6 Concluding Remarks and Future Challenges
In this review, we have provided a selective overview of theoretical and experimental studies on kinetic processes in block copolymer micellar systems. We have
demonstrated the strengths of time-resolved small-angle scattering techniques by
highlighting recent examples from the literature. Most of the available literature in
this field is either related to equilibrium exchange kinetics or micellization kinetics.
Although limited in number, the examples given illustrate impressively the potential and strength of these techniques. Primarily, as a common feature of both methods,
TR-SAXS and TR-SANS provide a structural resolution of the kinetic processes of
self-assembled systems on the nanometer scale. Individually, the strengths of
TR-SAXS for studying kinetic processes is the high flux available at synchrotron
sources, which allows experimenters to resolve fast kinetic processes such as block
copolymer micellization in the millisecond range on small sample volumes. The
advantage of TR-SANS on the other hand is the ability to exploit H/D contrast
variation, which makes this technique unique for studying fundamental aspects of
equilibrium kinetics like the influence of temperature, interfacial tension, and chain
length. Even fine details like the chain conformation during the expulsion process or
contributions from “hidden” processes like diffusion or fusion/fission are accessible.
Time-resolved techniques show promise for the future as improved technical
capabilities and more powerful neutron and X-ray sources emerge. For neutrons,
particularly promising is the planned construction of the powerful European Spallation Source (ESS), which will allow time resolutions approaching submilliseconds
as well as the use of smaller and more dilute samples. Although the flux at synchrotron sources will still be significant larger, H/D contrast variation provides enhanced
contrast over X-rays as well as a playground for performing studies of equilibrium
kinetics in soft matter systems that cannot easily be achieved by other techniques.
Additionally, neutrons may provide a significant advantage because beam radiation
damage, which is a concern in particular for synchrotron studies of aqueous systems,
is not an issue with SANS.
Increased flux and more powerful sources will also be particularly useful for the
study of biological or biohybrid materials, which often are only available in small
quantities. This demands a significant quality in terms of both flux and background
as biomaterials often are characterized by weak signals and/or low contrast. Combining time-resolved wide-angle and small-angle scattering techniques can be
expected to be particularly useful to bridge mesoscopic and microscopic scales,
providing full structural information and information on correlations between local
and global motions. This places an additional demand for low background and
accurate background subtraction that constitutes a significant challenge to instrumentation development in terms of stability and optimization.
Despite its rather short history, TR-SAS techniques have helped to resolve many
aspects of kinetic processes in micellar systems, in particularly the equilibrium
kinetics. However, many challenges remain for the future. For block copolymer
micelles, these include studies of morphological transitions, drug encapsulation and
152
R. Lund et al.
In this review, we have provided a selective overview of theoretical and experimental studies on kinetic processes in block copolymer micellar systems. We have
demonstrated the strengths of time-resolved small-angle scattering techniques by
highlighting recent examples from the literature. Most of the available literature in
this field is either related to equilibrium exchange kinetics or micellization kinetics.
Although limited in number, the examples given illustrate impressively the potential and strength of these techniques. Primarily, as a common feature of both methods,
TR-SAXS and TR-SANS provide a structural resolution of the kinetic processes of
self-assembled systems on the nanometer scale. Individually, the strengths of
TR-SAXS for studying kinetic processes is the high flux available at synchrotron
sources, which allows experimenters to resolve fast kinetic processes such as block
copolymer micellization in the millisecond range on small sample volumes. The
advantage of TR-SANS on the other hand is the ability to exploit H/D contrast
variation, which makes this technique unique for studying fundamental aspects of
equilibrium kinetics like the influence of temperature, interfacial tension, and chain
length. Even fine details like the chain conformation during the expulsion process or
contributions from “hidden” processes like diffusion or fusion/fission are accessible.
Time-resolved techniques show promise for the future as improved technical
capabilities and more powerful neutron and X-ray sources emerge. For neutrons,
particularly promising is the planned construction of the powerful European Spallation Source (ESS), which will allow time resolutions approaching submilliseconds
as well as the use of smaller and more dilute samples. Although the flux at synchrotron sources will still be significant larger, H/D contrast variation provides enhanced
contrast over X-rays as well as a playground for performing studies of equilibrium
kinetics in soft matter systems that cannot easily be achieved by other techniques.
Additionally, neutrons may provide a significant advantage because beam radiation
damage, which is a concern in particular for synchrotron studies of aqueous systems,
is not an issue with SANS.
Increased flux and more powerful sources will also be particularly useful for the
study of biological or biohybrid materials, which often are only available in small
quantities. This demands a significant quality in terms of both flux and background
as biomaterials often are characterized by weak signals and/or low contrast. Combining time-resolved wide-angle and small-angle scattering techniques can be
expected to be particularly useful to bridge mesoscopic and microscopic scales,
providing full structural information and information on correlations between local
and global motions. This places an additional demand for low background and
accurate background subtraction that constitutes a significant challenge to instrumentation development in terms of stability and optimization.
Despite its rather short history, TR-SAS techniques have helped to resolve many
aspects of kinetic processes in micellar systems, in particularly the equilibrium
kinetics. However, many challenges remain for the future. For block copolymer
micelles, these include studies of morphological transitions, drug encapsulation and
152
R. Lund et al.