Chain Length Dependence
In a study by Zhang et al. [177], the effect of hydrophobic chain length on the
micellization kinetics was investigated in a series of poly(ethylene oxide)–poly
(2-diethylamino) ethyl methacrylate) (PEO-PDEA) block copolymers with varying
PDEA molecular weight. Again, the time dependence of the scattered light was
approximated by a growth function consisting of a sum of two exponentials. Interestingly, the time constant of the second, slow process slightly decreases with increasing
hydrophobic block length. This was accompanied by a tendency for increasing
concentration dependence as well as a decreasing apparent activation energy with
chain length. These rather unexpected results were interpreted as an increasing
dominance of fusion and fission processes because of an increasing suppression of
pathways involving unimer exchange (increasing hydrophobicity). The activation
energy, E a , varied from about 8 to 3 kJ/mol, which seems rather low for a fusion
process. However, the measurements were performed on time scales from some
milliseconds to seconds and it is not clear whether the terminal relaxation reflects
the final equilibration to the true equilibrium state. As the final relaxation may be
exceedingly slow, it might be that the measured τ 2 only reflects an apparent value.
“Schizophrenic” Block Copolymer Micelle Systems
In a series of studies [8, 175], so-called schizophrenic systems were investigated.
These are diblock copolymers that may undergo micellization with either one block
or the other to form the micellar core, depending on pH and/or salt concentration. In
one study, for micelles formed by poly(4-vinylbenzoic acid)–poly(N-morpholino
ethyl methacrylate) (PVBA-PMEMA), the results show that the micellization
process induced by a pH-jump exhibited a fast initial growth and a slow terminal
growth, where the latter could not be described by a simple relaxation constant.
Consequently, only the initial part of the data was fitted to a two-exponential growth
function. With this description, the initial process was found to accelerate upon
increasing the concentration whereas the second process remained roughly constant. Salt-induced micellization, however, led to a τ 1 that was roughly independent
of concentration while τ 2 decreased. Interestingly, the micellar dissolution kinetics
induced by dilution or an inverse pH-jump led to a very fast decay that could be
described with a single exponential. Also, the remicellization kinetics (“inversion
kinetics”), whereby the systems are transformed from micelles with one coreforming block to the other, was investigated. The associated kinetics exhibited in
some cases an initial decay followed by a slower growth characterizing the reequilibration of the micelles.
In a work on a double stimuli-responsive PNIPAM-PDEA block copolymer,
micellization was triggered by either precipitating the PDEA upon a pH-jump or by
performing a T-jump to selectively precipitate the PNIPAM block. The kinetics upon
T-jumps has already been discussed above and was found to be represented by two
relaxation constants: τ 1 was found to decrease with concentration while τ 2 was
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
141
In a study by Zhang et al. [177], the effect of hydrophobic chain length on the
micellization kinetics was investigated in a series of poly(ethylene oxide)–poly
(2-diethylamino) ethyl methacrylate) (PEO-PDEA) block copolymers with varying
PDEA molecular weight. Again, the time dependence of the scattered light was
approximated by a growth function consisting of a sum of two exponentials. Interestingly, the time constant of the second, slow process slightly decreases with increasing
hydrophobic block length. This was accompanied by a tendency for increasing
concentration dependence as well as a decreasing apparent activation energy with
chain length. These rather unexpected results were interpreted as an increasing
dominance of fusion and fission processes because of an increasing suppression of
pathways involving unimer exchange (increasing hydrophobicity). The activation
energy, E a , varied from about 8 to 3 kJ/mol, which seems rather low for a fusion
process. However, the measurements were performed on time scales from some
milliseconds to seconds and it is not clear whether the terminal relaxation reflects
the final equilibration to the true equilibrium state. As the final relaxation may be
exceedingly slow, it might be that the measured τ 2 only reflects an apparent value.
“Schizophrenic” Block Copolymer Micelle Systems
In a series of studies [8, 175], so-called schizophrenic systems were investigated.
These are diblock copolymers that may undergo micellization with either one block
or the other to form the micellar core, depending on pH and/or salt concentration. In
one study, for micelles formed by poly(4-vinylbenzoic acid)–poly(N-morpholino
ethyl methacrylate) (PVBA-PMEMA), the results show that the micellization
process induced by a pH-jump exhibited a fast initial growth and a slow terminal
growth, where the latter could not be described by a simple relaxation constant.
Consequently, only the initial part of the data was fitted to a two-exponential growth
function. With this description, the initial process was found to accelerate upon
increasing the concentration whereas the second process remained roughly constant. Salt-induced micellization, however, led to a τ 1 that was roughly independent
of concentration while τ 2 decreased. Interestingly, the micellar dissolution kinetics
induced by dilution or an inverse pH-jump led to a very fast decay that could be
described with a single exponential. Also, the remicellization kinetics (“inversion
kinetics”), whereby the systems are transformed from micelles with one coreforming block to the other, was investigated. The associated kinetics exhibited in
some cases an initial decay followed by a slower growth characterizing the reequilibration of the micelles.
In a work on a double stimuli-responsive PNIPAM-PDEA block copolymer,
micellization was triggered by either precipitating the PDEA upon a pH-jump or by
performing a T-jump to selectively precipitate the PNIPAM block. The kinetics upon
T-jumps has already been discussed above and was found to be represented by two
relaxation constants: τ 1 was found to decrease with concentration while τ 2 was
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
141