Lattice Transition of Sphere-forming Block Copolymer Thin Films under Solvent Vapor Annealing

Block copolymer (BCP) can self-assemble into various nanostructures depending on the relative ratio of each block length. Especially, sphere-forming BCPs can exhibit various interesting structures beyond the classical lattice structure, such as the Frank-Kasper phase and Laves phase.

In this study, we report the lattice transition behavior of the sphere-forming BCP thin films under the solvent vapor annealing (SVA) process. Generally, sphere-forming BCP thin films have a hexagonally close-packed (HEX) symmetry. However, we found that the HEX symmetry developed at low solvent fraction transforms into face-centered orthorhombic (FCO) symmetry with increasing the solvent fraction. This structural transformation can be observed consistently in the thickness of 2-8 layers. We found that FCO symmetry can be a stable structure in the solvent swollen state and can be trapped with rapid solvent removal. Furthermore, we show that the transition path is varied with the solvent quality and discuss the overall lattice transitions that the sphere-forming BCP can experience from the interface to the bulk.