Embedding of gold nanoclusters on polystyrene surfaces: Influence of the surface modification on the glass transition

摘要

The thermal behavior of surfaces of polystyrene (PS) films of 80-100 nm thickness decorated with gold nanoclusters with radii ranging from 9 to 18 Angstrom has been investigated by in situ X-ray reflectivity measurements. Recently, the embedding of tiny clusters-driven by the large difference between the surface energies of the metal and the polymer-has been proposed to serve as a nanoprobe of the glass transition temperature V at the polymer surface. In the present work for the first time a systematic study of the 9 embedding process with a variable amount of metal has been carried out. Determining the density profile by X-ray reflectivity measurements, we monitored simultaneously the onset of the embedding at a temperature T-e and the glass transition temperature T-g(f) of the whole film, which corresponds to the bulk 9 value Y-g(b). We show that at sufficiently low coverages the gold clusters begin to embed at temperatures T-e well below which T-g(f), reveals the presence of a surface layer with an increased mobility of the polymer chains. Furthermore, it turned out that there is a strong effect due to the attractive polymer-metal interactions which again suppresses the kinetics at the surface and thus leads to an increase of T-e with higher gold coverages. Therefore, the embedding temperature T-e can be regarded as an upper limit for the glass transition temperature T-g(s) of the free polymer surface. In the present study we found for 9 polystyrene with molecular weight M-w = 220 kg/mol (M-w = 3.7 kg/mol) a reduction for the surface glass transition of more than 37 K (22 K) with respect to its bulk value. [References: 54]