Study On Structure And Properties Of Carbon Nanotubes Filled PVDF/PC Composites

Using multiwalled carbon nanotubes（MWNTs） as fillers, Poly（vinylidene fluoride）（PVDF）/Polycarbonate（PC） blend as matrices, conductive polymer composites were prepared.Effects of MWNT content, morphology and surface characteristic on the electrical performance, dynamic mechanical property and non-isothermal crystallization behaviors of the composites were studied systemically in this thesis.An in-situ method was used to study the kinetics of conductive formation of the composites by recording the variation of electrical resistivity with time during the isothermal treatment.Particular attention has been paid on the effect of filler types,the dispersion state of fillers in the matrix and external fields on conductive network formation of the composites,and a thermodynamic percolation model was proposed to fit the kinetics of conductive network formation.The room volume resistivity of carbon fillers filled PVDF composites showed percolation clearly.The percolation threshold of PVDF/MWNT was only 1.44 vol%,which was much lower than other systems.The enhancement of the electrical conductivity could be attributed to the large aspect ratio of MWNT.The percolation threshold of MWNT-filled PC/PVDF blends was much lower than those of MWNT-filled individual polymers because of the selective location of MWNTs in the PC phase.Dynamic mechanical analysis（DMA） results showed that PVDF/PC was a thermodynamic immiscible system.Adding MWNT into PC matrix induced sub transition of PC.Differential scanning calorimetry（DSC） results showed that MWNT played a role of heterogeneous nucleation during PVDF crystallization process.Parameters such as T_p,t_1/2,X_Tp of PVDF non-isothermal crystallization were changed because of the presence of MWNT.Jeziorny method and Mo equation were successful in describing the non-isothermal crystallization process.An in-situ method named as dynamic percolation was used to study the kinetics of conductive formation of the composites.The results showed that the activation energy of the conductive network formation（E_c） for PVDF/MWNT,PVDF/MWNT-COOH,PVDF/s-MWNT,PC/MWNT and PVDF/PC/MWNT was 108,128,60,150 and 143 kJ/mol,respectively. The E_c was related to the interfacial interaction of MWNT-polymer,the activation energy for MWNT-filled PC/PVDF blends was close to that of the PC/MWNT system,indicating that the conductive network formation was dominated by the characteristic of the filler-rich PC phase.Electric field controlled formation and dissociation of MWNT conductive pathways in a PC melt were investigated by the dynamic percolation measurement.The results showed that field induced MWNT alignment caused the decrease in the activation energy of conductive pathway formation and the percolation time.Based on the MWNT alignment in the PC melt,one-dimensional conductive material with the R_⊥/R_∥ratio near 10⁵ was obtained.The directional or disordered alignment of MWNT in the PC melt resulted in the transition from a conductor to an insulator as the electric field changes from 500 to 1 V/cm. This electric-controllable directional or disordered alignment technology was promising for the applications of voltage-switch devices.A thermodynamic percolation model was proposed to forecast the percolation time in the carbon nanotubes filled polymer system,i.e.,the thermodynamic percolation model can be used to fit the kinetics of the conductive network formation.