essentially independent of concentration. However, for the pH-induced micellization
process, both τ 1 and τ 2 were found to decrease with concentration. This difference
was attributed to electrostatic charges present in the corona during the thermally
induced micelization process, leading to a prevention of corona overlap and hence a
decreased probability of micellar fusion. Moreover, activation energies for both
processes were found to be very similar (about 35 and 38 kJ/mol, respectively) and
the time scales differed by a factor of about five. This was taken as an indication of a
predominant fusion/fission mechanism in both processes, although in this case these
time constants might reflect a continuum.
Influence of Salt Addition
Liu and coworkers investigated a series of A-B-C triblock copolymers, where the
C-block is selectively precipitated upon a pH-jump to alkaline conditions [47]. The
results showed very similar results as for the A-B diblock copolymers and was
again analyzed in terms of two relaxation constants, the first decreasing with
concentration and the second being concentration independent. Again, two very
similar apparent activation energies were found, where E a for the second, slower
process was even smaller than for the faster process. This indicates a larger entropic
barrier for the second process, leading to τ 2 > τ 1 . In a second paper [178], the effect
of addition of salt was studied. It was found that upon addition of salt, a gradual
increase in the concentration dependence of the second process was found. The
obtained results for τ 2 as a function of concentration are given for different added
salt amounts in Fig. 35. The results were interpreted as a signature of an increased
occurrence of fusion/fission due to screening of electrostatic interactions between
corona blocks. Interestingly, the micellization rate was faster at lower salt contents,
indicating that, although the fusion might occur in addition to unimer exchange, the
terminal process is slower, possibly due to higher interfacial energies and more
stable micelles. The latter may occur because addition of electrolytes is well known
to induce larger interfacial tensions between hydrocarbons and aqueous solutions
[180].
Ge et al. [179] investigated the formation kinetics of vesicles by pH-jumps in a
system consisting of a zwitterionic diblock copolymer, poly(2-(methacryloyloxy)
ethyl phosphorylcholine)–poly(2-(diisopropylamino)ethyl methacrylate) (PMPCPDPA) using a combination of stopped-flow and light scattering. The kinetics of
pH-induced formation was shown to be tentatively described by three distinct relaxation processes for the early stages of vesicle self-assembly (0–40 s). The kinetics of
vesicle formation in the later stage (for times longer than some minutes) was
investigated using dynamic light scattering. It was found that both the hydrodynamic
radius and the polydispersity decrease approximately exponentially with a single
characteristic relaxation time. The observed multiple time constants were interpreted
in terms of theoretical considerations, where the unimer-to-vesicle transition is proposed to proceed via a series of four steps: spherical micelles to cylindrical micelles to
lamellae platelets that are finally wrapped-up into vesicles [10, 181, 182]. Such a
142
R. Lund et al.
process, both τ 1 and τ 2 were found to decrease with concentration. This difference
was attributed to electrostatic charges present in the corona during the thermally
induced micelization process, leading to a prevention of corona overlap and hence a
decreased probability of micellar fusion. Moreover, activation energies for both
processes were found to be very similar (about 35 and 38 kJ/mol, respectively) and
the time scales differed by a factor of about five. This was taken as an indication of a
predominant fusion/fission mechanism in both processes, although in this case these
time constants might reflect a continuum.
Influence of Salt Addition
Liu and coworkers investigated a series of A-B-C triblock copolymers, where the
C-block is selectively precipitated upon a pH-jump to alkaline conditions [47]. The
results showed very similar results as for the A-B diblock copolymers and was
again analyzed in terms of two relaxation constants, the first decreasing with
concentration and the second being concentration independent. Again, two very
similar apparent activation energies were found, where E a for the second, slower
process was even smaller than for the faster process. This indicates a larger entropic
barrier for the second process, leading to τ 2 > τ 1 . In a second paper [178], the effect
of addition of salt was studied. It was found that upon addition of salt, a gradual
increase in the concentration dependence of the second process was found. The
obtained results for τ 2 as a function of concentration are given for different added
salt amounts in Fig. 35. The results were interpreted as a signature of an increased
occurrence of fusion/fission due to screening of electrostatic interactions between
corona blocks. Interestingly, the micellization rate was faster at lower salt contents,
indicating that, although the fusion might occur in addition to unimer exchange, the
terminal process is slower, possibly due to higher interfacial energies and more
stable micelles. The latter may occur because addition of electrolytes is well known
to induce larger interfacial tensions between hydrocarbons and aqueous solutions
[180].
Ge et al. [179] investigated the formation kinetics of vesicles by pH-jumps in a
system consisting of a zwitterionic diblock copolymer, poly(2-(methacryloyloxy)
ethyl phosphorylcholine)–poly(2-(diisopropylamino)ethyl methacrylate) (PMPCPDPA) using a combination of stopped-flow and light scattering. The kinetics of
pH-induced formation was shown to be tentatively described by three distinct relaxation processes for the early stages of vesicle self-assembly (0–40 s). The kinetics of
vesicle formation in the later stage (for times longer than some minutes) was
investigated using dynamic light scattering. It was found that both the hydrodynamic
radius and the polydispersity decrease approximately exponentially with a single
characteristic relaxation time. The observed multiple time constants were interpreted
in terms of theoretical considerations, where the unimer-to-vesicle transition is proposed to proceed via a series of four steps: spherical micelles to cylindrical micelles to
lamellae platelets that are finally wrapped-up into vesicles [10, 181, 182]. Such a
142
R. Lund et al.