Lamellar diblock copolymer thin films investigated by tapping mode atomic force microscopy:: Molar-mass dependence of surface ordering

Using tapping mode atomic force microscopy, we studied the influence of molar mass (13.9-183 kg/mol) on the lamellar orientation at the surface of thin films of symmetric polystyrene-polybutadiene diblock copolymers prepared via spin-coating and by slow solvent-casting. The ratio between film thickness and lamellar thickness was varied between 0.5 and 10. Whereas for film thicknesses between 1 and 10 times the lamellar thickness, a lamellar orientation parallel to the film surface is preferred for low molar masses (below similar to55 kg/mol), high molar mass samples (above similar to90 kg/mol) rather form lamellae oriented perpendicular to the surface. For film thicknesses equal to the lamellar thickness, the films do not exhibit any texture, whereas for film thicknesses equal to half the lamellar thickness, a weak surface structure could be observed. This is consistent with symmetric wetting, i.e., the same block adsorbs at both film interfaces, but their selectivity is only weak. Thus, entropic contributions like chain stretching along a wall and the enrichment of free chain ends at the interface become important, and both parallel and perpendicular orientations of lamellae in thin films can occur.