Shear-induced alignment of lamellae in thin films of diblock copolymers

Abstract

The shear alignment of lamellae-forming diblock copolymers in thin films is examined using coarse-grained Langevin dynamics simulations. We investigate how lamellar orientation is affected by the shear rate, and interactions between polymer segments and the confining surfaces. For neutral confining surfaces, we find that above a critical stress, lamellae melt and reform in the direction of the shear (perpendicular to the walls), irrespective of their initial orientation. The time needed for reorientation was four times longer when the initial configuration was parallel to the walls, relative to lamellae initially oriented perpendicular to both the walls and the shear direction. When the surface–block interactions become non-neutral, there is critical interaction strength above which the perpendicular orientation is no longer favored, because of enthalpic contributions of one block preferentially wetting the non-neutral surface. The formation of parallel lamellae in films under shear requires a smaller degree of preferential interfacial interactions if both confining surfaces are non-neutral. The film thickness affects the rate of lamellar reorientation as well as the lamellar periodicity. Overall, our results are in good agreement with experimental findings and provide insights into how to use shear to control alignment of lamellar structures in thin films.