3.2 Time-Resolved Small-Angle Scattering as a Technique
for Studying Micellar Kinetics
In addition to being a powerful tool for investigating the structural details of
nanostructures, small-angle scattering methods are very useful techniques for following structural changes over time, i.e., to follow phase transitions, morphological
transitions, etc. [13–15, 17, 18]. In addition to studying non-equilibrium kinetics
associated with structural changes and transformations over time, the sensitivity of
neutrons towards H/D, contrast variation, and SANS offers a study of subtle transport/
diffusive processes and kinetics under equilibrium conditions [18, 19], i.e., kinetics
can be probed without perturbing the system away from equilibrium.
Modern neutron instrumentation is an extensive subject that could cover a whole
book alone. Here we will just review some basic concepts related to the more
practical aspects and relevant principles related to studies of soft matter systems in
general and micellar systems in particular. In this section, we briefly review recent
modern methods used for time-resolved SAS studies.
3.2.1 Rapid Mixing Techniques: Stopped-Flow Methods
A versatile and classical method for studying kinetic reactions and other kinetic
phenomena on short time scales is the use of a stopped-flow apparatus (SFA) for
fast reproducible mixing and then to apply, e.g., spectroscopic methods for detection. In this technique, the reactants are rapidly mixed in a mixing chamber, usually
under full turbulent flow that ensures fast homogenization on length scales down to
nanometers [99]. Provided that short, synchronized acquisitions can be made, X-ray
or neutron scattering can be used to probe kinetic transitions and other processes
directly by measuring the temporal evolution of the intensity of the (mixed) sample.
The importance of a stopped-flow apparatus is to control the mixing of several
solutions in a short time, achieve precise synchronization between the mixing
process and the acquisition to ensure reproducibility of the experiment, and obtain
a well-defined kinetic time [18, 99]. For fast measurements, it is important to use
both fast mixing and short acquisition times while maintaining a reasonable statistics. This is more easily achieved with synchrotron sources, where the combination
of small mixing volumes and high brilliance of the beam easily allow kinetic times
of the order of a few milliseconds. However, with the more optimized neutron
instrumentation and, in particular, with the advent of more powerful neutron
sources such as spallation sources, there are a growing number of time-resolved
SANS experiments investigating kinetic processes on a typical time scale of about
100 ms and upwards [18]. With more powerful spallation sources such as the
planned European Spallation Source (ESS) in Lund, Sweden, time-resolved
SANS measurements are expected to approach that of TR-SAXS at current synchrotron sources.
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
99
for Studying Micellar Kinetics
In addition to being a powerful tool for investigating the structural details of
nanostructures, small-angle scattering methods are very useful techniques for following structural changes over time, i.e., to follow phase transitions, morphological
transitions, etc. [13–15, 17, 18]. In addition to studying non-equilibrium kinetics
associated with structural changes and transformations over time, the sensitivity of
neutrons towards H/D, contrast variation, and SANS offers a study of subtle transport/
diffusive processes and kinetics under equilibrium conditions [18, 19], i.e., kinetics
can be probed without perturbing the system away from equilibrium.
Modern neutron instrumentation is an extensive subject that could cover a whole
book alone. Here we will just review some basic concepts related to the more
practical aspects and relevant principles related to studies of soft matter systems in
general and micellar systems in particular. In this section, we briefly review recent
modern methods used for time-resolved SAS studies.
3.2.1 Rapid Mixing Techniques: Stopped-Flow Methods
A versatile and classical method for studying kinetic reactions and other kinetic
phenomena on short time scales is the use of a stopped-flow apparatus (SFA) for
fast reproducible mixing and then to apply, e.g., spectroscopic methods for detection. In this technique, the reactants are rapidly mixed in a mixing chamber, usually
under full turbulent flow that ensures fast homogenization on length scales down to
nanometers [99]. Provided that short, synchronized acquisitions can be made, X-ray
or neutron scattering can be used to probe kinetic transitions and other processes
directly by measuring the temporal evolution of the intensity of the (mixed) sample.
The importance of a stopped-flow apparatus is to control the mixing of several
solutions in a short time, achieve precise synchronization between the mixing
process and the acquisition to ensure reproducibility of the experiment, and obtain
a well-defined kinetic time [18, 99]. For fast measurements, it is important to use
both fast mixing and short acquisition times while maintaining a reasonable statistics. This is more easily achieved with synchrotron sources, where the combination
of small mixing volumes and high brilliance of the beam easily allow kinetic times
of the order of a few milliseconds. However, with the more optimized neutron
instrumentation and, in particular, with the advent of more powerful neutron
sources such as spallation sources, there are a growing number of time-resolved
SANS experiments investigating kinetic processes on a typical time scale of about
100 ms and upwards [18]. With more powerful spallation sources such as the
planned European Spallation Source (ESS) in Lund, Sweden, time-resolved
SANS measurements are expected to approach that of TR-SAXS at current synchrotron sources.
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
99