Influence of shear deformation on the electrical and rheological properties of combined filler networks in polymer melts: Carbon nanotubes and carbon black in polycarbonate

The influence of shear on combined filler systems containing multi-walled carbon nanotubes (MWNT) and carbon black (CB) in polycarbonate (PC) melts was investigated by time-resolved combined rheological and electrical measurements. Samples with different MWNT/CB ratios and total carbon filler contents (0.25-10 wt%) were studied. The morphology of the filler network was analyzed by TEM and SEM in charge contrast mode whereby a combined MWNT/CB filler network is indicated.Rheo-electric measurements in the quiescent melt after a defined shear deformation show an increase in electrical conductivity with recovery time for all combined filler systems. Similarly to earlier findings for composites containing only MWNT or CB, the time-dependent conductivity data for the combined filler systems can be described by an agglomeration model with one kinetic constant. This supports the assumption of a combined filler network. In steady shear experiments (1 rad/s) equilibrium values for conductivity and viscosity are reached. At a constant carbon filler content, the electrical conductivity increases on a logarithmic scale with the MWNT ratio, whereas the transient shear viscosity increases linearly. The conductivity of an MWNT/CB 50/50 composite is six orders of magnitude higher than the conductivity of a composite containing only CB. In comparison, the viscosity increases only by a factor of two. These experiments were carried out at a constant carbon filler content of 3 wt%. Equilibrium values of conductivity and viscosity are described by mixing laws based on effective medium approximations.