Conductive network formation and destruction in polypropylene/carbon nanotube composites via crystal control using supercritical carbon dioxide

This study reports on the effects of the matrix's crystal type (α or γ crystal) and chain mobility on the electrical properties of polypropylene (PP)/carbon nanotube (CNT) composites. Isothermal crystallization of the composites was performed at various temperatures and under atmospheric and elevated pressures with supercritical carbon dioxide (scCO2). Remarkably, experimental results indicated that the composites' isothermal annealing at 150 °C under a supercritical carbon dioxide (scCO2) pressure of 31 MPa reduced their percolation threshold by nearly 50%, via the formation of a significant amount of γ crystals. On the other hand, isothermal annealing of PP/CNT at 135 °C under the scCO2 conditions destructed the conductive network via promoting heterogeneous nucleation of α crystals on the CNT surface. Consequently, this led to a desirable combination of a high dielectric permittivity of ε' = 58.0 and a low dielectric loss of tan δ = 0.2 of PP/1.0 wt% CNT composites at a frequency of 100 Hz.