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

Abstract

During spin coating of very thin films from a solution of incompatible polymers quite interesting lateral structures are forming. From scanning force microscopy (SFM) and X-ray reflectometry it is concluded that a surface height modulation is present, which reflects at the surface the phase separated morphology of the blend in the film. Those structures depend critically on different parameters like relative concentration of the components, spinning parameters, solvent quality or compatibility of the blend. In blends of different statistical copolymers of poly-styrene-stat-para-bromo-styrene the compatibility between the blended polymers depends on the difference in the degree of bromination. This allows a variation of the interaction parameter within a wide range. Besides relatively incompatible materials, also weakly incompatible mixtures are of interest. Such a system with an almost vanishing but still positive interaction parameter is realized with blends of 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 homogeneous solution. The lateral structures created during the quench are examined with SFM. The SFM pictures are statistically analysed in terms of their Fourier components. From the power spectral density function the most prominent in-plane length scale is extracted. This analysis of a mean distance is complemented by the use of Minkowski measures.