Nanomosaic surfaces by lateral phase separation of a diblock copolymer

Ultrathin films of a symmetrical polystyrene-block-poly(2-vinylpyridine) diblock copolymer with a degree of polymerization of 600 were prepared by spin coating from a nonselective solvent, i.e., CHCl3 on mica. The concentration of the casting solutions was chosen to be too low to allow homogeneous coverage of the substrate without significant stretching of the macromolecules. Scanning force microscopy demonstrated the formation of uniform polystyrene clusters with a height of about 5 nm and a distance of about 100 nm between them. In between the substrate was covered by approximate to 1 nm thick film of poly(2-vinylpyridine). The large periodicity of the two-dimensional phase pattern is explained by the strong interaction of poly(2-vinylpyridine) with mica, favoring a highly stretched conformation. X-ray photoelectron spectroscopy showed that about every third poly(2-vinylpyridine) unit was in chemical contact with the substrate. When the film thickness was increased, the X-ray photoelectron spectroscopy N(1s) signal decreased in comparison to the C(1s) signal indicating that the adsorbed poly(2-vinylpyridine) got increasingly covered by polystyrene. An estimation of the free energy indicates that the lateral phase separation is mainly controlled by the length of the polystyrene blocks affecting the incompatibility of the polystyrene and the poly(2-vinylpyridine) covered mice.