Does Thermal Percolation Exist in Graphene-Reinforced Polymer Composites? A Molecular Dynamics Answer

Although the addition of graphene fillers can transform an electrical insulator to a conductor at a certain threshold of loading ratio, similar transition in thermal conductivity has not been confirmed yet. Here, we use molecular dynamics to investigate if a physical mechanism responsible for thermal percolation exists in a graphene–polymer composite system. We find that when the separation of two graphene flakes falls below 1.8 Å, their interaction transits from van der Waals force to covalent bonding force, which possibly acts as the underlying mechanism for thermal percolation. By constructing primitive graphene networks with different percolation states, we find that under ideal conditions the transition of inter-graphene interaction from van der Waals to covalent bond results in ≈150% increment in the overall thermal conductivity. An analytical model has also been proposed to describe the relation between the effective thermal conductivity of a graphene–polymer composite and the crystallographic orientations of graphene flakes forming the covalent inter-graphene junction. In sum, the formation of an appreciable amount of covalently bonded inter-graphene junctions is the key to take advantage of thermal percolation to significantly improve the thermal conductivity of graphene-reinforced polymer composites.