Thermal Conductivity of Polyethylene Chains Using Molecular Dynamics Simulations

Thermal Conductivity of Polyethylene Chains Using Molecular Dynamics Simulations

Asegun Henry and Gang Chen

Proceedings of 3rd Energy Nanotechnology International Conference
ENIC2008
August 10-14, 2008, Jacksonville, Florida USA

Purpose and Motivation

Polyethylene (and polymers in general) have low thermal conductivity. However, when mechanically stretched, polyethylene exhibits an order of magnitude increase in thermal conductivity. Since polyethylene is cheap, this motivates the study for its use in thermal applications.

The purpose of the study was to find the heat conductivity limit along a single chain of polyethylene.

Method

A modified molecular dynamics code, LAMMPS, was used to infer thermal conductivity. The polyethylene atoms were modeled as a chain with boundary conditions. The hydrogen and carbon atoms had their own degrees of freedom using the AIREBO potential.

The Green-Kubo relations were used to find thermal conductivity from the molecular trajectories.

Results

Infinite (theoretical) thermal conductivity was found for large systems. Faster divergence occurred with increased system size.

A finite thermal conductivity was calculated for small systems.

Conclusions

The resultant thermal conductivity figures arise from a physical aspect from the model (as opposed to numerical artifacts). In addition, the application of boundary conditions increases scattering of phonons; hence, smaller systems experienced slower divergence.

So, heat conduction in bulk polyethylene limited by disordered entanglement.

Notes:

LAMMPS: Large-scale Atomic/Molecular Massively Parallel Simulator developed by Sandia Labs.

AIREBO: Adaptive Intermolecular Reactive Empirical Bond Order potential is an empirical classical many-body potential.

Green-Kubo relations: give exact mathematical expression for transport coefficients in terms of integrals of time correlation functions.