Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene

Area of Science:

• Materials Science
• Polymer Science
• Condensed Matter Physics

Background:

• Thermally conductive polymer crystals exhibit high thermal conductivity, surpassing many metals.
• Recent studies show polyethylene fibers and films with high uniaxial thermal conductivity, prompting investigation into microscopic thermal transport.
• Previous ab initio calculations offered insights into perfect crystals, but experimental data for actual samples were lacking.

Purpose of the Study:

• To directly observe thermal phonons in semicrystalline polyethylene films.
• To determine the mean free paths of thermal phonons.
• To elucidate the microscopic origins of high thermal conductivity in polymers.

Main Methods:

• Transient grating spectroscopy was employed to directly observe thermal phonons.
• Small-angle X-ray scattering (SAXS) was used to measure crystalline domain sizes.
• Phonon mean free paths were correlated with crystalline domain sizes.

Main Results:

• Thermal phonons with mean free paths up to 200 nm were directly observed in semicrystalline polyethylene films.
• Observed phonon mean free paths exceeded crystalline domain sizes, indicating ballistic propagation.
• Phonon transmission across domain boundaries contributed approximately one-third to the total thermal conductivity.

Conclusions:

• This study provides the first direct measurement of thermal phonon propagation lengths in molecular solids.
• Ballistic phonon transport within and across nanocrystalline domains is crucial for high thermal conductivity in polyethylene.
• The findings offer critical insights into the microscopic mechanisms governing thermal transport in advanced polymer materials.