Observation of dynamical heterogeneities and their time evolution on the surface of an amorphous polymer

Although the formation of dynamic heterogeneities in glass-forming materials is believed to play an essential role in determining their properties as the glass transition is approached, direct imaging of these heterogeneities remains a challenge. Here, we report on a direct observation of nanoscale dynamic heterogeneities and their time evolution over similar to 10(3) s on the surface of a glassy polymer, polystyrene (PS), using atomic force microscopy with a 1 nm radius tip. The length scale of these heterogeneities was measured to be similar to 2.1 nm and the lifetime was determined to be similar to 10(2) s, in agreement with the length and time scales of heterogeneous dynamics reported for bulk polymers around the glass transition. These results are consistent with the existence of a very thin liquid-like layer at the glassy polymer surface. The validity of the method is confirmed by comparing the properties of surface dynamics of neat and plasticized films.