Tapping-mode atomic force microscopy study of the near-surface composition of a styrene-butadiene-styrene triblock copolymer film

Thin films of a styrene-butadiene-styrene triblock copolymer were examined by tapping-mode atomic force microscopy al different levels of tip-sample forces. The height images recorded at minimal forces show the surface topography of the topmost butadiene-rich layer. As the level of the tip-sample force is increased, the height and phase images gradually undergo local changes and eventually reveal patterns of a microphase separation which forms underneath the topmost butadiene-rich layer. The contrast variations of the phase and height images are caused by differences in the interaction of the tip with styrene and butadiene blocks, and their qualitative trends are explained by taking into account the change of the effective resonance frequency of the interacting cantilever. The relative contrast of styrene and butadiene blocks depends sensitively on the driving frequency in height images, but not in phase images. (C) 1997 Elsevier Science B.V.