Comparing surface and bulk flow of a molecular glass former

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.