Shear Forces Produce Long-Range Alignment of Spherical Nanodomains in Block Copolymer Thin Films

Nanofabrication increasingly relies on self-assembled templates - which utilize the chemical properties of materials such as polymers - to generate dense, regular patterns. When properly designed, block copolymers can spontaneously form spheres of one block in a matrix of the other, with a characteristic size of tens of nanometers. In thin films, the spheres pack hexagonally but the "crystal" is riddled with grain boundaries, rather than the long-range order needed for device applications. We have found that shearing thin bilayer films of spherical nanodomains can completely eliminate these grain boundaries and impart long-range orientational order to the spheres. Molecular dynamics simulations reveal that the alignment results from the tendency of the spheres to slide along paths of minimum frictional resistance against spheres in the opposing layer. This mechanism produces an exponential growth in the orientational order, which can quickly extend over the entire specimen.