Orientational order in sphere-forming block copolymer thin films aligned under shear

Shear can impart a high degree of orientational order to block copolymer thin films containing two or more layers of spherical domains, but the orientational order appears to plateau at a limited level with increasing shear stress. At high stresses, the only defects which remain in the film are isolated dislocations, and the orientational order in the film is thus uniquely and inversely correlated with the dislocation density. These dislocations preferentially orient normal to the shear direction, which facilitates the sliding of layers of spheres relative to each other during shearing. A simple elastic continuum model provides a good quantitative description of the impact of isolated dislocations on the films' orientational order. At low dislocation densities, the apparent orientational order is limited by uncertainties in locating the positions of the spheres by atomic force microscopy, an effect which is quantitatively captured in this work.