An energetic criterion to compare the evolution of thermally-excited surface disturbances and nanoindentation-induced defects on thin polymer films

A criterion is developed to predict the resulting evolution process of the following surface defects on thin (17 nm) polystyrene (PS) films on silicon (Si): (i) nanoindentation-induced indents which grow after being heated above the glass transition temperature of PS, Tg, leading to dewetting; (ii) nanoindentation-induced indents which level at temperatures above the Tg, resulting in a flat polymer surface and (iii) indents which are formed and grow spontaneously by thermal treatment above the Tg (thermal film break up). The criterion is based on the concept of the excess surface energy, ΔFγ, which was introduced in previous reports for cases (i) and (ii). Here, a similar energetic term is used which corresponds only to the effect of the depressions, ΔFγ(D). The effect of the rims which surround the depressions in cases (i) and (ii) is not taken into account. Measurements of ΔFγ(D), performed by atomic force microscopy, prior to any treatment above the Tg suggest that growing depressions (cases i and iii) correspond to ΔFγ(D) > 1.5 × 10−16 J while for healing depressions (case ii) ΔFγ(D) < 1.8 × 10−16 J. A critical region of ΔFγ(D) exists from 1.5 × 10−16 J to 1.8 × 10−16 J. Depressions which correspond to this, rather short, region can either grow or heal.