Study On Structure And Properties Of Carbon Fillers Filled PVDF Composites

Using carbon black (CB), vapor grown carbon fibers (VGCF) and multi-wall carbon nanotubes (MWNT) as fillers, electrical property, kinetics of conductive network formation, dynamic rheology property, and dynamic machine property of the polymer 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 structure and rheology property on the conductive network formation of the composites, and a thermodynamic percolation model was proposed to fit the dynamic process of the conductive network formation.The room volume resistivity of carbon fillers filled Poly (vinylidene fluoride) (PVDF) composites showed percolation clearly. The percolation threshold of the PVDF/MWNT was only 1.75 phr, which was much lower than others. The enhancement of the electrical conductivity could be attributed the large aspect ratio of MWNT. The percolation threshold of the composites decreased with the decrease of the PVDF viscosity.For high-structure carbon black (HCB) filled PVDF composites, storage modulus (G') and complex viscosity (η^*) of the composites increased with increasing the HCB content. Theη^* showed Newton platform at low frequency region. A rheological threshold and a solid-like behavior of the composites were found when the HCB content was higher than 6 phr. This phenomenon could be ascribed to the formation of network structure due to HCB aggregation.For the composites at a conductive filler concentration lower than the percolation threshold, the electrical resistivity decreased sharply at a critical time during the sample was annealed at a temperature higher than the melting point of the polymer matrix. This phenomenon is called dynamic percolation, and the critical time is called percolation time (t_p). SEM verified that the coagulation of conductive filler causes the conductive network formation. The activation energy of the conductive network formation (E_c) for PVDF/LCB,PVDF/HCB,PVDF/VGCF and PVDF/s-MWNT was 65,71,144 and 60 kJ/mol, respectively. The E_c was close to the activation energy of zero-shear-rate viscosity of the composites (E_η), but was higher than that of pure PVDF {E_η0). The room resistivity of the composites reached 10⁷Ω·cm after annealing treatment. This result indicated that the conductive network could remain.The fraction of the filler which join the conduction networks P(t) was used to calculate the coagulation velocity of the conductive fillers. The relationship between the coagulation velocity of conductive fillers S(t) and annealing time t for the conductive network formation was proposed.