Kinetic Pathway of the Cylinder-to-Sphere Transition in Block Copolymer Micelles Observed in Situ by Time-Resolved Neutron and Synchrotron Scattering.

Abstract

Here we present an in situ study of the nonequilibrium cylinder-to-sphere morphological transition kinetics on the millisecond range in a model block copolymer micelle system revealing the underlying mechanism and pathways of the process. By employing the stopped-flow mixing technique, the system was rapidly brought (≈100 μs) deep into the instability region, and the kinetics was followed on the time scale of milliseconds using both time-resolved small-angle neutron and X-ray scattering (TR-SANS and TR-SAXS, respectively). Due to the difference in contrast and resolution, SAXS and SANS provide unique complementary information. Our analysis shows that the morphological transition is characterized by a single rate constant indicating a two-state model where the transition proceeds through direct decomposition (fragmentation) of the cylinders without any transient intermediate structures. The cylindrical segments formed in the disintegration process subsequently grow into spherical micelles possibly through the molecular exchange mechanism until near equilibrium micelles are formed. The observation of a two-step kinetic mechanism, fluctuation-induced fragmentation and ″ripening″ processes, provides unique insight into the nonequilibrium behavior of block copolymer micelles in dilute solutions.