Unstable polymer bilayers .1. Morphology of dewetting

We present a detailed study of the morphology of dewetting of a polycarbonate (PC) melt from a poly(styrene-co-acrylonitrile) (SAN) melt supported on a rigid substrate. PC and SAN, which are immiscible, were spin coated to form 200 nm films for these studies. Optical microscopy is used to monitor the dewetting process. Cross-sectional scanning electron microscopy, atomic force microscopy, and Auger electron spectroscopy are used to study the detailed morphology of an isolated hole having a diameter of similar to 30 mu m. The rim surrounding the hole in the PC layer is steeper on the inside than the outside (i.e., asymmetric) and its near-surface composition is characteristic of PC. The maximum height of this rim is more than twice that of the original bilayer thickness. The floor of the hole uncovered by the PC is a SAN layer which is thinner than the original SAN layer. This thinning of the underlying SAN layer is much greater than the equilibrium PC/SAN interfacial width, 5 nm. During the initial stages of dewetting, a depression or ''dimple'' in the SAN layer was observed in the center of each hole which was similar to 10 mu m wide and 50 nm deep. These dimples persist until the late stages of dewetting when the PC broke up into droplets on the SAN.