We point out that the rate thus depends double exponentially on the interfacial
tension, the temperature, and the degree of polymerization of the core block. Hence,
these parameters are very effective for tuning the speed of chain exchange.
The expression for the expulsion rate constant further contains a pre-exponential
factor f(N A , N B ), which is determined by the micellar structure, i.e., crew-cut or
star-like. Tuning by N A will therefore be less effective because the rate depends
only single-exponentially. Importantly, one should be aware that changing one or
more of these parameters will also affect the underlying structure of the micelle.
Therefore, a study of exchange kinetics requires a complementary structural characterization for each set of parameters.
Several strategies have been used to control the kinetics in polymeric micelles,
including variation of temperature, change of the hydrophobic block size, and
adjustment of the interfacial tension between core block and solvent [62, 63,
100–103, 114, 119, 120, 146–152]. We will review and discuss the effect of tuning
on the kinetics by variation of γ because this parameter has been primarily used to
control and to modify the exchange dynamics in micellar aggregates.
The interfacial tension can be effectively varied by modifying the incompatibility between core block and selective solvent. In general, for block copolymers with
strong amphiphilicity in water, γ is very large such that even for short chains the
micelles are kinetically frozen on experimental time scales. This applies especially
for amphiphilic block copolymers with polybutadiene (PB), polyisoprene (PI),
polystyrene (PS), poly(ethylene-alt-propylene) (PEP), or poly(butylene oxide)
(PBO) as the hydrophobic block, having interfacial tensions against water typically
larger than 30 mN/m. Several strategies have been employed to reduce γ in order to
overcome the high barriers for chain exchange. One approach is the addition of
small surfactant molecules. In the work of Jacquin et al. [151], the melting of
kinetically frozen poly(butyl acrylate)-block-poly(acrylic acid) (PBA-PAA)
micelles was investigated. They observed structural transitions from polymeric
cylindrical micelles to spherical micelles as well as from large spherical to small
spherical micelles upon addition of surfactant. Pendant drop tensiometry on PBA
homopolymer in water and in surfactant solution revealed a drop in the interfacial
tension from 20 mN/m to 5–8 mN/m. The low value of γ was explained by the
incorporation of surfactant molecules into the interface, leading to the transition
from frozen polymeric micelles to equilibrated surfactant/block copolymer mixed
micelles.
Lejeune et al. [153] employed a chemical approach to lowering of interfacial
tension in poly(n-butyl acrylate)-(polyacrylic acid) (PnBA-PAA). PnBA-PAA
forms kinetically frozen micelles in water that are not able to reorganize over a
month. By statistical incorporation of hydrophilic acrylic acid (AA) units into the
hydrophobic PnBA block, P(nBA 50% -stat-AA 50% )-PAA, they could moderate the
hydrophobicity of the core block such that unimer exchange was promoted and
thermodynamic equilibrium was reached at shorter times.
A more straightforward and facile way to tune the kinetics via reduction of the
core–corona interfacial tension is by the addition of co-solvents. In the case of
PEP-PEO micelles, the use of DMF/water mixtures as selective solvent for PEO
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R. Lund et al.
tension, the temperature, and the degree of polymerization of the core block. Hence,
these parameters are very effective for tuning the speed of chain exchange.
The expression for the expulsion rate constant further contains a pre-exponential
factor f(N A , N B ), which is determined by the micellar structure, i.e., crew-cut or
star-like. Tuning by N A will therefore be less effective because the rate depends
only single-exponentially. Importantly, one should be aware that changing one or
more of these parameters will also affect the underlying structure of the micelle.
Therefore, a study of exchange kinetics requires a complementary structural characterization for each set of parameters.
Several strategies have been used to control the kinetics in polymeric micelles,
including variation of temperature, change of the hydrophobic block size, and
adjustment of the interfacial tension between core block and solvent [62, 63,
100–103, 114, 119, 120, 146–152]. We will review and discuss the effect of tuning
on the kinetics by variation of γ because this parameter has been primarily used to
control and to modify the exchange dynamics in micellar aggregates.
The interfacial tension can be effectively varied by modifying the incompatibility between core block and selective solvent. In general, for block copolymers with
strong amphiphilicity in water, γ is very large such that even for short chains the
micelles are kinetically frozen on experimental time scales. This applies especially
for amphiphilic block copolymers with polybutadiene (PB), polyisoprene (PI),
polystyrene (PS), poly(ethylene-alt-propylene) (PEP), or poly(butylene oxide)
(PBO) as the hydrophobic block, having interfacial tensions against water typically
larger than 30 mN/m. Several strategies have been employed to reduce γ in order to
overcome the high barriers for chain exchange. One approach is the addition of
small surfactant molecules. In the work of Jacquin et al. [151], the melting of
kinetically frozen poly(butyl acrylate)-block-poly(acrylic acid) (PBA-PAA)
micelles was investigated. They observed structural transitions from polymeric
cylindrical micelles to spherical micelles as well as from large spherical to small
spherical micelles upon addition of surfactant. Pendant drop tensiometry on PBA
homopolymer in water and in surfactant solution revealed a drop in the interfacial
tension from 20 mN/m to 5–8 mN/m. The low value of γ was explained by the
incorporation of surfactant molecules into the interface, leading to the transition
from frozen polymeric micelles to equilibrated surfactant/block copolymer mixed
micelles.
Lejeune et al. [153] employed a chemical approach to lowering of interfacial
tension in poly(n-butyl acrylate)-(polyacrylic acid) (PnBA-PAA). PnBA-PAA
forms kinetically frozen micelles in water that are not able to reorganize over a
month. By statistical incorporation of hydrophilic acrylic acid (AA) units into the
hydrophobic PnBA block, P(nBA 50% -stat-AA 50% )-PAA, they could moderate the
hydrophobicity of the core block such that unimer exchange was promoted and
thermodynamic equilibrium was reached at shorter times.
A more straightforward and facile way to tune the kinetics via reduction of the
core–corona interfacial tension is by the addition of co-solvents. In the case of
PEP-PEO micelles, the use of DMF/water mixtures as selective solvent for PEO
118
R. Lund et al.