We propose a theoretical explanation of the parallel and perpendicular lamellar orientations in free surface films of symmetric polystyrene-block-polybutadiene diblock copolymers on silicon substrates (with a native SiOx layer). Two approaches are developed: A correction to the strong segregation theory and a qualitative analysis of the intermediate segregation regime. We show that the perpendicular orientation of the lamellae formed by the molecules of high molecular weight is stabilized by A-B interfacial interactions. They are weaker in the case of the perpendicular orientation of the lamellae, whereas the surface tension coefficient of the A-B interface decreases with the increase of the molecular weight.
Potemkin, I. I., Busch, P., Smilgies, D-M., Posselt, D., & Papadakis, C. M. (2007). Effect of the Molecular Weight of AB Diblock Copolymers on the Lamellar Orientation in Thin Films: Theory and Experiment. Macromolecular Rapid Communications, 28(5), 579-584. https://doi.org/10.1002/marc.200600764