A Crystalline Micelle Intermediate Links Crystallization and Melting in Crystallization-driven Self-assembly

Crystallization-driven self-assembly (CDSA) provides a powerful strategy for constructing well-defined polymer nanostructures, yet the relationship between crystallization and melting remains poorly understood. In particular, it is unknown whether melting simply retraces the crystallization pathway or proceeds through distinct structural intermediates. Here we combine real-time small- and wide-angle X-ray scattering, transmission electron microscopy, and differential scanning calorimetry to resolve both the crystallization and melting pathways of poly(styrene)-block-poly(L-lactide) (PS- b -PLLA) platelet crystals. During self-seeding crystallization, residual seed crystallites first direct the epitaxial growth of crystalline cylindrical micelles, which assemble into highly ordered arrays before laterally coalescing into platelet crystals. Upon heating, the platelet crystals do not melt directly. Instead, they undergo edge fragmentation into ordered arrays of crystalline cylindrical micelles, generating a transient re-entrant ordered state before complete dissolution. Thus, crystallization and melting are linked by a common crystalline micelle intermediate but follow distinct microscopic pathways. The emergence and disappearance of this intermediate are marked by characteristic SAXS correlation peaks, while WAXS and calorimetry indicate that the intermediate exists within the PLLA melting interval, before crystallinity is completely lost. A scaling analysis of the competing interfacial and corona-stretching free energies places the platelet-to-micelle crossover at an effective PLLA–solvent interfacial tension of order 10 mN m ^− 1 , providing an energetic basis for the observed restructuring. These findings reveal a reversible yet pathway-dependent hierarchical assembly mechanism in CDSA, establish crystalline micelles as the key mesoscale building blocks governing both growth and disassembly, and provide a general framework for understanding reversible nonclassical crystallization and designing adaptive crystalline polymer nanomaterials.