Anomalous Temperature-Dependent Thermal Transport in Crystalline Polyethylene Driven by Strong Anharmonicity

Thermal conductivity typically decreases with increasing temperature along the three principal crystalline directions, primarily due to enhanced phonon anharmonicity. In this work, we conducted a comprehensive first-principles investigation of thermal transport in crystalline polyethylene by solving the Wigner transport equation, assisted with the stochastic self-consistent harmonic approximation. It is found that the thermal conductivity of crystalline polyethylene decreases along the chain direction, but increases nearly linearly in the out-of-chain directions. This anomalous contrasting behavior stems from the dominance of particle-like transport along the chain and wave-like transport in the out-of-chain directions. The strong anharmonicity facilitates phonon tunneling between high- and low-frequency modes in the out-of-plane directions. Therefore, further enhancement of thermal conductivity in those directions could benefit from increased anharmonicity and the introduction of additional disorder. These findings provide fundamental insights into the thermal transport mechanisms of anisotropic crystalline polymers, offering valuable guidance for rationally engineering their thermal properties.