Advantage of Edge versus Basal Plane bonding in enhancing k of polymer graphene nanocomposites
Ultra high thermal conductivity (k) of graphene exceeding 1000 W/mK makes it attractive for enhancing k of polymer-graphene nanocomposites. A limiting factor in the use of graphene is the interface thermal resistance between polymer and graphene. Covalently bonding the two enhances interface thermal conductance, providing new avenues to enhance composite k. To realize full potential of graphene, it is critical to gain understanding of covalent bonding schemes with superior interface thermal conductance. We show that bonding polymer chains on the edge of graphitic nanoplatelets offers distinct advantages compared to on the basal plane.
First, bonding on edge enables all graphene sheets within the nanoplatelet to be covalently bonded to the surrounding polymer matrix (seen below). This allows heat to be conducted from the polymer to the entire nanoplatelet, enabling the entire nanoplatelet to contribute towards enhancing heat conduction through the composite. In contrast bonding on the basal plane occurs only on the outermost layers of the nanoplatelet (shown in image below). This allows heat to be conducted by only part of the nanoplatelet, severely diminishing overall heat conduction through the nanoplatelet compared to the edge-bonding case.
We also demonstrated recently that bonding polymer chains on the edge provides almost 2-fold higher interface thermal conductance compared to bonding on the basal plane.[pdf] State-of-the art nonequilibrium Green’s approach along with force interactions derived from density-functional theory were used to derive above results. Using polarization specific extension of Green’s function method, the demonstrated higher interface thermal conductance for edge case was found to be due to efficient coupling between phonons of polymer and in-plane phonons of graphene for edge-bonding.
We studied the combined effect of above two advantages – coupling of all nanoplatelet graphene sheets with the surrounding polymer matrix and nearly 2-fold higher interface thermal conductance- in enhancing overall composite thermal conductivity by performing comprehensive molecular dynamics simulations of a system of polyethylene matrix embedded with an edge or basal-plane bonded nanoplatelet. Videos below demonstrate thermal transport through the composite for the two cases.
For the edge-case, the temperature profile clearly shows thermal mixing near the edges of the nanoplatelet – heating up the region near the cold boundary and cooling the region near the hot boundary. This thermal mixing is a direct consequence of efficient coupling of edge-bonded nanoplatelet with the surrounding polymer matrix, enabling it to efficiently conduct heat from hot to cold end, leading to high heat fluxes. For basal-plane case, however, a large temperature jump is observed at the boundaries between polymer and graphene nanoplatelet (GnP), indicative of large interface thermal resistance between the two. This prevents the nanoplatelet to be effective in heat transfer, preventing large enhancement in composite thermal conductivity. Some modification of temperature profile is observed above and below the GnP, however the effect is minimal due to the participation of only the outermost layers of GnP in heat conduction. Above temperature profiles highlight overall advantage of edge-bonding in enhancing polymer-composite k.
Alignment of polymer chains and embedded graphene sheets through strain
Thermal conductivity along the polymer chain axis can be very high, even exceeding that of metals. Random orientation of polymer chains, however, diminishes overall k of the polymer. Applying strain can align polymer chains enhancing k. We have performed molecular dynamics simulations of polyethylene being strained. Video below shows gradual alignment of polymer chains as strain is applied.
Strain can also align embedded filler material such as graphene nanoplatelets (GnPs). Alignment of planar surface of GnPs along the heat transfer direction takes full advantage of their ultra high in-plane k (>1000 W/mK). Simultaneous alignment of polymer chains and GnPs can lead to very high k polymer composites. Video below demonstrates alignment of embedded graphene sheet upon applying strain to the composite material.