In soft matter and materials physics, self-assembly is not only a fascinating phenomenon (i.e. spontaneous ordering of molecules on mesoscopic scales) but also serves as a versatile toolbox to synthesize and control nano-structures with well-defined sizes and geometries. Accurate control of the properties (structure and dynamics) of these nanoparticles is essential for the performance relevant to a wide range of fields including biomedical and nanotechnological applications.  A suitable way of controlling the structure and morphology of nanoparticles is by manipulating the kinetic pathways, i.e. the pathway by which the particles are formed.

However such strategies require a detailed knowledge of the kinetic pathways and basic mechanisms of the self-assembly process. In particular, for an efficient and predictive design, a detailed quantitative understanding is needed.   
 
In this study we seek to understand how nanoscale shape transformations occur in systems of amphiphilic block copolymers in water. These polymers contain both hydrophilic (water soluble) and hydrophobic (water insoluble) domains and undergo self-assembly into a wide range of nanostructures in solution, including cylinders, spheres, discs and vesicles.

Here we present a direct observation of the non-equilibrium cylinder-to-sphere transition in a model block copolymer system using time-resolved small-angle neutron and X-ray scattering (TR-SANS and TR-SAXS, respectively).  Taking advantage of the state-of-the-art instruments, D11 at ILL, and ID02 at ESRF, the kinetics could be followed on the time scale of milliseconds providing sampling of the process in real time. Due to the difference in contrast and resolution, SAXS and SANS provide unique complementary information on the overall shape and internal structure respectively. Our combined analysis shows that the transition occurs via a two-step process. First the shape fluctuations become sufficiently important for the cylinders to break-up. Then from the fragments spherical micelles grow slowly and equilibrate (″ripening process”). The fluctuation-induced decomposition scenario is consistent with earlier works for both surfactant liquid crystals systems and block copolymer melts where this mechanism has been suggested although not directly observed. The subsequent "ripening-type" mechanism, however, has not been observed in earlier works and could only be revealed due to the unique combination of the high resolution of synchrotron SAXS combined with the contrast of SANS.

Reidar Lund


Re.: ACS Macro Lett 2, 1082 – 1087 (2013) dx.doi.org/10.1021/mz400521p
R. Lund, L. Willner, D. Richter, P. Lindner, T. Narayanan

Nov. 2013