Increasing the Thermal Conductivity of Graphene-Polyamide-6,6 Nanocomposites by Surface-Grafted Polymer Chains: Calculation with Molecular Dynamics and Effective-Medium Approximation

Abstract

By employing reverse nonequilibrium molecular dynamics simulations in a full atomistic resolution, the effect of surface-grafted chains on the thermal conductivity of graphene-polyamide-6.6 (PA) nanocomposites has been investigated. The interfacial thermal conductivity perpendicular to the graphene plane is proportional to the grafting density, while it first increases and then saturates with the grafting length. Meanwhile, the intrinsic in-plane thermal conductivity of graphene drops sharply as the grafting density increases. The maximum overall thermal conductivity of nanocomposites appears at an intermediate grafting density because of these two competing effects. The thermal conductivity of the composite parallel to the graphene plane increases with the grafting density and grafting length which is attributed to better interfacial coupling between graphene and PA. There exists an optimal balance between grafting density and grafting length to obtain the highest interfacial and parallel thermal conductivity. Two empirical formulas are suggested, which quantitatively account for the effects of grafting length and density on the interfacial and parallel thermal conductivity. Combined with effective medium approximation, for ungrafted graphene in random orientation, the model overestimates the thermal conductivity at low graphene volume fraction (f < 10%) compared with experiments, while it underestimates it at high graphene volume fraction (f > 10%). For unoriented grafted graphene, the model matches the experimental results well. In short, this work provides some valuable guides to obtain the nanocomposites with high thermal conductivity by grafting chain on the surface of graphene.

Thermal conductivity of polymers and polymer nanocomposites

“…However, surface functionalization of 2D fillers does not always lead to a smaller interfacial thermal resistance. The effect of surface-grafted chains on the thermal conductivity of graphene@polyamide-6.6 nanocomposites has been studied using MD simulations [209]. It turns out that the thermal conductivity perpendicular to the graphene plane is proportional to the grafting density, while the in-plane thermal conductivity of graphene drops sharply as the grafting density increases.…”

Recent Advances in Graphene-Based Free-Standing Films for Thermal Management: Synthesis, Properties, and Applications

“…This may be due to the ultralow thermal conductivity of the polymer (~0.1 W/m·K) compared to that of graphene (>100 W/m·K), transforming the ballistic heat transfer in graphene to the diffusive heat transfer in polymers [76,77]. Moreover, a number of theoretical studies have demonstrated that the interfacial thermal resistance (often known as the Kapitza resistance) between graphene and polymer also lowers the thermal conductivity of the composite films [78][79][80][81].…”

“…However, excessive loading of grafted polymer may reduce the thermal conductivity of the graphene sheets, thus inducing lower effective thermal conductivity for the composite film. Therefore, the grafting density should be well-controlled to achieve a higher thermal conductivity of the composite film than the conductivity of the polymer matrix [81]. More discussion about nanoscale thermal transport based on phonon propagation can be found in other reviews [82,83].…”

Edge‐Grafted Molecular Junctions between Graphene Nanoplatelets: Applied Chemistry to Enhance Heat Transfer in Nanomaterials

“…The effect of chemical functionalization on thermal properties of junctions between nanoflakes was studied by MD calculations on a simplified model system, made of two adjacent graphene sheets edge functionalized with phenols or with the covalent molecular junctions ( Figure ), using a well‐known method previously reported. [12c]…”

“…To tailor the properties of nanopapers, the exploitation of molecular junctions between GRM nanoflakes is currently a promising possibility, yet experimentally challenging. In fact, while the conductance of molecular junctions was widely studied by molecular dynamics and density functional theory, the experimental exploitation of organic molecular junctions was only recently reported by Han et al for the thermal coupling of the graphene–graphene oxide and the graphene oxide–silica surfaces. In this work, both covalent and non‐covalent molecular junctions were designed and synthetized to create GRM‐based nanopapers with inherently low contact thermal resistance between nanoplatelets.…”