Polymeric Self-Assembled Monolayers Anomalously Improve Thermal Transport across Graphene/Polymer Interfaces
L Zhang and L Liu, ACS APPLIED MATERIALS & INTERFACES, 9, 28949-28958 (2017).
Ultralow thermal conductivities of bulk polymers greatly limit their applications in areas demanding fast heat dissipation, such as flexible electronics and microelectronics. Therefore, polymeric composites incorporating highly thermally conductive filler materials (e.g., graphene and carbon nanotubes) have been produced to address the issue. However, despite some enhancement, thermal conductivities of these materials are still far below theoretical predictions, mainly due to the inefficient thermal transport across material interfaces. Here, using molecular dynamics simulations, we demonstrate that polyethylene (PE) self-assembled monolayer (SAM) functionalized graphene surfaces at a high grafting density can drastically improve interfacial thermal conduction between graphene and the matrix of poly(methyl methacrylate) (PMMA). In contrast to abrupt temperature drop across pristine graphene/PMMA interfaces, temperature field in the vicinity of a PE grafted graphene/PMMA interface is continuous with a smoother transition and higher thermal conductance. This anomalous improvement is attributed to three factors that closely relate to the grafting density of the SAM of PE. First, the SAM with high grafting densities features highly extended chains that enhance along-chain thermal conduction. Second, the strong covalent bonding between the SAM and the graphene facilitates heat transfer at their joints. Third, the SAM and the PMMA matrix are both organic materials, leading to enhanced interfacial vibrational coupling. Molecular mechanisms underpinning these phenomena are systematically elucidated by analyzing the temperature field, density distribution, Herman's orientation factor, the vibrational density of states, cumulative correlation factor, the integrated autocorrelation of interfacial heat power, and interfacial adhesion. All results suggest the incorporation of SAMs at a high grafting density or extremely extended polymer brushes for drastically improved interfacial thermal transport between hard and soft materials toward a wide range of applications.
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