Hydrodynamic phonon transport in bulk crystalline polymers

Hydrodynamic phonon transport in solids exhibits unique thermal transport behaviors, such as second sound, the Poiseuille flow, and ultrahigh thermal conductivity. However, those have been limited up to the cryogenic temperature (∼1 K) for a few materials. In this work, by employing the phonon Boltzmann transport equation, we demonstrate hydrodynamic phonon transport in organic systems such as bulk crystalline polymers. Remarkable hydrodynamic phonon transport up to 50 K is demonstrated for both polyacene (-C4H2-)n and polyacetylene (-C2H2-)n crystals. More interestingly, a weak phonon hydrodynamic behavior takes place in crystalline polyethylene (-C2H4-)n in the intermediate-temperature range around 120 K, different from the observed hydrodynamics in most systems. The spectral phonon analysis reveals that the torsional motion of the A2 mode causes this unique hydrodynamic behavior. A modified criterion for the emergence of hydrodynamic phonon transport is proposed which agrees quantitatively with the results from thermal conductivity and phonon drifting component calculations. This study provides physical insights into the understanding of phonon hydrodynamics in organic materials and also a reliable criterion to probe the hydrodynamic phonon transport in complex systems.