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

Abstract

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.

Molecular Dynamics Simulations of Polyamide-6 Composite with Covalently Bonded Graphene Network for Thermal Conductivity Enhancement

“…Microscopically, however, they might still interact with Van der Waals molecular force. According to our previous work, when the separation between a pair of graphene nanosheets ϕ is smaller than ϕ c = 1.8 Å, the intergraphene interaction shifts from Van der Waals force to sp 3 -hybridized C–C bonds . Only under the latter scenario, the graphene nanosheets are considered as interconnected.…”

“…According to our previous work, when the separation between a pair of graphene nanosheets ϕ is smaller than ϕ c = 1.8 Å, the intergraphene interaction shifts from Van der Waals force to sp 3 -hybridized C−C bonds. 27 Only under the latter scenario, the graphene nanosheets are considered as interconnected. Recently, it has been an interesting topic of investigating the thermal properties of an interpenetrating network (IPN) of reinforcement fillers, such as carbon nanotubes (CNTs), graphene nanosheets, etc.…”

Efficient Preconstruction of Three-Dimensional Graphene Networks for Thermally Conductive Polymer Composites

“…The thermal conductivities of graphene/polymer composites defer to the percolation theory, and the content of graphene is the decisive factor determining the thermal conductivity of polymer composites [ 63 , 64 ]. A higher graphene content means more contact between the sheets and more heat transfer pathways in the composite, thus ensuring higher thermal conductivity [ 65 – 67 ]. In conventional compounding methods by directly mixing graphene with polymers, high graphene contents usually cause the formation of agglomerates/clusters, which hinders the enhancement in thermal conductivity.…”

Thermal Conductivity Enhancement of Graphene/Epoxy Nanocomposites by Reducing Interfacial Thermal Resistance

“…When the graphene forms the network in the epoxy matrix, the thermal transport of composites is dominated by the graphene–graphene ITR, which is even larger than the graphene–epoxy ITR . To enhance the heat transfer efficiency of the graphene network, the graphene–graphene heat transfer mechanism has also been revealed. − …”