How alignment controls heat transport in polymer chains with kinks?

Abstract

Thermal transport in long polymer molecules is commonly attributed to ballistic propagation of long-wavelength acoustic phonons, which act as Goldstone modes arising from translational symmetry, while the transport of other phonons is suppressed by Anderson localization. This mechanism leads to thermal conductivity that increases with molecular length. Consistent with this picture, strongly aligned polymers exhibit exceptionally high thermal conductivity, whereas poorly aligned polymers are orders of magnitude less conductive and function as thermal insulators. Here we show that this strong sensitivity to molecular alignment originates from phonon scattering by molecular kinks. Even in the long-wavelength limit, the kink scattering remains strong because kinks break translational symmetry both for longitudinal and transverse phonons. As a result, randomly oriented kinks cause a rapid decrease in thermal conductivity with increasing molecular length. These findings identify alignment control by means of kink engineering as a route for tuning thermal transport in polymers.