Sphere-to-cylinder transitions in thin films of diblock copolymers under shear: The role of wetting layers

Shear-induced sphere-to-cylinder transitions in diblock copolymer thin films have been studied using coarse-grained Langevin dynamics simulations. Parameters of the coarse-grained model were chosen to represent a polystyrene-polyisoprene copolymer with molecular weights of the blocks equal to 68 and 12 kg/mol, respectively, matching the system studied experimentally by Hong et al. [Soft Matter2009, 5, 1687]. At zero-shear conditions and below the order-disorder transition temperature, thin films form a monolayer or bilayer of spheres. The minority block has higher affinity for the confining surfaces, thus forming wetting layers whose chains interpenetrate those forming the microdomain layer(s). Once a shear field is applied and above a critical shear rate, the spheres elongate and merge with their neighbors to form cylinders. We find that shear-induced cylinder formation is closely related to stretching of individual diblock chains. Our simulations suggest that a higher stress is required to achieve the sphere-to-cylinder transition in monolayer versus bilayer thin films because the wetting layers transfer momentum into the film by stretching the chains, which in turn causes higher shear stress for a given surface velocity. This observation is in agreement with experimental findings. In addition to the effects of shear, the impact of temperature was investigated with respect to chain stretching and the formation of cylinders under shear.