Shear-induced sphere-to-cylinder transition in diblock copolymer thin films

Applying sufficiently strong shear to thin films of a sphere-forming polystyrene-polyisoprene diblock copolymer is shown to induce an order-order transition to cylinders. The transformation is not continuous or epitaxial, as the intercylinder spacing is ca. 10% greater than the spacing between close-packed lines of spheres. The transition is facilitated when the block copolymer has a composition which places it close to the "zero-field" (no shear) sphere/cylinder phase boundary; the shear-induced transformation is more difficult and less effective for a polymer further from this boundary. Applying a modest shear stress to a polymer close to the boundary distorts the hexagonal lattice formed by the spheres without forming cylinders; the mechanical anisotropy produced by this distortion is sufficient to permit a film containing only a single layer of spherical domains to align in shear.