Graphene nanoplatelets (GNP) dispersed in high-density polyethylene (HDPE) were found to significantly enhance nucleation of PE crystallization, and thermal/mechanical properties of HDPE-GNP nanocomposites. Excellent dispersion of GNPs was achieved by combining solution blending and twin-screw extrusion homogenization. The dispersion state was confirmed by bimodal AFM and also by the resulting improvements in thermal, mechanical, and transport properties. The crystallization rates were shown to markedly increase at low GNP content (0.1 wt%) and continued to increase with GNP loading up to 15 wt%. The size of secondary structures, or spherulites, decreased with GNP loading. Smaller spherulite sizes and faster kinetics suggested enhanced nucleation with GNP addition. 30-40% reductions in permeability and up to 60% higher Young's moduli were found in these HDPE-GNP nanocomposites. Restricted polyethylene mobility and enhanced thermal stability with increasing GNP loading were also indicated in these HDPE-GNP nanocomposites. (C) 2016 Elsevier Ltd. All rights reserved.