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Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene

Robbins, Andrew B. and Drakopoulos, Stavros X. and Martin-Fabiani, Ignacio and Ronca, Sara and Minnich, Austin J. (2019) Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene. Proceedings of the National Academy of Sciences of the United States of America, 116 (35). pp. 17163-17168. ISSN 0027-8424. PMCID PMC6717268. doi:10.1073/pnas.1905492116.

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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 W⋅m^(-1)⋅K^(-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 nanocrystalline domains; those transmitting across domain boundaries contribute nearly one-third of the thermal conductivity. Our work provides a direct determination of thermal phonon propagation lengths in molecular solids, yielding insights into the microscopic origins of their high thermal conductivity.

Item Type:Article
Related URLs:
URLURL TypeDescription Information CentralArticle
Robbins, Andrew B.0000-0002-8328-1762
Drakopoulos, Stavros X.0000-0002-6798-0790
Martin-Fabiani, Ignacio0000-0002-1977-7659
Ronca, Sara0000-0003-3434-6352
Minnich, Austin J.0000-0002-9671-9540
Additional Information:© 2019 National Academy of Sciences. Published under the PNAS license. Edited by Timothy M. Swager, Massachusetts Institute of Technology, Cambridge, MA, and approved July 10, 2019 (received for review April 6, 2019). PNAS first published August 12, 2019. A.B.R. and A.J.M. were supported by an Office of Naval Research Young Investigator Award under Grant N00014-15-1-2688. S.R. acknowledges funding from Engineering and Physical Sciences Research Council, Grant EP/K034405/1. We thank Dr. Steven Huband and Dr. Tara Schiller at the University of Warwick X-ray facility for assistance during the SAXS/WAXS measurements and analysis; and Dr. Georgios C. Psarras from the Department of Materials Science, University of Patras, for providing the ZnO nanoparticles. We thank Dr. Giuseppe Forte for providing disentangled UHMWPE. Author contributions: A.B.R., S.R., and A.J.M. designed research; A.B.R., S.X.D., and I.M.-F. performed research; A.B.R., S.X.D., and I.M.-F. analyzed data; and A.B.R. and A.J.M. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. Data deposition: Wide-angle and small-angle X-ray scattering data have been deposited at the Loughborough University Repository ( and, respectively). The raw transient grating data from this article have been deposited in CaltechDATA ( This article contains supporting information online at
Funding AgencyGrant Number
Office of Naval Research (ONR)N00014-15-1-2688
Engineering and Physical Sciences Research Council (EPSRC)EP/K034405/1
Subject Keywords:polymers; polyethylene; thermal conductivity; mean free path; ballistic transport
Issue or Number:35
PubMed Central ID:PMC6717268
Record Number:CaltechAUTHORS:20190812-154046597
Persistent URL:
Official Citation:Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene. Andrew B. Robbins, Stavros X. Drakopoulos, Ignacio Martin-Fabiani, Sara Ronca, Austin J. Minnich. Proceedings of the National Academy of Sciences Aug 2019, 116 (35) 17163-17168; DOI: 10.1073/pnas.1905492116
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:97801
Deposited By: Tony Diaz
Deposited On:12 Aug 2019 22:57
Last Modified:16 Feb 2022 00:51

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