(102a) Layering Transitions In Thin Films Of Spherical-Domain Block Copolymers

The packing of spherical block copolymer domains confined to a thin film is investigated with grazing-incidence small-angle x-ray scattering (GISAXS) and transmission electron microscopy. We find a complex transition from hexagonal symmetry in 2D to body-centered cubic packing in 3D as a function of film thickness. The monolayer hexagonal symmetry persists through three layers, with close-packed AB and ABA stacking in films two and three layers thick. At four layers, coexistence between an equilibrium hexagonal close-packed symmetry and a metastable face-centered orthorhombic structure is observed. As the number of layers is further increased from four to 23, the hexagonal phase vanishes, and the symmetry of the face-centered orthorhombic phase deforms to approach asymptotically a structure similar to a body-centered cubic lattice with the closest-packed (110) plane oriented parallel to the substrate. Our approach to analysis of GISAXS data will be described.