chromatography [107–112] and ultracentrifugation [113, 114] as investigative tools.
Since both techniques involve strong flow fields, the experiments yield information
under quasi-static conditions rather than true equilibrium kinetics. Therefore, these
works will not be further reviewed here. Kinetic studies have additionally been
performed using ultrasonic absorption techniques [115–117] but after careful analysis
it was concluded that the ultrasonic relaxation observed in micellar solution cannot be
associated with the Aniansson and Wall mechanism of single chain exchange [117].
More relevant in the context of this chapter are relaxation experiments under quasiequilibrium conditions, i.e., after small perturbations from equilibrium such as small
temperature-jump (T-jump) experiments with light scattering detection in triblock
copolymer micelles of the Pluronics type, PEO-PPO-PEO. Regarding this work, we
will devote a small section (Sect. 4.1) to a brief summary of the main results. Other
techniques like fluorescence quenching or nonradiative energy transfer [118–122]
and transmission electron microscopy [123–125] have been used to assess micellar
kinetics. However, strong perturbations or the incorporation of bulky labels are
necessary to be able to monitor kinetic processes, which are often accompanied by
reorganization of micellar structures. In any case these classical works have already
been discussed extensively before in, e.g., the review of Zana [16]. Therefore, we will
mainly restrict this section to the more recent results obtained by the H/D contrast
variation/TR-SANS technique because this method was shown to access the chain
exchange dynamics under true equilibrium conditions. By the combination of proper
spatial and temporal resolution and by using suitable block copolymer/solvent
systems, these experiments have given new insights into the mechanisms of chain
exchange and allowed a profound discussion of the relaxation behavior within the
framework of the existing theories.
This section on equilibrium kinetics is organized as follows: We will first focus on
the T-jump experiments with light scattering detection on Pluronics polymers
(Sect. 4.1). Thereafter, a chronological review of kinetic experiments on PEP-PEO
block copolymers in aqueous solution is presented, including a summary of the
morphological properties of this system (Sect. 4.2). A special focus is given on the
tuning of kinetics by variation of mainly the interfacial tension. The main outcome of
these experiments was the observation of an unexpected logarithmic time decay. This
is discussed in more detail (Sect. 4.2.4). Further, we will review experiments obtained
on block copolymers in organic solvents and discuss the dramatic effect of polydispersity on the kinetics, which finally allows an explanation of the log-time dependence (Sect. 4.3). Subsequently, a summary of kinetic experiments on n-alkyl-PEO
block copolymers with monodisperse core blocks will be given, supporting the
polydispersity effect by directly demonstrating the enormous influence of the
n-alkyl chain length and the observed single exponential decay (Sect. 4.4). Finally,
two special sections are devoted to the exchange kinetics at higher concentrations in
ordered diblock copolymer micelles (Sect. 4.5) and to the influence of the morphology by comparing kinetics in spherical and cylindrical micelles (Sect. 4.6).
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
109
Since both techniques involve strong flow fields, the experiments yield information
under quasi-static conditions rather than true equilibrium kinetics. Therefore, these
works will not be further reviewed here. Kinetic studies have additionally been
performed using ultrasonic absorption techniques [115–117] but after careful analysis
it was concluded that the ultrasonic relaxation observed in micellar solution cannot be
associated with the Aniansson and Wall mechanism of single chain exchange [117].
More relevant in the context of this chapter are relaxation experiments under quasiequilibrium conditions, i.e., after small perturbations from equilibrium such as small
temperature-jump (T-jump) experiments with light scattering detection in triblock
copolymer micelles of the Pluronics type, PEO-PPO-PEO. Regarding this work, we
will devote a small section (Sect. 4.1) to a brief summary of the main results. Other
techniques like fluorescence quenching or nonradiative energy transfer [118–122]
and transmission electron microscopy [123–125] have been used to assess micellar
kinetics. However, strong perturbations or the incorporation of bulky labels are
necessary to be able to monitor kinetic processes, which are often accompanied by
reorganization of micellar structures. In any case these classical works have already
been discussed extensively before in, e.g., the review of Zana [16]. Therefore, we will
mainly restrict this section to the more recent results obtained by the H/D contrast
variation/TR-SANS technique because this method was shown to access the chain
exchange dynamics under true equilibrium conditions. By the combination of proper
spatial and temporal resolution and by using suitable block copolymer/solvent
systems, these experiments have given new insights into the mechanisms of chain
exchange and allowed a profound discussion of the relaxation behavior within the
framework of the existing theories.
This section on equilibrium kinetics is organized as follows: We will first focus on
the T-jump experiments with light scattering detection on Pluronics polymers
(Sect. 4.1). Thereafter, a chronological review of kinetic experiments on PEP-PEO
block copolymers in aqueous solution is presented, including a summary of the
morphological properties of this system (Sect. 4.2). A special focus is given on the
tuning of kinetics by variation of mainly the interfacial tension. The main outcome of
these experiments was the observation of an unexpected logarithmic time decay. This
is discussed in more detail (Sect. 4.2.4). Further, we will review experiments obtained
on block copolymers in organic solvents and discuss the dramatic effect of polydispersity on the kinetics, which finally allows an explanation of the log-time dependence (Sect. 4.3). Subsequently, a summary of kinetic experiments on n-alkyl-PEO
block copolymers with monodisperse core blocks will be given, supporting the
polydispersity effect by directly demonstrating the enormous influence of the
n-alkyl chain length and the observed single exponential decay (Sect. 4.4). Finally,
two special sections are devoted to the exchange kinetics at higher concentrations in
ordered diblock copolymer micelles (Sect. 4.5) and to the influence of the morphology by comparing kinetics in spherical and cylindrical micelles (Sect. 4.6).
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
109