This study reports on the effects of the matrix's crystal type (alpha or gamma 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 (scCO(2)). Remarkably, experimental results indicated that the composites' isothermal annealing at 150 degrees C under a supercritical carbon dioxide (scCO(2)) pressure of 31 MPa reduced their percolation threshold by nearly 50%, via the formation of a significant amount of gamma crystals. On the other hand, isothermal annealing of PP/CNT at 135 degrees C under the scCO(2) conditions destructed the conductive network via promoting heterogeneous nucleation of alpha crystals on the CNT surface. Consequently, this led to a desirable combination of a high dielectric permittivity of epsilon' = 58.0 and a low dielectric loss of tan delta = 0.2 of PP/1.0 wt% CNT composites at a frequency of 100 Hz. (C) 2017 Elsevier Ltd. All rights reserved.