Direct Measurements of Heterogeneous Viscosity Distributions in Ultrathin Polymer Films

We will present direct measurements of depth dependence of the polymer chain dynamics in single ultrathin films via the diffusive motion of gold nanoparticles dispersed in the films. The technique used was x-ray photon correlation spectroscopy (XPCS) which serves a fingerprint of the Brownian motion of the gold nanoparticles. By intensifying the probing electrical field in the region of interest, we could measure the viscosity at the surface and at the position close to the center of the film separately. The XPCS results showed that most chains had at least some direct contacts with an immobile layer (absorbed with a substrate) up to 3$R_{g}$ thick from the substrate, where R$_{g}$ is the radius of polymer gyration, and hence the lateral dynamics of the polymer chains was significantly reduced.