copolymer (numbers denote approximate molecular weight in kg/mol) was done by
SANS using contrast variation and model fitting [87]. Fit results simultaneously
obtained on four different contrasts revealed a spherical shape of the micelles,
consisting of a compact solvent-free micellar core and a highly swollen PEO
corona. It was found that the micelles have unusually large aggregation numbers
of P ¼ 2,430 and rather large dimensions. It was shown from thermodynamic
calculations based on a mean-field model of Nagarajan and Ganesh [28] that this
is a consequence of the large interfacial tension between PEP and water.
In a series of experiments, aqueous dispersions of symmetric PEP-PEO block
copolymers were studied over a wide range of molecular weights, always keeping
the ratio between the volumes of the blocks constant [142]. The scattering behavior
of the solutions showed that a morphological transition takes place upon lowering
the molecular weight. The high molecular weight materials all formed spherical,
almost monodisperse, micelles with large aggregation numbers. At low molecular
weights, however, cylindrical micelles were observed. An interesting intermediate
case is represented by the PEP2-PEO2 system. Here spherical micelles were found
at higher concentrations while cylinders occurred at larger dilutions.
The effect of a growing soluble block on the morphology of the micelles was
investigated by varying the molecular weight of the PEO from about 5 to 120 kg/mol
while keeping the PEP block constant at 5 kg/mol for all polymers [44]. Thus, a
systematic study of the aggregation number and the corona shape became possible
over a large range of compositional asymmetry (1:1 ! 1:24). Partial labeling of the
block copolymers allowed highlighting corona structures individually by matching
out core contributions using D 2 O/H 2 O mixtures. Data analysis using a spherical
core–shell model with variable density profile indicated a crossover from a practically
homogeneous corona profile for more symmetric diblocks to a star-like profile at
larger asymmetries. Notably was the observation that PEP-PEO block copolymers
aggregate into micelles even in a large compositional asymmetry, realizing thereby
micelles with a star-like profile. It was concluded from free energy considerations that
this is only possible because of the high interfacial energy as the main contribution.
In contrast to the study of Willner et al. [44], a cylinder-to-sphere transition was
observed by Jensen et al. [143, 144] upon increasing the PEO block molecular
weight in a similar molecular weight range. By SAXS and SLS it was found that
PEP5-PEO5, PEP5-PEO10, and PEP5-PEO20 form cylindrical micelles whereas
PEP5-PEO40 micelles are spherical. However, no explanation was given for the
different molecular weight dependence of the morphology found in this work as
compared to the work by Willner et al. [44].
The effect of interfacial tension γ on the micellar structure of a highly asymmetric PEP1-PEO20 block copolymer was examined using binary solvent mixtures of
water and DMF as selective solvents [45, 145]. DMF and water are both good
solvents for PEO and non-solvents for PEP, but exhibit a large difference in γ with
respect to the insoluble core block. The micellar characteristics were obtained by
SANS and subsequent fitting with a spherical core–shell form factor. Scattering
curves together with model fits for selected water/DMF mixtures are depicted in
Fig. 19a. As anticipated from the large asymmetry in block composition, the
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
113
SANS using contrast variation and model fitting [87]. Fit results simultaneously
obtained on four different contrasts revealed a spherical shape of the micelles,
consisting of a compact solvent-free micellar core and a highly swollen PEO
corona. It was found that the micelles have unusually large aggregation numbers
of P ¼ 2,430 and rather large dimensions. It was shown from thermodynamic
calculations based on a mean-field model of Nagarajan and Ganesh [28] that this
is a consequence of the large interfacial tension between PEP and water.
In a series of experiments, aqueous dispersions of symmetric PEP-PEO block
copolymers were studied over a wide range of molecular weights, always keeping
the ratio between the volumes of the blocks constant [142]. The scattering behavior
of the solutions showed that a morphological transition takes place upon lowering
the molecular weight. The high molecular weight materials all formed spherical,
almost monodisperse, micelles with large aggregation numbers. At low molecular
weights, however, cylindrical micelles were observed. An interesting intermediate
case is represented by the PEP2-PEO2 system. Here spherical micelles were found
at higher concentrations while cylinders occurred at larger dilutions.
The effect of a growing soluble block on the morphology of the micelles was
investigated by varying the molecular weight of the PEO from about 5 to 120 kg/mol
while keeping the PEP block constant at 5 kg/mol for all polymers [44]. Thus, a
systematic study of the aggregation number and the corona shape became possible
over a large range of compositional asymmetry (1:1 ! 1:24). Partial labeling of the
block copolymers allowed highlighting corona structures individually by matching
out core contributions using D 2 O/H 2 O mixtures. Data analysis using a spherical
core–shell model with variable density profile indicated a crossover from a practically
homogeneous corona profile for more symmetric diblocks to a star-like profile at
larger asymmetries. Notably was the observation that PEP-PEO block copolymers
aggregate into micelles even in a large compositional asymmetry, realizing thereby
micelles with a star-like profile. It was concluded from free energy considerations that
this is only possible because of the high interfacial energy as the main contribution.
In contrast to the study of Willner et al. [44], a cylinder-to-sphere transition was
observed by Jensen et al. [143, 144] upon increasing the PEO block molecular
weight in a similar molecular weight range. By SAXS and SLS it was found that
PEP5-PEO5, PEP5-PEO10, and PEP5-PEO20 form cylindrical micelles whereas
PEP5-PEO40 micelles are spherical. However, no explanation was given for the
different molecular weight dependence of the morphology found in this work as
compared to the work by Willner et al. [44].
The effect of interfacial tension γ on the micellar structure of a highly asymmetric PEP1-PEO20 block copolymer was examined using binary solvent mixtures of
water and DMF as selective solvents [45, 145]. DMF and water are both good
solvents for PEO and non-solvents for PEP, but exhibit a large difference in γ with
respect to the insoluble core block. The micellar characteristics were obtained by
SANS and subsequent fitting with a spherical core–shell form factor. Scattering
curves together with model fits for selected water/DMF mixtures are depicted in
Fig. 19a. As anticipated from the large asymmetry in block composition, the
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
113