Revisiting Vertical Orientation in Block Copolymer Complexes through Controlled Solvent Vapor Annealing

Block copolymers (BCPs) are macromolecules made up of two or more chemically different polymer blocks that are covalently bound together. Their mutual incompatibility causes them to separate into nanoscale microphases, which allows the blocks to self-assemble into ordered shapes, such as lamellae, spheres, and cylinders. The ability to self-assemble offers a versatile route for forming well-ordered nanoscale architectures relevant to nanolithography, membranes, and functional materials. Incorporating hydrogen-bonding small molecules to form supramolecular BCP complexes expands structural diversity by altering domain size, interfacial energies, and chain mobility. These complexes expand the range of parameters for precise control over nanoscale morphology while also providing highly ordered nanostructures that can be used as soft templates for the formation of inorganic films and porous networks. However, the added complexity raises challenges in obtaining vertically oriented nanostructures, which are crucial in nanopatterning applications. In the case of solvent vapor annealed films, the effect of swelling parameters on vertical orientation has not been thoroughly investigated. Here, we demonstrate that swelling-induced surface neutrality promotes vertical orientation in PS-b-P4VP-(PDP) thin films, where P4VP-(PDP) is the majority block and PS the minority block. Morphological transitions were observed, including horizontal, mixed, and vertical orientations, as film thickness, PDP ratio, and swelling ratio were varied. A swelling ratio window ranging from ∼ 1.6 to 1.8 that reliably stabilizes vertical orientation was discovered. Significantly, vertical structures require a balance between selective swelling for surface energy neutrality and sufficient chain mobility for domain re-organization. These findings depart from previous reports where the formation of hierarchical lamellar structures within P4VP-(PDP) was described as a key requirement for vertical orientation. Overall, this study identified solvent and wetting parameters that influence the BCP orientation, thereby advancing supramolecular block copolymer thin films for nanopatterning and functional material applications.