Oberflächenempfindliche Röntgenstreuung an Polymer-Metall-Grenzflächen : Strukturelle Eigenschaften und thermisches Verhalten
Polymer-metal nanocluster interfaces have been examined by means of surface sensitive x-ray scattering techniques to study the thermal behavior of polymer films in the bulk and at the surface. The evaporation of gold atoms onto thin polystyrene layers of thickness 10-100 nm on a Si/SiOx substrate leads to the formation of nanoclusters at the polymer surface. Samples with polymer molecular weight Mw ranging between 3.7 k and 226 k have been examined by specular and diffuse x-ray scattering methods using Synchrotron radiation. At sufficiently low viscosities an embedding process of the clusters into the polymer can be observed, driven by the lowering of the surface energy. On annealing the samples, the onset of this process at the embedding temperature, Te, and the glass transition temperature of the entire film, Tg, can be simultaneously monitored. Normally cluster embedding occurs only at temperatures T>Tg, however, here the gold clusters begin to embed at temperatures Te well below Tg. This indicates an increased mobility of the polymer chains at the surface. Additionally, the amount of gold dispersed onto the polymer surface has a strong influence on the embedding temperature. High gold coverage resulted in a pronounced increase of Te which could be even higher than Tg. This effect is due to the attractive interaction between the metal and the polystyrene, which again suppresses the increased chain mobility.