Classically, studies of micellization kinetics involve monitoring the time-resolved
response of micelles using either fluorescence spectroscopy or light scattering
methods. SAX/SANS techniques have not been used very extensively so far but are
becoming increasingly popular because the technical feasibility has increased significantly over the last few years due to more advanced instrumentation and more
powerful sources. The main limitation of light scattering and fluorescence spectroscopy methods is the lack of relevant structural resolution. Consequently, the structural evolution of micellar systems, whose typical sizes are of the order of 1–50 nm,
cannot be followed during the course of the kinetic process. An exception is large
particles of the order of several hundreds of nanometers, which thus enter into the
window of light scattering. Consequently, most studies discuss relaxation times,
which cannot be straightforwardly related to specific kinetic processes.
As mentioned in the Introduction, there is some unclear terminology related to
kinetics of micelles: often the terms “micelle dynamics” or “micelle kinetics” are
used interchangeably for equilibrium kinetics (exchange kinetics), relaxation kinetics
(micelle–micelle relaxation kinetics) and micellization kinetics (unimer–micelle
transition). We will focus on results related to micellization kinetics of block
copolymers starting from unimers, although in some cases we will also mention
results more related to re-equilibration kinetics. The term “dynamics” will be
associated with molecular level diffusion, rotations or elemental rotations/vibration,
etc. and will be left out of this review.
5.1.1 Temperature-Jump Experiments
As in experiments related to equilibrium or near-equilibrium kinetics, micellar
growth can be induced using larger amplitude T-jumps, thereby perturbing the system
from a unimer to micellar state. This requires temperature-sensitive polymers that
undergo micellization upon heating or cooling. A particularly well-studied block
copolymer system is poly(ethylene oxide)–poly(propylene-oxide)–poly(ethylene
oxide) (PEO-PPO-PEO; Pluronics) triblock copolymer in aqueous dispersions. PPO
exhibits a LCST and is generally not water soluble at ambient temperature. PEOPPO-PEO undergoes a unimer–micelle transition in a range of approximately
10–35
C, depending on molecular weight, composition, and concentration [165].
The transition temperature is referred to as the critical micellization temperature
(cmt), which is equivalent to the cmc at constant temperature.
The first applications of the T-jump method to study kinetics related to micelles
is due to Kreschek et al. [166] and Eyring and coworkers [167, 168] in the late
1960s to early 1970s. The rate of dissociation of various ionic surfactants was
measured by suddenly increasing the temperature using electrical resistance heating
and capacitor discharge following the change in time using scattered light. While
the experiments showed qualitatively that the kinetics occurred on a typical millisecond timescale, not much more information could be obtained due to a lack of
angular resolution. Block copolymer systems are generally more robust towards
environmental changes than surfactants. Consequently, polymer systems where at
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R. Lund et al.
response of micelles using either fluorescence spectroscopy or light scattering
methods. SAX/SANS techniques have not been used very extensively so far but are
becoming increasingly popular because the technical feasibility has increased significantly over the last few years due to more advanced instrumentation and more
powerful sources. The main limitation of light scattering and fluorescence spectroscopy methods is the lack of relevant structural resolution. Consequently, the structural evolution of micellar systems, whose typical sizes are of the order of 1–50 nm,
cannot be followed during the course of the kinetic process. An exception is large
particles of the order of several hundreds of nanometers, which thus enter into the
window of light scattering. Consequently, most studies discuss relaxation times,
which cannot be straightforwardly related to specific kinetic processes.
As mentioned in the Introduction, there is some unclear terminology related to
kinetics of micelles: often the terms “micelle dynamics” or “micelle kinetics” are
used interchangeably for equilibrium kinetics (exchange kinetics), relaxation kinetics
(micelle–micelle relaxation kinetics) and micellization kinetics (unimer–micelle
transition). We will focus on results related to micellization kinetics of block
copolymers starting from unimers, although in some cases we will also mention
results more related to re-equilibration kinetics. The term “dynamics” will be
associated with molecular level diffusion, rotations or elemental rotations/vibration,
etc. and will be left out of this review.
5.1.1 Temperature-Jump Experiments
As in experiments related to equilibrium or near-equilibrium kinetics, micellar
growth can be induced using larger amplitude T-jumps, thereby perturbing the system
from a unimer to micellar state. This requires temperature-sensitive polymers that
undergo micellization upon heating or cooling. A particularly well-studied block
copolymer system is poly(ethylene oxide)–poly(propylene-oxide)–poly(ethylene
oxide) (PEO-PPO-PEO; Pluronics) triblock copolymer in aqueous dispersions. PPO
exhibits a LCST and is generally not water soluble at ambient temperature. PEOPPO-PEO undergoes a unimer–micelle transition in a range of approximately
10–35
C, depending on molecular weight, composition, and concentration [165].
The transition temperature is referred to as the critical micellization temperature
(cmt), which is equivalent to the cmc at constant temperature.
The first applications of the T-jump method to study kinetics related to micelles
is due to Kreschek et al. [166] and Eyring and coworkers [167, 168] in the late
1960s to early 1970s. The rate of dissociation of various ionic surfactants was
measured by suddenly increasing the temperature using electrical resistance heating
and capacitor discharge following the change in time using scattered light. While
the experiments showed qualitatively that the kinetics occurred on a typical millisecond timescale, not much more information could be obtained due to a lack of
angular resolution. Block copolymer systems are generally more robust towards
environmental changes than surfactants. Consequently, polymer systems where at
134
R. Lund et al.