It can be seen that cylinders are formed in water and in water/DMF mixtures with
low DMF fractions, i.e., high interfacial tensions, whereas spherical micelles occur
above $50 mol % DMF at low interfacial tensions. TR-SANS experiments were
performed at different DMF/water compositions for both cylindrical and spherical
geometries at 41%, 47%, 60%, and 75% DMF mole fraction. As already observed
for PEP1-PEO20 micelles [102], chain exchange became increasingly faster with
growing DMF content due to lower interfacial tensions (see Fig. 22). In order to
access the initial part of the relaxation, a stopped-flow apparatus for rapid mixing
was used. Technical details of the stopped flow method are given in Sect. 3.2.1.
However, with 75% DMF in the selective solvent mixture, chain exchange was
already too fast to be resolved even with the stopped-flow fast mixing technique.
The relaxation curves obtained for the other solvent compositions showed an
extended decay over several orders of magnitude in time, which for intermediate
and long time scales was again quasi-logarithmic. Accordingly, the data were
analyzed using Eq. 117 by taking into account the polydispersity of the PEP
block by a Schulz–Zimm distribution (Eq. 121). In accord with the model of
Choi, Bates, and Lodge [63], the longest Rouse time was used as pre-exponential
factor as defined in Eq. 120. Independent of the morphology, good fits were only
obtained by taking β ¼ 1 implying a stretched conformation of the insoluble block
during the expulsion process (see Sect. 2.2.6.).
The parameter α assumed values that were about a factor of three smaller than
expected from the geometrical estimate. Parallel to the transition from cylinders to
spheres, α slightly decreases. However, this does not clearly reflect any influence of
the morphology because the fits are very sensitive to α and other influences (e.g.,
arising from experimental uncertainties) cannot be fully excluded. Therefore, in order
to delineate any factors arising from the different morphologies, Lund et al. have
exploited the fact that at 50% DMF fraction the transition can be also induced by
heating. Dissolution of the polymer at room temperature leads first to stable cylindrical
micelles, which transform into spherical entities after annealing for several hours at
70
C. Importantly, the transition is irreversible, meaning that the spherical shape is
preserved at low temperatures. The exchange kinetics was then measured in both
morphologies on the same specimen by TR-SANS under exactly the same conditions.
The authors found that the kinetics in spherical morphology is slightly but unambiguously faster than for cylinders, as shown in Fig. 30. With respect to the fit
parameters, the faster kinetics is reflected by a slightly smaller α while β ¼ 1 was
kept constant. This was found to be in accordance with the slightly more pronounced decrease in α for varying DMF composition. The value of β ¼ 1was in
contrast to β ¼ 2/3 found for PEP1-PEO20 star-like micelles in which the PEP1
chain assumes a completely segregated spherical bud. It was argued that a stretched
conformation facilitates passage through the more dense corona of the spherical and
cylindrical crew-cut type micelles obtained from PEP1-PEO1 block copolymers.
An interpretation of the small differences in α is, however, difficult. It was
speculated that this has its origin in small variances in the local coronal structure,
which may be different for the cylindrical and spherical morphologies.
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
131
low DMF fractions, i.e., high interfacial tensions, whereas spherical micelles occur
above $50 mol % DMF at low interfacial tensions. TR-SANS experiments were
performed at different DMF/water compositions for both cylindrical and spherical
geometries at 41%, 47%, 60%, and 75% DMF mole fraction. As already observed
for PEP1-PEO20 micelles [102], chain exchange became increasingly faster with
growing DMF content due to lower interfacial tensions (see Fig. 22). In order to
access the initial part of the relaxation, a stopped-flow apparatus for rapid mixing
was used. Technical details of the stopped flow method are given in Sect. 3.2.1.
However, with 75% DMF in the selective solvent mixture, chain exchange was
already too fast to be resolved even with the stopped-flow fast mixing technique.
The relaxation curves obtained for the other solvent compositions showed an
extended decay over several orders of magnitude in time, which for intermediate
and long time scales was again quasi-logarithmic. Accordingly, the data were
analyzed using Eq. 117 by taking into account the polydispersity of the PEP
block by a Schulz–Zimm distribution (Eq. 121). In accord with the model of
Choi, Bates, and Lodge [63], the longest Rouse time was used as pre-exponential
factor as defined in Eq. 120. Independent of the morphology, good fits were only
obtained by taking β ¼ 1 implying a stretched conformation of the insoluble block
during the expulsion process (see Sect. 2.2.6.).
The parameter α assumed values that were about a factor of three smaller than
expected from the geometrical estimate. Parallel to the transition from cylinders to
spheres, α slightly decreases. However, this does not clearly reflect any influence of
the morphology because the fits are very sensitive to α and other influences (e.g.,
arising from experimental uncertainties) cannot be fully excluded. Therefore, in order
to delineate any factors arising from the different morphologies, Lund et al. have
exploited the fact that at 50% DMF fraction the transition can be also induced by
heating. Dissolution of the polymer at room temperature leads first to stable cylindrical
micelles, which transform into spherical entities after annealing for several hours at
70
C. Importantly, the transition is irreversible, meaning that the spherical shape is
preserved at low temperatures. The exchange kinetics was then measured in both
morphologies on the same specimen by TR-SANS under exactly the same conditions.
The authors found that the kinetics in spherical morphology is slightly but unambiguously faster than for cylinders, as shown in Fig. 30. With respect to the fit
parameters, the faster kinetics is reflected by a slightly smaller α while β ¼ 1 was
kept constant. This was found to be in accordance with the slightly more pronounced decrease in α for varying DMF composition. The value of β ¼ 1was in
contrast to β ¼ 2/3 found for PEP1-PEO20 star-like micelles in which the PEP1
chain assumes a completely segregated spherical bud. It was argued that a stretched
conformation facilitates passage through the more dense corona of the spherical and
cylindrical crew-cut type micelles obtained from PEP1-PEO1 block copolymers.
An interpretation of the small differences in α is, however, difficult. It was
speculated that this has its origin in small variances in the local coronal structure,
which may be different for the cylindrical and spherical morphologies.
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
131