Real-Time Observation of Order-Disorder Transitions in Block Copolymer Nanostructures via In Situ TEM

Block copolymers, which contain two or more chemically distinct but covalently linked blocks, microphase separate into periodically ordered nanostructures such as lamellae, hexagonally packed cylinders, the bicontinuous gyroid, and body centered cubic spheres, with domain spacings of tens of nanometers. The equilibrium morphology is set by three parameters. The Flory Huggins interaction parameter χ quantifies the unfavorable enthalpy of mixing for monomer contact, the degree of polymerization N scales the loss of conformational entropy, and the volume fraction f biases interfacial curvature. Their product χN measures the segregation strength, and because χ generally varies inversely with temperature, heating lowers χN and can drive the system across the order disorder transition (ODT). These transitions are not merely thermodynamic endpoints. Their kinetic pathways govern grain size, defect density, and the long range order that ultimately controls macroscopic properties.

Small angle X ray scattering and rheology have reliably mapped these phase boundaries, yet both are ensemble averaged probes that integrate over many grains and cannot resolve the local, real space dynamics of a transition. Fundamental questions consequently remain open. Whether an ODT advances by nucleation and growth, with discrete ordered domains expanding into a disordered matrix, or by a spinodal like process of continuous, system wide modulation, and how individual defects nucleate, migrate, and annihilate as order develops.

Here we use in situ heating TEM to follow thermally induced ODTs in real time. By directly imaging nucleation sites, interface propagation, and defect evolution as a function of temperature, we resolve the transition pathway at the grain scale and establish a mechanistic picture of how these pathways select the final nanostructure