2134/38316
Andrew B. Robbins
Andrew B.
Robbins
Stavros Drakopoulos
Stavros
Drakopoulos
Ignacio Martin-Fabiani
Ignacio
Martin-Fabiani
Sara Ronca
Sara
Ronca
Austin J. Minnich
Austin J.
Minnich
Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene
Loughborough University
2019
Polymers
Polyethylene
Thermal conductivity
Mean free path
Ballistic transport
Materials Engineering not elsewhere classified
2019-07-11 15:36:31
Journal contribution
https://repository.lboro.ac.uk/articles/journal_contribution/Ballistic_thermal_phonons_traversing_nanocrystalline_domains_in_oriented_polyethylene/9236714
Thermally conductive polymer crystals are of both fundamental and practical interest for their high thermal conductivity that exceeds that of many metals. In particular, polyethylene fibers and oriented films
with uniaxial thermal conductivity exceeding 50 Wm−1K−1 have been reported recently, stimulating interest into the underlying microscopic thermal transport processes. While ab-initio calculations have provided insight into microscopic phonon properties for perfect crystals, such properties of actual samples have remained experimentally inaccessible. Here, we report the direct observation of thermal phonons with mean free paths up to 200 nm in semicrystalline polyethylene films using transient grating spectroscopy. Many of the mean free paths substantially exceed the crystalline
domain sizes measured using small-angle x-ray scattering, indicating that thermal phonons propagate ballistically within and across the nano-crystalline domains, with those transmitting across domain boundaries contributing nearly a third of the thermal conductivity. Our work provides the first direct determination of thermal phonon propagation lengths in molecular solids, providing insights into the microscopic origins of their high thermal conductivity.