Real-time observation of order-order transitions in block copolymer nanostructures via in situ TEM

Abstract

Block copolymers self-assemble into periodically ordered nanostructures — lamellae, cylinders, gyroids, or spheres — governed by the Flory-Huggins interaction parameter, chain architecture, and volume fraction. Temperature variation alters the effective segregation strength, driving order-order transitions (OOTs) between these equilibrium morphologies. Understanding the kinetic pathway and structural mechanism of OOTs is essential for designing functional nanostructured materials, as the transition pathway determines the resulting grain structure, defect density, and long-range spatial ordering. Extensive studies using small-angle X-ray scattering (SAXS) and rheology have mapped thermodynamic phase boundaries and identified key transition temperatures. However, these bulk-averaged, reciprocal-space techniques provide limited insight into how OOTs progress in real space at the level of individual grains and domain boundaries. Critical questions remain unresolved: whether OOTs proceed via nucleation and growth or through a spinodal-like continuous transformation, what intermediate structures form during the transition, and how defects nucleate, propagate, and annihilate as one morphology evolves into another. Here, we employ in situ TEM with a heating cell to directly observe thermally induced OOTs in block copolymer films in real time. Ultramicrotomed specimens are subjected to controlled heating within the electron microscope, enabling continuous imaging of nanostructural evolution across the transition. This approach captures nucleation sites and growth fronts of the emerging phase, epitaxial orientation relationships between parent and daughter morphologies, transient intermediate structures, and the evolution of topological defects including dislocations and grain boundaries. Correlating real-space structural progression with known thermodynamic phase behavior establishes a mechanistic picture of OOTs and clarifies how transition pathway selection governs the final nanostructure