Free Surface Relaxations of Star-Shaped Polymer Films

Emmanouil Glynos, Kyle J. Johnson, Bradley Frieberg, Alexandros Chremos, Suresh Narayanan, Georgios Sakellariou, and Peter F. Green

Phys. Rev. Lett. 119, 227801 – Published 28 November 2017

Abstract

The surface relaxation dynamics of supported star-shaped polymer thin films are shown to be slower than the bulk, persisting up to temperatures at least 50 K above the bulk glass transition temperature $T_{g}^{bulk}$ . This behavior, exhibited by star-shaped polystyrenes with functionality $f = 8$ arms and molecular weights per arm $M_{arm} < M_{e}$ ( $M_{e}$ is the entanglement molecular weight), is shown by molecular dynamics simulations to be associated with a preferential localization of these macromolecules at the free surface. This new phenomenon is in notable contrast to that of linear-chain polymer thin film systems, where the surface relaxations are enhanced in relation to the bulk; this enhancement persists only for a limited temperature range above the bulk $T_{g}^{bulk}$ . Evidence of the slow surface dynamics, compared to the bulk, for temperatures well above $T_{g}$ and at length and time scales not associated with the glass transition has not previously been reported for polymers.

Received 16 April 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.227801

Physics Subject Headings (PhySH)

Transport phenomena

Polymers Thin films

Polymers & Soft Matter

Authors & Affiliations

Emmanouil Glynos^1,*, Kyle J. Johnson^2,3, Bradley Frieberg^3,4,†, Alexandros Chremos⁵, Suresh Narayanan⁶, Georgios Sakellariou⁷, and Peter F. Green^2,3,4,8,‡

¹Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, P.O. Box 1385, 711 10 Heraklion, Crete, Greece
²Department of Material Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
³Biointeraces Institute, University of Michigan, Ann Arbor, Michigan 48109, USA
⁴Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
⁵Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
⁶X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
⁷Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografou, 15771 Athens Greece
⁸National Renewable Energy Laboratory (NREL), Golden, Colorado 80401, USA

^*Corresponding author. eglynos@iesl.forth.gr
^†Present address: Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
^‡Corresponding author. pfgreen@umich.edu; peter.green@NREL.gov