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Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene

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posted on 11.07.2019 by Andrew B. Robbins, Stavros Drakopoulos, Ignacio Martin-Fabiani, Sara Ronca, Austin J. Minnich
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.

Funding

ONR Young Investigator Award under Grant No. N00014-15-1-2688 and EPSRC, grant EP/K034405/1.

History

School

  • Aeronautical, Automotive, Chemical and Materials Engineering

Department

  • Materials

Published in

Proceedings of the National Academy of Sciences

Volume

116

Issue

35

Pages

17163-17168

Citation

ROBBINS, A.B. ... et al, 2019. Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene. Proceedings of the National Academy of Sciences, 116(35), pp. 17163-17168.

Publisher

National Academy of Sciences

Version

AM (Accepted Manuscript)

Publisher statement

This paper was accepted for publication in the journal Proceedings of the National Academy of Sciences and the definitive published version is available at https://doi.org/10.1073/pnas.1905492116

Acceptance date

10/07/2019

Publication date

2019-08-12

Copyright date

2019

ISSN

0027-8424

eISSN

1091-6490

Language

en

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