Thermal conductance of Teflon and Polyethylene: Insight from an atomistic, single-molecule level

Marius Buerkle,Yoshihiro Asai

doi:10.1038/srep41898

Marius Buerkle, Yoshihiro Asai

Open Access

https://doi.org/10.1038/srep41898

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Abstract

The thermal transport properties of teflon (polytetrafluoroethylene) and its polyethylene counterparts are, while highly desirable and widely used, only superficially understood. Here, we aim therefore to provide rigorous insight from an atomistic point of view in context of single-molecule devices. We show that for vinyl polymers adsorbed on metal-surfaces the thermal transport strongly depends on the properties of the metal-molecule interface and that the reduced thermal conductance observed for teflon derivatives originates in a reduced phonon injection life time. In asymmetric molecules phonon blocking on the intra molecular interface leads to a further reduction of thermal conductance. For hetrojunctions with different electrode materials we find that thermal conductance is suppressed due to a reduced overlap of the available phonon modes in the different electrodes. A detailed atomistic picture is thereby provided by studying the transport through perfluorooctane and octane on a single-molecule level using first principles transport calculations and nonequilibrium molecular dynamic simulations.

Highlights

Energy window available for phonon propagation, which in turn limits the phonon thermal conductance largely by filtering out the molecular vibrations at higher energies
For the modes above 30 meV the reduced injection life time confines the vibrations on the CF2 units giving rise to the very sharp resonances in the high energy end of the transmission which only give a negligible contribution to the overall thermal transport
While for low temperature the reverse nonequilibrium molecular dynamic simulations (RNEMD) approach is expected to fail due to missing quantum mechanical effects[9] we find that for higher temperatures (T > 200 K), where the thermal conductance tends to saturate towards the classical limit, the RNEMD results agree very well with the first principles calculations reproducing the absolute values as well as the weak temperature dependence of κ(T)

Summary

Introduction

Energy window available for phonon propagation, which in turn limits the phonon thermal conductance largely by filtering out the molecular vibrations at higher energies. The thermal conductance will be mainly determined by the interface resistance and the contribution of the intrinsic thermal resistance of the molecular bridge remains comparably small for the molecules studied here and in the limit of infinitely long chains both contribution are expected to be comparable. The thermal transport properties are thereby obtained by atomistic first principles calculations in combination with nonequilibrium Green’s functions and classical nonequilibrium molecular dynamics (NEMD) simulations using accurate reactive force fields

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Results

Conclusion