Electrically Conductive Polypropylene Nanocomposites with Negative Permittivity at Low Carbon Nanotube Loading Levels

Polypropylene (PP)/carbon nanotubes (CNTs) nanocomposites were prepared by coating CNTs on the surface of gelated/swollen soft PP pellets. The electrical conductivity (σ) studies revealed a percolation threshold of only 0.3 wt %, and the electrical conductivity mechanism followed a 3-d variable range hopping (VRH) behavior. At lower processing temperature, the CNTs formed the network structure more easily, resulting in a higher σ. The fraction of γ-phase PP increased with increasing the pressing temperature. The CNTs at lower loading (0.1 wt %) served as nucleating sites and promoted the crystallization of PP. The CNTs favored the disentanglement of polymer chains and thus caused an even lower melt viscosity of nanocomposites than that of pure PP. The calculated optical band gap of CNTs was observed to increase with increasing the processing temperature, i.e., 1.55 eV for nanocomposites prepared at 120 °C and 1.70 eV prepared at 160 and 180 °C. Both the Drude model and interband transition phenomenon have been used for theoretical analysis of the real permittivity of the nanocomposites.