Crystalline Polymers with Exceptionally Low Thermal Conductivity Studied using Molecular Dynamics

TL;DR

This study uses molecular dynamics to reveal that crystalline polynorbornene exhibits extremely low thermal conductivity due to strong anharmonic scattering, with heat transported by diffusons rather than phonons, informing thermoelectric material design.

Contribution

It uncovers the intrinsic low thermal conductivity mechanism in crystalline polynorbornene caused by anharmonic scattering and diffuson-mediated heat transport.

Findings

Crystalline polynorbornene has near-amorphous thermal conductivity.

Strong anharmonic scattering prevents phonon propagation.

Heat is carried by diffusons instead of phonons.

Abstract

Semi-crystalline polymers have been shown to have greatly increased thermal conductivity compared to amorphous bulk polymers due to effective heat conduction along the covalent bonds of the backbone. However, the mechanisms governing the intrinsic thermal conductivity of polymers remain largely unexplored as thermal transport has been studied in relatively few polymers. Here, we use molecular dynamics simulations to study heat transport in polynorbornene, a polymer that can be synthesized in semi-crystalline form using solution processing. We find that even perfectly crystalline polynorbornene has an exceptionally low thermal conductivity near the amorphous limit due to extremely strong anharmonic scattering. Our calculations show that this scattering is sufficiently strong to prevent the formation of propagating phonons, with heat being instead carried by non-propagating, delocalized vibrational modes known as diffusons. Our results demonstrate a mechanism for achieving intrinsically low thermal conductivity even in crystalline polymers that may be useful for organic thermoelectrics.