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Issue 7, 2012
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Comparing surface and bulk flow of a molecular glass former

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In this work we measure the response of the molecular glass former 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (TNB Tg = 347 K) to the presence of 20 nm gold nanoparticles placed on the material surface. At times ranging from a few minutes to many hundreds of minutes at temperatures below Tg − 2 K the surface evolves with no change in the apparent height of the nanoparticle. At temperatures Tg − 9 K < T < Tg, and after sufficiently long times, the nanospheres are observed to embed into the material. We employ a simple model for embedding in order to estimate a bulk material viscosity (the material properties ∼10–20 nm into the film) and obtain good agreement with previously reported values over the temperature range 338–345 K. The surface evolution that is observed prior to nanoparticle embedding has a much weaker temperature dependence than the embedding process. The surface evolution is modelled as a thin film with uniformly enhanced mobility, and alternately as surface diffusion. In the context of a decreased viscosity in the entire film, the measured time scales correspond to a viscosity value of 107–1010 Pa·s. Restricting the surface flow to a smaller layer results in correspondingly decreased viscosity values. In the context of a surface diffusion model, the timescale for surface evolution corresponds to a range of surface diffusion coefficients of Ds from 10−14 (at 318 K) to 10−11 m2/s (at 345 K). By measuring both surface and bulk dynamics we provide a quantitative measure for the enhancement of surface dynamics relative to the bulk.

Graphical abstract: Comparing surface and bulk flow of a molecular glass former

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Publication details

The article was received on 25 Sep 2011, accepted on 23 Nov 2011 and first published on 06 Jan 2012

Article type: Paper
DOI: 10.1039/C2SM06826E
Soft Matter, 2012,8, 2206-2212

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    Comparing surface and bulk flow of a molecular glass former

    C. R. Daley, Z. Fakhraai, M. D. Ediger and J. A. Forrest, Soft Matter, 2012, 8, 2206
    DOI: 10.1039/C2SM06826E

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