Comparing surface and bulk flow of a molecular glass former

Abstract

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.