Unveiling metastable intermediates of block copolymer self-assembly in colloids via graphene liquid cell imaging.

The self-assembly of block copolymers (BCPs) in solution is a primary method for creating functional nanostructures, however the real-time kinetic pathways-especially the formation of transient, metastable intermediates-remain difficult to observe. In this study, we use in-situ graphene liquid cell (GLC) transmission electron microscopy to directly visualize the phase separation dynamics of two vesicle-forming block copolymers, PS154-b-PAA49 and PS144-b-PAA22, in a DMF/water mixture. Our results reveal that the transition from spherical micelles to complex vesicles follows a three-stage kinetic pathway, rather than the traditional two-step model. We identify a critical transformation in which initial disordered aggregates undergo a "swallowing" phase followed by a "self-hollowing" stage. During this second stage, internal reorganization driven by interfacial energy minimization leads to the formation of metastable hollowed structures prior to reaching equilibrium. Molecular dynamics simulations and deep-learning-assisted image analysis support these observations, demonstrating that "self-hollowing" arises from the interplay between solvent-selective block interactions and local density fluctuations. By providing a high-resolution kinetic map of these morphological transitions, this work offers a clearer understanding of how to control BCP assembly to design precise porous nanomaterials.