macrophase separation into a block-copolymer-rich phase and a water-rich phase
takes place. These experiments revealed the appearance of a third process with a
positive amplitude. This was assigned to intermicellar interactions, indicating the
onset of the macrophase separation. We note that in these cases PPO was the major
compound in the triblock copolymer. The increasing hydrophobicity at higher
temperatures may thus lead to the observed clustering and macrophase separation
in water. In a subsequent paper by Waton et al. [130], it was argued that the second
and third relaxation processes are identical and are both due to the formation and
breakup of micellar entities at the point where the sign of the amplitude changes
from negative to positive by increasing the temperature. This was shown to be a
consequence of the relative size of the micelles after the fast initial growth and at
equilibrium. If micelles are larger after the first process than at the end of the slow
process, the scattered light decreases and vice versa. Because this changes with
temperature, the change in the sign of the amplitude becomes obvious.
The above discussed experiments were performed under quasi-equilibrium
conditions inherently showing a rather complex relaxation behavior that is
governed by processes from equilibrium and non-equilibrium kinetics. Additionally, in light of the discussion above, the presence of an apparent third mode is
controversial. With respect to the understanding of equilibrium kinetics in general,
the outcome of these experiments is limited as it does not provide deep insight into
the exchange mechanism and the dependence of system-specific parameters like the
interfacial tension, core chain length, and polydispersity. A more thorough understanding was only possible after the advent of the TR-SANS technique, as will be
shown in the subsequent sections.
4.2 PEP-PEO Block Copolymers in Aqueous Solution
The study of the structure and kinetics of hydrocarbon-PEO amphiphilic diblock
copolymers in selective solvents has received increased attention within the last
25 years. For example, polystyrene-block-poly(ethylene oxide) (PS-PEO) was intensively investigated in the 1990s [3, 131–136]. More recently, the micellar properties
of block copolymers containing either polydienes like 1,4-polyisoprene (PI); 1,2- or
1,4-polybutadiene (PB); or their saturated analogues poly(ethylene-alt-propylene)
(PEP) and poly(ethylethylene) (PEE) as the hydrocarbon block have attracted the
attention of many research groups because of their potential technical and biomedical
applications. The micellar properties of PE-PEO block copolymers, with PE the
saturation product of 1,4-PB, have only rarely been investigated [137, 138]. In
particular, equilibrium kinetics have to the best of our knowledge not been studied
so far most likely due to complications arising from unwanted coupling between
crystallization of PE and chain exchange dynamics at moderate temperatures. The
chemical design of amphiphilic block copolymers resembles that of the well-known
low molecular weight oligo(ethylene oxide)-monoalkyl ether [C n (EO) m ] surfactants,
with subscript n being the number of carbon atoms of the hydrophobic n-alkyl-moiety
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
111
takes place. These experiments revealed the appearance of a third process with a
positive amplitude. This was assigned to intermicellar interactions, indicating the
onset of the macrophase separation. We note that in these cases PPO was the major
compound in the triblock copolymer. The increasing hydrophobicity at higher
temperatures may thus lead to the observed clustering and macrophase separation
in water. In a subsequent paper by Waton et al. [130], it was argued that the second
and third relaxation processes are identical and are both due to the formation and
breakup of micellar entities at the point where the sign of the amplitude changes
from negative to positive by increasing the temperature. This was shown to be a
consequence of the relative size of the micelles after the fast initial growth and at
equilibrium. If micelles are larger after the first process than at the end of the slow
process, the scattered light decreases and vice versa. Because this changes with
temperature, the change in the sign of the amplitude becomes obvious.
The above discussed experiments were performed under quasi-equilibrium
conditions inherently showing a rather complex relaxation behavior that is
governed by processes from equilibrium and non-equilibrium kinetics. Additionally, in light of the discussion above, the presence of an apparent third mode is
controversial. With respect to the understanding of equilibrium kinetics in general,
the outcome of these experiments is limited as it does not provide deep insight into
the exchange mechanism and the dependence of system-specific parameters like the
interfacial tension, core chain length, and polydispersity. A more thorough understanding was only possible after the advent of the TR-SANS technique, as will be
shown in the subsequent sections.
4.2 PEP-PEO Block Copolymers in Aqueous Solution
The study of the structure and kinetics of hydrocarbon-PEO amphiphilic diblock
copolymers in selective solvents has received increased attention within the last
25 years. For example, polystyrene-block-poly(ethylene oxide) (PS-PEO) was intensively investigated in the 1990s [3, 131–136]. More recently, the micellar properties
of block copolymers containing either polydienes like 1,4-polyisoprene (PI); 1,2- or
1,4-polybutadiene (PB); or their saturated analogues poly(ethylene-alt-propylene)
(PEP) and poly(ethylethylene) (PEE) as the hydrocarbon block have attracted the
attention of many research groups because of their potential technical and biomedical
applications. The micellar properties of PE-PEO block copolymers, with PE the
saturation product of 1,4-PB, have only rarely been investigated [137, 138]. In
particular, equilibrium kinetics have to the best of our knowledge not been studied
so far most likely due to complications arising from unwanted coupling between
crystallization of PE and chain exchange dynamics at moderate temperatures. The
chemical design of amphiphilic block copolymers resembles that of the well-known
low molecular weight oligo(ethylene oxide)-monoalkyl ether [C n (EO) m ] surfactants,
with subscript n being the number of carbon atoms of the hydrophobic n-alkyl-moiety
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
111