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 silicon (Si) substrates. Indents with depths of penetration higher than the film thicknesses were imposed. Upon heating above the bulk glass transition temperature (T-g) of PS, some of the indents healed, resulting in a flat polymer surface, and some grew laterally and dewetted the substrate. Our goal is to establish a criterion that can predict the final result (healing or growing) of the indent evolution. We find that the applied force during the nanoindentation and the substrate area exposed to the air after the imposition 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) > DeltaF(gamma ,crit), and it heals when DeltaF(gamma) < <Delta>F-gamma ,F-crit.