least one block is thermosensitive are used and micellization is induced by crossing
the cmt via a sudden temperature jump or quench. A disadvantage is, however, that
systems are usually in the proximity of the microphase boundary and thus typically
not well segregated, complicating comparisons with theories that often assume
strong segregation.
Honda et al. [169, 170] employed a thermosensitive poly(α-methylstyrene)–poly
(vinyl phenethyl alcohol) (PαMS-PVPA) block copolymer system with two molecular weights that micellize in benzyl alcohol below 24
C and 50
C, respectively. In
the experiments, the system was completely dissolved into unimers at 60
C and
subsequently abruptly quenched to various temperatures in the micellar region of
the phase diagram using a pre-thermostated sample and measuring cells. Subsequently, the system was monitored in real time using simultaneously time-resolved
static and dynamic light scattering. By investigating two molecular weights and
various temperatures, a thorough and systematic investigation of the system was
made. Interestingly, the micellization process was found to be very slow and
occurred on a time scale of hours. For the lowest molecular weight (8.1 kg/mol),
a single exponential growth was found whereas for the higher molecular weight
block copolymer (12.5 kg/mol) the kinetics was slower and could be fitted with a
double exponential decay. This behavior was interpreted in terms of nucleation and
growth type kinetics and a clear distinction was made with respect to the Aniansson
and Wall-type kinetics that predicts a double exponential behavior close to equilibrium. By comparing the information obtained from both dynamic and static light
scattering, it was observed that the radius of gyration and the hydrodynamic radius
increase more rapidly in the beginning compared to the molecular mass. The
polydispersity, obtained using a cumulant expansion, appeared to decrease towards
the end of the micellization process. The micellization process was therefore
pictured as having a rapid initial process that bears some similarity to a nucleation
and growth process and is characterized by an increase in the number of micelles.
The time constant of the first process was observed to decrease with increasing
concentration. This process is followed by a slower reorganization process, independent of concentration, reflecting an equilibration mechanism where the number
of micelles decreases but the micelles increase in overall size.
Small Temperature-Jump Studies of Pluronics
Hecht and Hoffman [126] investigated the kinetics of Pluronics micelles using a
capacitor discharge in the micellar solution containing electrolytes to increase the
conductivity and amplify the temperature jumps. Using this method, T-jumps ranging
from 0.05 to 2.4 K were obtained. The micellization was followed using light
scattering at a fixed angle of 90
. T-jumps were performed both below and above
cmt at different concentrations without any clear distinction between the nature of the
kinetic process. In other words premicellization, micellization and micelle–micelle
equilibration kinetics were investigated, leaving a theoretical comparison difficult.
This study thus indicates the existence of “pre-critical” micelles. Interestingly, the
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
135
the cmt via a sudden temperature jump or quench. A disadvantage is, however, that
systems are usually in the proximity of the microphase boundary and thus typically
not well segregated, complicating comparisons with theories that often assume
strong segregation.
Honda et al. [169, 170] employed a thermosensitive poly(α-methylstyrene)–poly
(vinyl phenethyl alcohol) (PαMS-PVPA) block copolymer system with two molecular weights that micellize in benzyl alcohol below 24
C and 50
C, respectively. In
the experiments, the system was completely dissolved into unimers at 60
C and
subsequently abruptly quenched to various temperatures in the micellar region of
the phase diagram using a pre-thermostated sample and measuring cells. Subsequently, the system was monitored in real time using simultaneously time-resolved
static and dynamic light scattering. By investigating two molecular weights and
various temperatures, a thorough and systematic investigation of the system was
made. Interestingly, the micellization process was found to be very slow and
occurred on a time scale of hours. For the lowest molecular weight (8.1 kg/mol),
a single exponential growth was found whereas for the higher molecular weight
block copolymer (12.5 kg/mol) the kinetics was slower and could be fitted with a
double exponential decay. This behavior was interpreted in terms of nucleation and
growth type kinetics and a clear distinction was made with respect to the Aniansson
and Wall-type kinetics that predicts a double exponential behavior close to equilibrium. By comparing the information obtained from both dynamic and static light
scattering, it was observed that the radius of gyration and the hydrodynamic radius
increase more rapidly in the beginning compared to the molecular mass. The
polydispersity, obtained using a cumulant expansion, appeared to decrease towards
the end of the micellization process. The micellization process was therefore
pictured as having a rapid initial process that bears some similarity to a nucleation
and growth process and is characterized by an increase in the number of micelles.
The time constant of the first process was observed to decrease with increasing
concentration. This process is followed by a slower reorganization process, independent of concentration, reflecting an equilibration mechanism where the number
of micelles decreases but the micelles increase in overall size.
Small Temperature-Jump Studies of Pluronics
Hecht and Hoffman [126] investigated the kinetics of Pluronics micelles using a
capacitor discharge in the micellar solution containing electrolytes to increase the
conductivity and amplify the temperature jumps. Using this method, T-jumps ranging
from 0.05 to 2.4 K were obtained. The micellization was followed using light
scattering at a fixed angle of 90
. T-jumps were performed both below and above
cmt at different concentrations without any clear distinction between the nature of the
kinetic process. In other words premicellization, micellization and micelle–micelle
equilibration kinetics were investigated, leaving a theoretical comparison difficult.
This study thus indicates the existence of “pre-critical” micelles. Interestingly, the
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
135