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 ∼103 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 ∼2.1 nm and the lifetime was determined to be ∼102 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.