Non-native square packing and structural arrangement of ultrahigh molecular weight diblock copolymer from one-step solvent vapor annealing

Block copolymer self-assembly provides a promising method of generating nanoscale periodic structures with long-range order, serving as a template for advanced nano-lithographic applications. However, their intrinsic tendency towards free-energy minimization typically limits the accessible morphologies to thermodynamically preferred arrangements such as spheres on cubic lattices, hexagonally packed cylinders, or alternating lamellae. In this study, we demonstrate the formation of unconventional well-ordered square-packed arrays of vertical cylinders through one-step solvent annealing of ultrahigh molecular weight (UHMW) block copolymers (BCPs). The square-packed morphology observed in this study emerges from the interplay between low solvent vapor pressure, sluggish chain dynamics, and molecular polydispersity in UHMW BCPs, which collectively and kinetically trap the system in a metastable, nonequilibrium state favoring square symmetry. Furthermore, the introduction of a homopolymer facilitates defect healing, enhances structural stability, and enlarges the domain size by approximately 30%. These findings establish a pathway for generating square arrays as soft templates for advanced nanofabrication, offering direct compatibility with conventional integrated circuit design and fabrication.