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.

Kinetically Trapped Block Copolymer Nano‐Objects with Cylinder to Sphere Shape Transition Properties

“…However, the smaller spherical micelles with diameters of 18 ± 1 nm (PS 114 ‐ b ‐PNIPAM 209 ) and 27 ± 1 nm (PS 227 ‐ b ‐PNIPAM 470 ) seemed to be formed through another pathway. Since there would be some BCP chains released during the decomposition process[4a] and it was difficult for the BCP chains to insert into the spherical micelles with a PS core, the smaller spherical micelles might be formed by the self‐assembly of these released BCP chains in ethanol. To prove that, self‐assembly of PS 114 ‐ b ‐PNIPAM 209 and PS 227 ‐ b ‐PNIPAM 470 BCPs were conducted in ethanol.…”

“…However, it is often neglected that the kinetically trapped assemblies could show abilities quite alike the stimuli‐responsive systems. Once the kinetic barrier is removed under certain condition, the thermodynamic unstable nature could drive the assemblies to their thermodynamic equilibrium state through a morphology transition process . There are several advantages of kinetically trapped systems, which make them important complements to the stimuli‐responsive systems.…”

Probing the Self-Assembly of Unilamellar Vesicles Using Time-Resolved SAXS

“…Both sample and background (water) measurements were done at the same spot on the stopped-flow capillary cell to ensure an accurate subtraction of the background. In order to obtain a good background scattering curve, multiple frames (20) were acquired and averaged after normalization and azimuthal integration. In the following, the scattered intensity after the subtraction of the background is denoted by I(q).…”

“…This allowed probing the millisecond range structural dynamics involved in micellization of copolymers in selective solvents [18], self-assembly of surfactant micelles [19], morphological transformation of block-copolymer micelles [20], self-organization of DNA and lipid membranes [21], etc. Nevertheless, the potential of this approach in the elucidation of self-assembly processes is only beginning to be explored and this chapter is intended to provide a brief overview of experimental requirements.…”

Coarse-graining in simulations of multicomponent polymer systems

“…A precise understanding of the morphologies of self-assembly, 4 their kinetics of formation, 5 and their properties is essential for fruition of many of these applications. In this context, theoretical models and computer simulation tools have emerged as an efficient option to study such phenomena over a wide range of parametric conditions.…”