Interference microscopy on thin diblock copolymer films

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Abstract
At equilibrium thin films of symmetric diblock copolymers exhibit a morphology where the lamellar microdomains are oriented parallel to the external surfaces. Interference microscopy has been used to investigate this orientation which results in a quantization of the film thickness. Solution cast films of the symmetric polystyrene/polymethylmethacrylate P(S-b-MMA) diblock copolymers exhibit a single interference color characteristic of a film of uniform thickness with a free surface that is flat and smooth. Interference microscopy on these films annealed at 170°C for 24 hours and more shows the development of two distinct interference colors. The variations in the colors are discrete rather than continuous indicating step discontinuities in the film thickness with a height equal to one period of the underlying oriented lamellar multilayer. The free surface topology is such there are patches of one color spread across the surface on a uniform background, i.e., there are islands or depressions on the surface. The average size and spatial distribution of these islands or depressions are found to vary with annealing time ; the appearance of the free surface is reminiscent of a two-dimensional phase coarsening process. The time dependent change in the case of depressions is reported herein