  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.
