Thermal phonons with micron-scale mean free paths in ultra-drawn polyethylene

Heat conduction in highly oriented polymers is of fundamental and practical interest. It is well-known that the thermal conductivity of certain polymers such as polyethylene (PE) increases by orders of magnitude with drawing, but the microscopic properties of phonons responsible for heat conduction have remained difficult to access experimentally. Here, we report the observation of thermal phonons with micron-scale mean free paths (MFPs) in ultradrawn polyethylene using transient grating spectroscopy. The MFPs are comparable to those in covalent single crystals such as Si despite the imperfect nature of the sample’s microstructure. Further, the sample exhibits a decrease in thermal conductivity with increasing temperature above 200 K, indicating that anharmonic scattering is dominant over reflections from domains between crystallites. Our work provides new insights into the microscopic origin of high uniaxial thermal conductivity of ultradrawn polymers.