cylinder-to-gyroid [199] transition kinetics. Because we are dealing here with
micellar solutions, these examples fall out of the scope of this review.
It has been widely reported that amphiphilic diblock copolymers can self-assemble
into various nanoscale morphologies such as spheres, cylinders, and vesicles
[46, 200]. The type of morphology that is obtained in a certain block copolymer
solvent system was found to depend on the hydrophilic fraction of the block copolymer [201, 202], the overall molecular weight [142], solvent quality [48, 203], and
temperature [204, 205]. Also, the polymer concentration was found to affect the
morphology and the size of the aggregates [203]. The transitions between different
morphologies of strong amphiphilic block copolymer aggregates in water are generally irreversible. In particular, vesicles are non-equilibrium structures in a trapped
metastable state analogous to frozen spherical micelles [46]. The kinetics of the
morphological transitions can be accelerated by the use of co-solvents with higher
compatibility to the insoluble block or by variation of temperature such that the
transitions are reversible [203–205]. Hence, it was argued that the speed of single
chain exchange also determines the reversibility of the transition process. However, in
a recent work by Lund et al. [48], it was shown that the transition from cylinders to
spheres is very fast whereas the reverse process will not take place on finite time
scales, although the single unimer exchange was measured to be very fast in the
transition region. We note that both morphologies are thermodynamically stable
within their range of existence, as was explicitly shown from model calculations
taking into account the variation of structural parameters with the interfacial tension
[48]. The irreversibility of the cylinder-to-sphere transition is thus due to a kinetic
hindrance, which generally implies that for morphological transitions mechanisms
other than pure chain exchange are active.
Unfortunately, kinetic studies of morphological transitions in block copolymer
micellar systems are very rare. Most important in this context is the work of
Eisenberg and colleagues, who explored the kinetics and mechanisms of various
transitions of polystyrene 310 -b-poly(acrylic acid) 52 (PS 310 -b-PAA 52 ) block copolymer in water/1,4-dioxane mixtures. A morphological phase diagram of the ternary
system, PS 310 -b-PAA 52 /water/1,4-dioxane was elaborated by Shen and Eisenberg
[203] employing freeze-drying transmission electron microscopy (TEM), turbidity
measurements, and static and dynamic light scattering. Starting from molecularly
dissolved block copolymer chains in 1,4-dioxane they found upon addition of water
the following sequence of morphologies: spheres, coexisting spheres and rods, rods,
coexisting rods and vesicles, and vesicles at all polymer concentrations. It was
found that in this system the appearance of the morphologies is reversible. The
formation of the coexisting mixtures of morphologies is thermodynamically controlled, as concluded from the fact that they are identically formed independent of
the pathway. Burke and Eisenberg [206] studied the kinetics and mechanisms
involved in the sphere-to-rod and rod-to-sphere transitions of this system by TEM
and turbidity measurements. The transition was induced by adding small amounts
of water close to the morphological boundary for the forward transition and
1,4-dioxane for the reverse transition. Analysis by double exponential equations
revealed two relaxation steps in both directions. The observed time scales in
150
R. Lund et al.
micellar solutions, these examples fall out of the scope of this review.
It has been widely reported that amphiphilic diblock copolymers can self-assemble
into various nanoscale morphologies such as spheres, cylinders, and vesicles
[46, 200]. The type of morphology that is obtained in a certain block copolymer
solvent system was found to depend on the hydrophilic fraction of the block copolymer [201, 202], the overall molecular weight [142], solvent quality [48, 203], and
temperature [204, 205]. Also, the polymer concentration was found to affect the
morphology and the size of the aggregates [203]. The transitions between different
morphologies of strong amphiphilic block copolymer aggregates in water are generally irreversible. In particular, vesicles are non-equilibrium structures in a trapped
metastable state analogous to frozen spherical micelles [46]. The kinetics of the
morphological transitions can be accelerated by the use of co-solvents with higher
compatibility to the insoluble block or by variation of temperature such that the
transitions are reversible [203–205]. Hence, it was argued that the speed of single
chain exchange also determines the reversibility of the transition process. However, in
a recent work by Lund et al. [48], it was shown that the transition from cylinders to
spheres is very fast whereas the reverse process will not take place on finite time
scales, although the single unimer exchange was measured to be very fast in the
transition region. We note that both morphologies are thermodynamically stable
within their range of existence, as was explicitly shown from model calculations
taking into account the variation of structural parameters with the interfacial tension
[48]. The irreversibility of the cylinder-to-sphere transition is thus due to a kinetic
hindrance, which generally implies that for morphological transitions mechanisms
other than pure chain exchange are active.
Unfortunately, kinetic studies of morphological transitions in block copolymer
micellar systems are very rare. Most important in this context is the work of
Eisenberg and colleagues, who explored the kinetics and mechanisms of various
transitions of polystyrene 310 -b-poly(acrylic acid) 52 (PS 310 -b-PAA 52 ) block copolymer in water/1,4-dioxane mixtures. A morphological phase diagram of the ternary
system, PS 310 -b-PAA 52 /water/1,4-dioxane was elaborated by Shen and Eisenberg
[203] employing freeze-drying transmission electron microscopy (TEM), turbidity
measurements, and static and dynamic light scattering. Starting from molecularly
dissolved block copolymer chains in 1,4-dioxane they found upon addition of water
the following sequence of morphologies: spheres, coexisting spheres and rods, rods,
coexisting rods and vesicles, and vesicles at all polymer concentrations. It was
found that in this system the appearance of the morphologies is reversible. The
formation of the coexisting mixtures of morphologies is thermodynamically controlled, as concluded from the fact that they are identically formed independent of
the pathway. Burke and Eisenberg [206] studied the kinetics and mechanisms
involved in the sphere-to-rod and rod-to-sphere transitions of this system by TEM
and turbidity measurements. The transition was induced by adding small amounts
of water close to the morphological boundary for the forward transition and
1,4-dioxane for the reverse transition. Analysis by double exponential equations
revealed two relaxation steps in both directions. The observed time scales in
150
R. Lund et al.