Dynamics of Polymer/Metal Nanocomposite Films at Short Times As Studied by X-ray Standing Waves

X-ray standing waves generated by total external reflection from a silver mirror were used to monitor the mobility of metal particles in ultrathin polymer matrices with subnanometer spatial resolution in the direction perpendicular to the film surface. The particular model system consisted of gold nanoparticles deposited onto poly(tert-butyl acrylate) (PtBA) by thermal evaporation. Particles remained at the free surface of a PtBA film during annealing treatments above the polymer glass transition temperature but were able to diffuse when a second polymer layer was added to create sandwich samples with a buried gold layer. In these sandwich samples the gold distribution broadened with time and moved toward the polymer layer with the highest mobility. The polymer mobility is affected by the molecular weight of the polymer itself and by the substrate with which the layer is in contact. Values of the gold particle diffusion coefficients obtained from the broadening of the distribution are coupled to the terminal relaxation time of the polymer. An effective step size, defined as the average distance a gold particle moves within the relaxation time of the polymer, is equal to a few percent of the tube diameter.