An Investigation into Modeling Heat Transfer in Polyethylene Molecular Crystal Systems with Molecular Dynamics

Abstract

Polymer materials can be an effective resource for thermal management applications. Polymers are generally thermal insulators in their bulk form (amorphous or partially crystalline), but they can have a large range of thermal conductivities depending on their structure and crystallinity. For example, research into the thermal conductivity of polymer fibers revealed surprisingly high thermal conductivities. Investigations into the thermal properties of polymers have given various results and an exact understanding of thermal transport in polymers is still lacking. Atomistic scale studies are useful in understanding the properties and mechanisms of nanoscale materials. Molecular dynamics (MD) techniques to calculate the thermal conductivity of materials are divided into two categories. The first being nonequilibrium molecular dynamics (NEMD), and the second being equilibrium molecular dynamics (EMD) using the Green-Kubo formalism. Despite the array of tools available for understanding thermal properties of polymers, a review of published work into the simplest of polymers, polyethylene (PE), results in a wide range of predicted properties. Part of the discrepancy may be accounted for by differing simulation methods and parameters between studies. Further obscuring results is the choice of atomistic force fields used in molecular dynamics. Some studies favor reactive potentials such as AIREBO, while others use potentials such as DREIDING or COMPASS.

In this study, an analysis of atomistic potentials comparing phonon spectra with density functional theory phonon spectra and a comparison of thermal conductivity values are used to judge the effectiveness of these potentials in modeling thermal transport. Next, NEMD and EMD methods of calculating thermal transport are investigated and compared. Lastly, a study of the effect of sampling parameters used in the Green-Kubo method is made. Using this data, recommended guidelines are presented.