Enhancing the electrical conductivity of PP/CNT nanocomposites through crystal-induced volume exclusion effect with a slow cooling rate

Polymer crystallization on CNT surfaces has been considered to be a critical reason for the relatively high percolation thresholds obtained in melt-blended polymer/CNT nanocomposites (e.g., generally > 1 wt% in most polyolefin/CNT systems). In this work, the crystal-nucleating capability of CNTs in polypropylene (PP) was controlled through addition of a sorbitol-based external nucleating agent (NA) and via controlling the cooling rate. Despite the competition between CNT and NA for crystal nucleation, little increase in conductivity was obtained as compared to PP/CNT with fast cooling (~150 °C/min). However, with a slow cooling process (~1.5 °C/min), large PP crystallites were induced with only a small number of nuclei. Therefore, the majority of the CNT particles were prevented from participating in the nucleation of PP crystals so that they could be concentrated to form conductive networks through an enhanced volume exclusion effect. This significantly increased the conductivity of the PP/CNT nanocomposites, and the percolation threshold was greatly reduced from 0.75 wt% to 0.36 wt%. The work highlights the crucial influence of controlling CNT nucleation capability and the polymer crystallite size on the conductivity using slow cooling in addition to annealing treatment.