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 SAXS gives access to
spatiotemporal resolution of the order of only micro- to 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 Institut Laue Langevin (ILL), it is now also
possible to access time scales down to about 100 ms with SANS. With even more
powerful spallation sources such as the SNS, Oak Ridge in the USA and the future
European Spallation Source (ESS) at Lund, Sweden, SANS is expected to catch up
with the synchrotron and more easily give time resolutions approaching milliseconds
and microseconds. Here in particular, neutron scattering will be useful as selective
deuteration of molecules or parts of molecules, opening up many possibilities for
contrast variation to label and highlight specific parts of the structure or to watch
specific kinetic processes. In this review, we will focus on how these methods can be
applied to investigate kinetics in micellar systems, in particular block copolymer
systems.
Amphiphilic molecules such as surfactants and block copolymers containing
hydrophobic (water-insoluble) and hydrophilic (water-soluble) parts, serve as simple synthetic model systems for understanding self-assembly. Micellization is a
common self-assembly process whereby amphiphilic molecules spontaneously
aggregate into various nanostructures that are usually of spherical, ellipsoidal,
cylindrical, or vesicular shapes [1, 2]. These processes usually occur in selective
solvents, i.e., solvents that are good for one part but poor for the other. Here, selfassembly is primarily driven by the incompatibility of the insoluble (hydrophobic
or more generally “solvophobic”) part with water or other solvents, and is mainly
counteracted by repulsions or unfavorable configurations experienced in the
swollen corona of the resulting micelles.
Block copolymer micelles are macromolecular analogues to ordinary surfactant
micelles. These systems generally consist of two or more distinct types of polymeric
blocks covalently linked together. Because of the wealth of possible combinations of
chemistry and compositions of such polymers, the possibilities for tailoring selfassembly and resulting structures are virtually endless, leading to a wide range of
applications. Common for micellization is that the aggregates are formed above a
certain threshold concentration, called the critical micelle concentration (cmc). As
block copolymers often contain large insoluble blocks, the cmc in these systems can
be almost immeasurably small and micelles are spontaneously formed at almost any
concentration in solvents where the interfacial tension is high, such as in water. An
idealized example of how micelles are formed from block copolymers is shown in
Fig. 1. Here, a sudden micellization is induced from a homogeneous solution of
dissolved block copolymer chains by suddenly altering the conditions (change in
solubility by addition of co-solvents, salts, etc.). This micelle formation process is a
typical non-equilibrium kinetic process. Once micelles are formed, the classical view
for micellar solutions is that the system attains its equilibrium by continuously
exchanging the constituting chains. An example of such scenario is also shown in
Fig. 1, where a single chain (unimer) is released and reabsorbed into the micelle. In
much the same way as in chemical reactions, such a unimer exchange process is a
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
55
such as the third-generation synchrotrons, state-of-the-art SAXS gives access to
spatiotemporal resolution of the order of only micro- to 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 Institut Laue Langevin (ILL), it is now also
possible to access time scales down to about 100 ms with SANS. With even more
powerful spallation sources such as the SNS, Oak Ridge in the USA and the future
European Spallation Source (ESS) at Lund, Sweden, SANS is expected to catch up
with the synchrotron and more easily give time resolutions approaching milliseconds
and microseconds. Here in particular, neutron scattering will be useful as selective
deuteration of molecules or parts of molecules, opening up many possibilities for
contrast variation to label and highlight specific parts of the structure or to watch
specific kinetic processes. In this review, we will focus on how these methods can be
applied to investigate kinetics in micellar systems, in particular block copolymer
systems.
Amphiphilic molecules such as surfactants and block copolymers containing
hydrophobic (water-insoluble) and hydrophilic (water-soluble) parts, serve as simple synthetic model systems for understanding self-assembly. Micellization is a
common self-assembly process whereby amphiphilic molecules spontaneously
aggregate into various nanostructures that are usually of spherical, ellipsoidal,
cylindrical, or vesicular shapes [1, 2]. These processes usually occur in selective
solvents, i.e., solvents that are good for one part but poor for the other. Here, selfassembly is primarily driven by the incompatibility of the insoluble (hydrophobic
or more generally “solvophobic”) part with water or other solvents, and is mainly
counteracted by repulsions or unfavorable configurations experienced in the
swollen corona of the resulting micelles.
Block copolymer micelles are macromolecular analogues to ordinary surfactant
micelles. These systems generally consist of two or more distinct types of polymeric
blocks covalently linked together. Because of the wealth of possible combinations of
chemistry and compositions of such polymers, the possibilities for tailoring selfassembly and resulting structures are virtually endless, leading to a wide range of
applications. Common for micellization is that the aggregates are formed above a
certain threshold concentration, called the critical micelle concentration (cmc). As
block copolymers often contain large insoluble blocks, the cmc in these systems can
be almost immeasurably small and micelles are spontaneously formed at almost any
concentration in solvents where the interfacial tension is high, such as in water. An
idealized example of how micelles are formed from block copolymers is shown in
Fig. 1. Here, a sudden micellization is induced from a homogeneous solution of
dissolved block copolymer chains by suddenly altering the conditions (change in
solubility by addition of co-solvents, salts, etc.). This micelle formation process is a
typical non-equilibrium kinetic process. Once micelles are formed, the classical view
for micellar solutions is that the system attains its equilibrium by continuously
exchanging the constituting chains. An example of such scenario is also shown in
Fig. 1, where a single chain (unimer) is released and reabsorbed into the micelle. In
much the same way as in chemical reactions, such a unimer exchange process is a
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
55