and m of the hydrophilic EO units. Accordingly, amphiphilic block copolymers can
form micelles in aqueous media as well as in nonpolar solvents.
In this part of the review we will focus on kinetics in aqueous dispersions because
water-based systems have been more commonly investigated. Among the amphiphilic block copolymers mentioned above, PEP-PEO takes a prominent position
because it possesses four main features that turn it into an ideal model system for
studying fundamental aspects of block copolymer micellization: (1) The synthesis of
narrowly distributed PEP-PEO with predefined molecular weight and composition is
easily feasible by well-established living anionic polymerization techniques
[139–141]. Importantly, as a prerequisite for TR-SANS studies, PEP-PEO can be
synthesized fully deuterated. (2) Compared to polydienes, the aliphatic PEP block is
chemically and thermally stable, which facilitates sample handling and preparation.
The addition of stabilizing agents such as antioxidants is not necessary. (3) PEO is
highly water-soluble and electrostatically neutral. (4) PEP is an amorphous material
with a low T g of À56
C. Specific influences from glassy or crystallized micellar cores
need not be considered. (5) PEP is highly incompatibility with water, as reflected by a
large value for the interfacial tension, γ,of about 46 mN/m [45]. This high value of
γ is the important physical quantity determining the aggregation behavior of PEPPEO block copolymers in aqueous solutions. Moreover, the tuning of micellar
structure and kinetics by varying γ through the addition of less incompatible cosolvents, e.g., DMF or ethanol becomes very effective. As the static properties are an
important prerequisite for discussing exchange kinetics, the PEP-PEO micellar
structure was characterized as a function of molecular weight, block composition,
and solvent quality. A brief summary of the main structural features of these
investigations is given in the beginning of Sect. 4.2.1.
The main part is then devoted to the equilibrium exchange kinetics of selected
PEP-PEO micellar systems. We report on TR-SANS measurements in pure water
that, independently of block copolymer molecular weight, composition, and temperature, revealed frozen micelles. This review further concerns the effect of tuning the
kinetics by addition of co-solvents, i.e., reduction of γ. The relaxation behavior of
some selected systems revealing chain exchange dynamics that can be resolved by
TR-SANS will be presented, followed by a discussion of the main observation,
namely, the unexpected appearance of a pseudo-logarithmic time decay of the
relaxation function.
4.2.1 Morphological Behavior of PEP-PEO Block Copolymers
in Aqueous Solution
The static structure of PEP-PEO block copolymer micelles in aqueous solution have
been studied by small angle scattering techniques, primarily SANS [44, 45, 87, 104,
139, 142], and in one case by a combination of SAXS and static light scattering
(SLS) [143]. In water, PEP-PEO block copolymers self-assemble into a variety of
micellar structures depending on molecular weight and composition. A thorough
structural characterization of micelles formed by a symmetric PEP5-PEO5 diblock
112
R. Lund et al.
form micelles in aqueous media as well as in nonpolar solvents.
In this part of the review we will focus on kinetics in aqueous dispersions because
water-based systems have been more commonly investigated. Among the amphiphilic block copolymers mentioned above, PEP-PEO takes a prominent position
because it possesses four main features that turn it into an ideal model system for
studying fundamental aspects of block copolymer micellization: (1) The synthesis of
narrowly distributed PEP-PEO with predefined molecular weight and composition is
easily feasible by well-established living anionic polymerization techniques
[139–141]. Importantly, as a prerequisite for TR-SANS studies, PEP-PEO can be
synthesized fully deuterated. (2) Compared to polydienes, the aliphatic PEP block is
chemically and thermally stable, which facilitates sample handling and preparation.
The addition of stabilizing agents such as antioxidants is not necessary. (3) PEO is
highly water-soluble and electrostatically neutral. (4) PEP is an amorphous material
with a low T g of À56
C. Specific influences from glassy or crystallized micellar cores
need not be considered. (5) PEP is highly incompatibility with water, as reflected by a
large value for the interfacial tension, γ,of about 46 mN/m [45]. This high value of
γ is the important physical quantity determining the aggregation behavior of PEPPEO block copolymers in aqueous solutions. Moreover, the tuning of micellar
structure and kinetics by varying γ through the addition of less incompatible cosolvents, e.g., DMF or ethanol becomes very effective. As the static properties are an
important prerequisite for discussing exchange kinetics, the PEP-PEO micellar
structure was characterized as a function of molecular weight, block composition,
and solvent quality. A brief summary of the main structural features of these
investigations is given in the beginning of Sect. 4.2.1.
The main part is then devoted to the equilibrium exchange kinetics of selected
PEP-PEO micellar systems. We report on TR-SANS measurements in pure water
that, independently of block copolymer molecular weight, composition, and temperature, revealed frozen micelles. This review further concerns the effect of tuning the
kinetics by addition of co-solvents, i.e., reduction of γ. The relaxation behavior of
some selected systems revealing chain exchange dynamics that can be resolved by
TR-SANS will be presented, followed by a discussion of the main observation,
namely, the unexpected appearance of a pseudo-logarithmic time decay of the
relaxation function.
4.2.1 Morphological Behavior of PEP-PEO Block Copolymers
in Aqueous Solution
The static structure of PEP-PEO block copolymer micelles in aqueous solution have
been studied by small angle scattering techniques, primarily SANS [44, 45, 87, 104,
139, 142], and in one case by a combination of SAXS and static light scattering
(SLS) [143]. In water, PEP-PEO block copolymers self-assemble into a variety of
micellar structures depending on molecular weight and composition. A thorough
structural characterization of micelles formed by a symmetric PEP5-PEO5 diblock
112
R. Lund et al.