forward and backward directions were found to be comparable to each other. In the
forward direction, the evaluation of TEM micrographs showed the formation of an
irregular pearl necklace structure by adhesive collision of spheres, which reorganize in a second slow step into smooth rods. The inverse transition starts with the
formation of bulbs at the end of the rods, which in the rate-determining second step
are released from the ends of the cylindrical body. The kinetics of both transitions
was found to depend on the initial solvent composition, the magnitude of the
solvent jump, and the initial polymer concentration. Systematic studies have
shown that the kinetics are mostly affected by the initial conditions, i.e., location
in the morphological phase diagram.
Using the same techniques as described above, Chen et al. [207] studied the
rod-to-vesicle transition by adding water to the ternary system, PS 310 -b-PAA 52 /
water/1,4-dioxane in the corresponding region of the phase diagram. Similar to
the observation in the sphere-to-rod transition, the kinetics were governed by
two consecutive steps. The first step involves the flattening of short rods to
circular lamellae followed by the second step, which is the closing of the
vesicles. The rates of the shape transformation crucially depend on the initial
water content. The studies revealed that the larger the initial water content, the
slower the relaxation times. This was also the case for increasing polymer
concentration, while the size of the jump did not change the relaxation times
significantly.
The corresponding vesicle-to-rod transition was investigated by Burke and
Eisenberg [208]. The transition, monitored by turbidity measurements, could be
described by a single relaxation step. The TEM micrographs revealed that the
reorganization to rods proceeds through a sequence of intermediate structures:
first the deformation of a vesicle to a “bowtie” shape structure was seen, which
afterwards develops into dumbbells. Elongation of the long axis finally leads to the
formation of thin rods. It was argued that the driving force is the increase in the
curvature energy due to the change in the bilayer thickness. The instability is then
relaxed by the transformation to thinner rods which, apparently, is the energetically
favorable morphology for the final solvent composition. In summary, the work by
Eisenberg and colleagues impressively demonstrates that the transition between
different morphologies proceeds via different pathways, including the formation of
different irregular intermediate structures. Thus this work reveals that, besides
single chain exchange, other mechanisms are important for a shape transformation.
However, reorganization of structures will not take place whenever unimer
exchange is not existent, i.e., aggregates are in a frozen metastable state. We note
that, except for the studies of Eisenberg and colleagues, there is a lack of systematic
studies on the kinetics and mechanisms of morphological transitions. Time resolved
small angle scattering on suitable block copolymer/solvent systems may help to
obtain a clear picture of the general mechanisms governing morphological
transitions in block copolymer micelles.
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
151
forward direction, the evaluation of TEM micrographs showed the formation of an
irregular pearl necklace structure by adhesive collision of spheres, which reorganize in a second slow step into smooth rods. The inverse transition starts with the
formation of bulbs at the end of the rods, which in the rate-determining second step
are released from the ends of the cylindrical body. The kinetics of both transitions
was found to depend on the initial solvent composition, the magnitude of the
solvent jump, and the initial polymer concentration. Systematic studies have
shown that the kinetics are mostly affected by the initial conditions, i.e., location
in the morphological phase diagram.
Using the same techniques as described above, Chen et al. [207] studied the
rod-to-vesicle transition by adding water to the ternary system, PS 310 -b-PAA 52 /
water/1,4-dioxane in the corresponding region of the phase diagram. Similar to
the observation in the sphere-to-rod transition, the kinetics were governed by
two consecutive steps. The first step involves the flattening of short rods to
circular lamellae followed by the second step, which is the closing of the
vesicles. The rates of the shape transformation crucially depend on the initial
water content. The studies revealed that the larger the initial water content, the
slower the relaxation times. This was also the case for increasing polymer
concentration, while the size of the jump did not change the relaxation times
significantly.
The corresponding vesicle-to-rod transition was investigated by Burke and
Eisenberg [208]. The transition, monitored by turbidity measurements, could be
described by a single relaxation step. The TEM micrographs revealed that the
reorganization to rods proceeds through a sequence of intermediate structures:
first the deformation of a vesicle to a “bowtie” shape structure was seen, which
afterwards develops into dumbbells. Elongation of the long axis finally leads to the
formation of thin rods. It was argued that the driving force is the increase in the
curvature energy due to the change in the bilayer thickness. The instability is then
relaxed by the transformation to thinner rods which, apparently, is the energetically
favorable morphology for the final solvent composition. In summary, the work by
Eisenberg and colleagues impressively demonstrates that the transition between
different morphologies proceeds via different pathways, including the formation of
different irregular intermediate structures. Thus this work reveals that, besides
single chain exchange, other mechanisms are important for a shape transformation.
However, reorganization of structures will not take place whenever unimer
exchange is not existent, i.e., aggregates are in a frozen metastable state. We note
that, except for the studies of Eisenberg and colleagues, there is a lack of systematic
studies on the kinetics and mechanisms of morphological transitions. Time resolved
small angle scattering on suitable block copolymer/solvent systems may help to
obtain a clear picture of the general mechanisms governing morphological
transitions in block copolymer micelles.
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
151