Square packing and structural arrangement of ABC triblock copolymer spheres in thin films

Nanoporous thin films were prepared from poly(ethylene oxide)-b-poly(methyl methacrylate)-b-polystyrene (PEO-b-PMMA-b-PS) ABC triblock copolymer by solvent annealing under high relative humidity followed by UV degradation and acid washing. Ordered half-spheres at the surface that template ordering of spheres below the surface in thin films were formed as a result of the interaction between the hydrophilic PEO segments and water vapor during processing. The spherical block copolymer domains exhibit complex packing behavior on the surface and in the interior. A half-sphere "monolayer" and a half-sphere plus whole sphere "bilayer" were formed in thin films with thicknesses of 43 and 71 nm, respectively, and have hexagonal lattice symmetry. For half-sphere plus two whole sphere "trilayers" with a nominal thickness of 117 nm, coexistence of regions of hexagonal and square packing was observed by transmission electron microscopy, scanning force microscopy, scanning electron microscopy, and grazing-incidence small-angle X-ray scattering. Square packing was consistent with a surface-truncated unit cell of a body-centered cubic lattice with the (100) plane parallel to the surface.