Decoupling of surface diffusion and relaxation dynamics of molecular glasses

Yue Zhang and Zahra Fakhraai
PNAS published ahead of print April 3, 2017 https://doi.org/10.1073/pnas.1701400114

  1. Edited by Pablo G. Debenedetti, Princeton University, Princeton, NJ, and approved March 13, 2017 (received for review January 25, 2017)

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Significance

Diffusion on the surfaces of molecular glasses is observed to be greatly enhanced compared with the bulk diffusion with lower activation energies. However, the physical nature of the fast surface diffusion and its relation to the glassy dynamics remain unclear. Relaxation dynamics of these glasses can be enhanced by up to 14 orders of magnitude by reducing their film thickness below 50 nm. By investigating the relationship between surface diffusion and relaxation times of ultrathin films, we find that the fast surface diffusion remains invariant of the films’ relaxation dynamics even when the activation energy of the film becomes lower than the activation energy for the surface diffusion, indicating a complete decoupling of the relaxation dynamics and surface diffusion.

Abstract

Tobacco mosaic virus is used as a probe to measure surface diffusion of ultrathin films of N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) (12 nm <h< 53 nm, where h is the film thickness) at various temperatures below the glass transition temperature, 𝐓𝐠, of all films. As the film thickness is decreased, 𝐓𝐠 decreases rapidly and the average film dynamics are enhanced by 6–14 orders of magnitude. We show that the surface diffusion is invariant of the film thickness decrease and the resulting enhanced overall mobility. The values of the surface diffusion coefficient and its temperature dependence are invariant of film thickness and are the same as the corresponding bulk values (h=400 nm). For the thinnest films (h<20 nm), the effective activation energy for rearrangement (temperature dependence of relaxation times) becomes smaller than the activation energy for surface diffusion. These results suggest that the fast surface diffusion is decoupled from film relaxation dynamics and is a solely free surface property.

Footnotes

  • ↵1To whom correspondence should be addressed. Email: fakhraai{at}sas.upenn.edu.

  • Author contributions: Y.Z. and Z.F. designed research; Y.Z. performed research; Y.Z. and Z.F. analyzed data; and Y.Z. and Z.F. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1701400114/-/DCSupplemental.

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Surface diffusion decoupled from bulk relaxation
Yue Zhang, Zahra Fakhraai
Proceedings of the National Academy of Sciences Apr 2017, 201701400; DOI: 10.1073/pnas.1701400114
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