Early dewetting stages of thin polymer films initiated by nanoindentation

We study the effect of nanoindentation induced defects on thin (h = 50 nm) and thick (h = 100 nm) polystyrene (PS) films, spin cast on nonwettable sil icon (Si) substrates. Indents with depths of penetration higher than the fi lm thicknesses were imposed. Upon heating above the bulk glass transition t emperature (T-g) of PS, some of the indents healed, resulting in a flat pol ymer surface, and some grew laterally and dewetted the substrate. Our goal is to establish a criterion that can predict the final result (healing or g rowing) of the indent evolution. We find that the applied force during the nanoindentation and the substrate area exposed to the air after the imposit ion of an indent can only be used as crude rules, since they do not provide a "cutoff' border, that distinguishes the growing from the healing indents . On the contrary the excess surface energy (DeltaF(gamma)) of the system, that is added to the initially flat coating with the formation of an indent , provides a clear critical value, DeltaF(gamma ,crit), which separates the growing from the healing indents. An indent grows when DeltaF(gamma) > Del taF(gamma ,crit), and it heals when DeltaF(gamma) < <Delta>F-gamma ,F-crit.