Evidence of a Transition Layer between the Free Surface and the Bulk

Abstract
The free surface, a very thin layer at the interface between polymer and air, is considered the main source of the perturbations in the properties of ultrathin polymer films, i.e., nanoconfinement effects. The structural relaxation of such a layer is decoupled from the molecular dynamics of the bulk. The free surface is, in fact, able to stay liquid even below the temperature where the polymer resides in the glassy state. Importantly, this surface layer is expected to have a very sharp interface with the underlying bulk. Here, by analyzing the penetration of n-hexane into polystyrene films, we report on the existence of a transition region, not observed by previous investigations, extending for 12 nm below the free surface. The presence of such a layer permits reconciling the behavior of interfacial layers with current models and has profound implications on the performance of ultrathin membranes. We show that the expected increase in the flux of the permeating species is actually overruled by nanoconfinement.

Citation
Ogieglo W, Tempelman K, Napolitano S, Benes NE (2018) Evidence of a Transition Layer between the Free Surface and the Bulk. The Journal of Physical Chemistry Letters 9: 1195–1199. Available: http://dx.doi.org/10.1021/acs.jpclett.8b00076.

Acknowledgements
The authors acknowledge financial support of the Dutch Institute for Sustainable Process Design (ISPT). S.N. acknowledges financial support from the Fonds de la Recherche Scientifique (FNRS) under Grant No. T.0147.16 “TIACIC”.

Publisher
American Chemical Society (ACS)

Journal
The Journal of Physical Chemistry Letters

DOI
10.1021/acs.jpclett.8b00076

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