Complex pattern formation by phase separation of polymer blends in thin films

During spin coating of very thin films from a solution of incompatible poly mers quite interesting lateral structures are forming. From scanning force microscopy (SFM) and X-ray reflectometry it is concluded that a surface hei ght modulation is present, which reflects at the surface the phase separate d morphology of the blend in the film. Those structures depend critically o n different parameters like relative concentration of the components, spinn ing parameters, solvent quality or compatibility of the blend. In blends of different statistical copolymers of poly-styrene-stat-para-bromo-styrene t he compatibility between the blended polymers depends on the difference in the degree of bromination. This allows a variation of the interaction param eter within a wide range. Besides relatively incompatible materials, also w eakly incompatible mixtures are of interest. Such a system with an almost v anishing but still positive interaction parameter is realized with blends o f poly-styrene and poly-para-methyl-styrene. Ultra thin polymer films with thicknesses between 2 to 4 R-G were prepared by solvent quenches from homog eneous solution. The lateral structures created during the quench are exami ned with SFM. The SFM pictures are statistically analysed in terms of their Fourier components. From the power spectral density function the most prom inent in-plane length scale is extracted. This analysis of a mean distance is complemented by the use of Minkowski measures.