Scattering Techniques Based on Neutrons, Light and X-rays
Structure and dynamics of a wide range of self-assembled systems can most conveniently be studied using X-ray and neutron scattering techniques as described in our recent review. 5 However, experimental observation of the kinetic processes of self-assembly is generally very challenging because the wide range of both temporal and spatial scales involved. Typically, the nucleation processes take place on short time scales of the order of microsecond-millisecond, which contrasts the very slow time scales of activated diffusion and structural reorganization processes. Often the faith of the final structure is sealed already under the nucleation process, which occurs typically in the millisecond or sub-millisecond regime. Thus, ideally we would also like to see the structure evolution of the process to keep track of possible metastable intermediates in analogue with what is commonly observed under chemical reactions. For this, time-resolved scattering techniques are ideal as the structure is encoded in the angular dependence of the scattered intensity. In fact, with the advent of ever more powerful x-ray sources such as the third-generation synchrotrons, state-of-the-art small-angle x-ray scattering give access to spatiotemporal resolution of the order of microseconds- milliseconds up to hours, directly on the relevant length scales of about 1-100 nm.
With more powerful neutron sources such as the high-flux reactor at ILL it is nowadays also possible to access time scales down to about 50 ms with SANS. With even more powerful spallation sources such as at J-PARC in Japan, SNS at Oak Ridge in USA, and the future European Spallation Source (ESS) at Lund, Sweden, SANS is slowly approaching the millisecond range opened up by synchrotrons. Neutron scattering is particularly useful as selective deuteration of molecules or parts of molecules, opens up many possibilities for contrast variation in order to label and highlight specific parts of the structure or to watch specific kinetic processes. In addition, with more powerful sources, neutron spin-echo spectrometers are capable of approaching the microsecond regime. The gap to modern time-resolved small-angle scattering thus starts to close and opens up a possibility to study the direct link between polymer dynamics, diffusion and kinetic transport processes.
Through collaboration through large-scale facilities, e.g. MLZ (Germany), MAXIV (Sweden), ESRF (France) we are actively trying to expand the capabilities of these techniques by help developing new sample environment and analysis methods.
- Narayanan, T., Wacklin, H., Konovalov, O. & Lund, R. Recent applications of synchrotron radiation and neutrons in the study of soft matter. Crystallography Reviews 23, 160–226 (2017).
- Lund, R., Willner, L. & Richter, D. Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods. Adv Polym Sci 259, 51–158 (2013).