Direct imaging of nanoparticle embedding to probe viscoelasticity of polymer surfaces.

Atomic force microscopy was used to study the embedding of gold nanoparticles into the surface of polystyrene films. The rate of embedding was determined at temperatures near the bulk glass transition temperature T(g) by measuring the apparent nanosphere height as a function of annealing time. In particular, relative height measurements of the adhered particles were made at temperatures below the bulk T(g) value. In the absence of enhanced surface dynamics or yield processes no embedding is expected to occur for T<T(g). Measurements on 10 and 20 nm particles both indicated that the particles did embed 3-4 nm into the polymer for T<T(g). Both the extent and time frame for engulfment appear to be independent of the particle diameter. The results suggest a more mobile surface region on the order of 3-4 nm thick, with a lower glass transition temperature than the bulk.