does not significantly change with the solvent size because γ stays almost constant.
Therefore, in order to explain the effect of solvent chain length, it was considered that
PS in the bulk state has a glass transition temperature (T g ) of 105
C but when swollen
with n-alkanes, T g is significantly reduced. The swelling ability and solvent quality,
however, depend drastically on the number of carbons, n, of the n-alkyl solvent,
which in turn also changes T g . For example, for a bulk PS containing 18 vol%
n-heptane, the glass transition is reduced from 105
C to À11
C whereas with
15 vol% n-octane T g is only lowered to 40
C [156]. Quintana et al. [157, 158] have
studied the micellization behavior of PS-PEP polymers in n-alkanes. They reported
that the dependence of micellar properties on the temperature is decreased for longer
n-alkanes. This was explained by a higher T g of the PS core that is swollen to a lesser
extent as n increases. This trend was further confirmed by structural studies of PS-PEP
micelles in squalane by Choi et al. [159]. Squalane is a natural hydrocarbon with the
molecular formula C 30 H 62 . Dynamic light scattering and SAXS experiments showed
that below 100
C micellar cores are practically unswollen. Clear penetration of
solvent into the PS core was observed only above 100
C close to the bulk T g of PS.
Nevertheless, the glass transition of PS-rich domains of PS-PI polymers in squalane
was found to be considerably reduced as measured by Lai et al. [160]. They found
that, depending on the PS weight fraction in solution, T g is reduced to 70
C. Based on
these findings it is reasonable to assume that the measured decrease in the exchange
dynamics of PS-PB polymers in n-alkanes is directly related to the increase in T g of
the PS core.
In order to be independent of any feature related to the glass transition, the kinetics
of PS-PB micelles formed in DMF were additionally studied by Lund et al. [101].
DMF is selective for PS such that the micelles consist of a melt-like PB core with a T g
of À95
C and a swollen PS corona. Structural studies by SANS have shown that the
PB core is solvent-free and the corona has a compact structure with a constant
polymer density distribution [155]. Hence, in DMF these micelles are inverted
analogues to those in n-alkanes but with unswollen cores. Exchange kinetics could
be conveniently measured by TR-SANS in a time range of 10 h until an almost
statistical distribution of proteated and deuterated block copolymers across the
micelles was reached. A thorough evaluation of the relaxation kinetics of the
symmetric PS-PB block copolymer micelles in n-hexadecane and in DMF shows in
agreement with the exchange kinetics in PEP-PEO micelles in water/DMF, i.e., a
logarithmic time dependence indicating an extremely broad distribution of relaxation
rates (Figs. 23 and 24).
Because this observation was obtained independently from three structurally
different types of micelles, it was concluded that the broad relaxation is an inherent
property of block copolymer micelles. Consistent with these findings is the almost
linear dependence of R(t) on a log-time scale of PS-PEP micelles in squalane
presented by Choi et al. [63]. They used TR-SANS to study two pairs of PS-PEP
micelles, d-PS–h-PEP-1/h-PS–h-PEP-1 and d-PS–h-PEP-2/h-PS–h-PEP-2 with different PS degrees of polymerization: pair 1, N PS % 255 and pair 2, N PS % 412. Each
specimen was measured at three different temperatures. Individual master curves
for R(t) were obtained by time–temperature superposition principles. A comparison
of R(t) of the two PS-PEP samples was done at a reference temperature of 125
C and
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
123
Therefore, in order to explain the effect of solvent chain length, it was considered that
PS in the bulk state has a glass transition temperature (T g ) of 105
C but when swollen
with n-alkanes, T g is significantly reduced. The swelling ability and solvent quality,
however, depend drastically on the number of carbons, n, of the n-alkyl solvent,
which in turn also changes T g . For example, for a bulk PS containing 18 vol%
n-heptane, the glass transition is reduced from 105
C to À11
C whereas with
15 vol% n-octane T g is only lowered to 40
C [156]. Quintana et al. [157, 158] have
studied the micellization behavior of PS-PEP polymers in n-alkanes. They reported
that the dependence of micellar properties on the temperature is decreased for longer
n-alkanes. This was explained by a higher T g of the PS core that is swollen to a lesser
extent as n increases. This trend was further confirmed by structural studies of PS-PEP
micelles in squalane by Choi et al. [159]. Squalane is a natural hydrocarbon with the
molecular formula C 30 H 62 . Dynamic light scattering and SAXS experiments showed
that below 100
C micellar cores are practically unswollen. Clear penetration of
solvent into the PS core was observed only above 100
C close to the bulk T g of PS.
Nevertheless, the glass transition of PS-rich domains of PS-PI polymers in squalane
was found to be considerably reduced as measured by Lai et al. [160]. They found
that, depending on the PS weight fraction in solution, T g is reduced to 70
C. Based on
these findings it is reasonable to assume that the measured decrease in the exchange
dynamics of PS-PB polymers in n-alkanes is directly related to the increase in T g of
the PS core.
In order to be independent of any feature related to the glass transition, the kinetics
of PS-PB micelles formed in DMF were additionally studied by Lund et al. [101].
DMF is selective for PS such that the micelles consist of a melt-like PB core with a T g
of À95
C and a swollen PS corona. Structural studies by SANS have shown that the
PB core is solvent-free and the corona has a compact structure with a constant
polymer density distribution [155]. Hence, in DMF these micelles are inverted
analogues to those in n-alkanes but with unswollen cores. Exchange kinetics could
be conveniently measured by TR-SANS in a time range of 10 h until an almost
statistical distribution of proteated and deuterated block copolymers across the
micelles was reached. A thorough evaluation of the relaxation kinetics of the
symmetric PS-PB block copolymer micelles in n-hexadecane and in DMF shows in
agreement with the exchange kinetics in PEP-PEO micelles in water/DMF, i.e., a
logarithmic time dependence indicating an extremely broad distribution of relaxation
rates (Figs. 23 and 24).
Because this observation was obtained independently from three structurally
different types of micelles, it was concluded that the broad relaxation is an inherent
property of block copolymer micelles. Consistent with these findings is the almost
linear dependence of R(t) on a log-time scale of PS-PEP micelles in squalane
presented by Choi et al. [63]. They used TR-SANS to study two pairs of PS-PEP
micelles, d-PS–h-PEP-1/h-PS–h-PEP-1 and d-PS–h-PEP-2/h-PS–h-PEP-2 with different PS degrees of polymerization: pair 1, N PS % 255 and pair 2, N PS % 412. Each
specimen was measured at three different temperatures. Individual master curves
for R(t) were obtained by time–temperature superposition principles. A comparison
of R(t) of the two PS-PEP samples was done at a reference temperature of 125
C and
Kinetics of Block Copolymer Micelles Studied by Small-Angle Scattering Methods
123