Polymer film rupturing in comparison with leveling and dewetting

The break up of a thin polystyrene (PS) film applied on silicon (Si) upon heating above the bulk glass transition temperature (T_g) of the polymer is studied using atomic force microscopy (AFM). In a 17-nm thick PS film rupturing occurs with the formation of indent-like surface disturbances pointing toward the Si substrate and leads to the standard nucleated dewetting process. Indent growth toward the substrate is accompanied by an increase of the indent radius at the level of the free, unperturbed, PS surface and a decrease of the radius of curvature at the indent bottom. The two radii are comparable and are 1-3 orders of magnitude larger than the film thickness. The area induced in the system upon indent formation was measured for several indents and found to be almost negligible comparing to the total area of the system. Rupturing (growing indents) is discussed with respect to the opposing leveling (healing indents) process of artificially induced indents. Also, a comparison of the indent growth rate toward the substrate, calculated using previously published data, with the dewetting rate of an expanded dry patch along the substrate is provided, which suggests that the former is about 2-3 orders of magnitude lower than the latter.