Shear-aligned block copolymer thin films as nanofabrication templates

Block copolymers spontaneously self-assemble into highly regular nanostructures; in bulk, these structures include spheres packed on a body-centered cubic lattice and cylinders arranged on a hexagonal lattice, where the diameter of the spheres or cylinders (typically 10-100 nm) is readily tunable through the polymer's molecular weight. In ultrathin films containing a monolayer of these nanostructures, similar morphologies are adopted: two-dimensional hexagonally-packed sphere arrays, and striped patterns formed by in-plane cylinders. By exploiting the chemical differences between the two blocks, such films can be used as masks, where the patterns formed by the spheres or cylinders are ultimately replicated in another material (perhaps inorganic), such as semiconductor dots (spheres) or metal wires (cylinders). But while self-assembly can produce highly-regular local structures, it does not provide any global order or orientation; for example, the coherence length for a block copolymer cylinder is typically less than a micron. This limits the application of the arrays formed by block copolymer templating. Recently, we have developed methods to shear-align the spheres or cylinders in block copolymer thin films, where the nanodomains are oriented in a common direction over centimeter lengths and square-centimeter areas. Shear-aligned films of cylinder-formers, when converted to arrays of metal nanowires, provide broadband optical polarizers which function down into the ultraviolet. By directing the path of a nonsolvent fluid flowing over the block copolymer film, we can shape the direction of the nanodomains, creating complex user-defined patterns of cylinder orientation on the millimeter length scale.