreased significantly with increasing temperature while that of the latter didn’t change obviously with φ and temperature changing.2) The microstructure of MWCNTs/PS composites before and after isothermal annealing was observed, the time-dependent dynamic modulus percolation (DMP) behaviors of MWCNTs/PS and MWCNTs/HDPE composites during isothermal annealing at ω=1 rad s-1 were investigated, the dependence of the ratio of the complex modulus of the composites to that of the polymer matrix (GR=|Gc*|/|Gm*|) on φ was discussed, and time-temperature superposition (TTS) principle was adopted to build DMP main curves of two composites. The filled melts behaved liquid-like DMP during annealing and the dispersion state of the fillers had almost no change after annealing with φ below percolation threshold (φc). And the dependence of GR on φ followed Guth formula in the same φ range. Furthermore, at φ<φc, the modulus activation energies (EG’ and EG’’) of the G’ and G" were higher and lower than those of viscous flow (Eω) of matrix melts, respectively, but were in agreement with the dynamic modulus activation energies of dynamic moduli of matrix melt at ω=1 rad s"1, these dynamic moduli having been obtained through the dynamic frequency experiment. On the other hand, at φ>φc, annealing caused MWCNTs to aggregate further so much as that the melts changed from the liquid-like to the solid-like and the dependence of GR on (p followed percolation theory formula GR～(φ-φc)α In this case, both EG and EG" were smaller than Eω and decreased with increasin φ, while dynamic time activation energies (Et and Et") of liquid-like to solid-like transition, appearing exclusively in the φ range of conductor state, were significantly higher than Eω. Moreover, EG’ and EG" exhibited a sharp decrease in the percolation transition region, but had almost no change beyond the region. It was suggested that the isothermal time-dependent DMP disclosed to following TTS for the first time was closely related to the formation of the MWCNTs aggregation-induced construction of three-dimensional elastic network while the terminal relaxation of polymer matrix macromolecules was not the determinant factor for DMP.3) The relations between the microstructure, conductive and rheological behaviors of MWCNTs filled poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blends were investigated. The CP φc of MWCNTs/PVDF/PMMA composites was smaller than that of MWCNTs/PVDF, which could be attributed to the promoted aggregation of MWCNTs and the reduced viscosity of blends with the addition of PMMA. The crystallization and dynamic mechanical behaviors indicated that MWCNTs didn’t destroyed the good compatibility of PVDF and PMMA in the composites and they played the role of heterogeneous nucleation agent, having accelerated the crystallization of PVDF. Interestingly, MWCNTs changed barely the melting temperatures (Tm) and glass transition temperature (Tg) of the composites. With the addition of MWCNTs, the thermal stability of MWCNTs/PVDF composite improved obviously while that of MWCNTs/PVDF/PMMA composite deteriorated, both having exhibited the best thermal stability at φ=0.04.