3 research outputs found
Chain Exchange Kinetics in Diblock Copolymer Micelles in Ionic Liquids: The Role of χ
Chain exchange kinetics of diblock
copolymer micelles with lower
critical micellization temperature (LCMT) phase behavior were investigated
using time-resolved small-angle neutron scattering (TR-SANS). Three
nearly identical isotopically substituted pairs of poly(methyl methacrylate)-<i>block</i>-poly(<i>n</i>-butyl methacrylate) (PMMA-<i>b</i>-PnBMA) diblocks were used in mixtures of the room temperature
ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.
In this case, the <i>h-</i>PnBMA and <i>d</i><sub>9</sub>-PnBMA blocks form the micellar cores. The results are consistent
with previous measurements in other systems, in that the barrier to
chain extraction scales linearly with the core block length. By varying
the ratio of the two homologous solvents in the mixture, the value
of χ between the core block and the solvent is varied systematically.
The results show that the solvent selectivity has a remarkable effect
on the chain exchange rate, as anticipated by a previous theory. However,
in contrast to an assumption in previous studies, we find that the
barrier to chain exchange is not simply proportional to χ. Accordingly,
we propose a more elaborate function of χ for the energy barrier,
which is rationalized by a calculation in the spirit of Flory–Huggins
theory. This modification can account for the chain exchange behavior
when χ is relatively modest, i.e., in the vicinity of the critical
micelle temperature
Poly(methyl methacrylate)-<i>block</i>-poly(<i>n</i>-butyl methacrylate) Diblock Copolymer Micelles in an Ionic Liquid: Scaling of Core and Corona Size with Core Block Length
The structure of poly(methyl methacrylate)-<i>block</i>-poly(<i>n</i>-butyl methacrylate) (PMMA-<i>b</i>-PnBMA) micelles in the room temperature ionic liquid 1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), a selective solvent
for the PMMA block, has been studied using dynamic light scattering
(DLS) and small-angle X-ray scattering (SAXS). A series of seven PMMA-<i>b</i>-PnBMA diblock copolymers were prepared by reversible addition–fragmentation
chain-transfer (RAFT) polymerization, in which the degree of polymerization
of the PMMA block was kept constant while the PnBMA block length was
varied. All the polymers formed spherical micelles at ambient temperature
in dilute solution; their hydrodynamic radius (<i>R</i><sub>h</sub>) and core radius (<i>R</i><sub>c</sub>) were obtained
by DLS and SAXS, respectively. It was found that <i>R</i><sub>c</sub> and the degree of polymerization of the core block, <i>N</i><sub>B</sub>, followed a power law relationship in which <i>R</i><sub>c</sub> ∼ <i>N</i><sub>B</sub><sup>0.71±0.01</sup>. The corona thickness (<i>L</i><sub>corona</sub>), given by the difference of <i>R</i><sub>h</sub> and <i>R</i><sub>c</sub>, does not show any apparent
dependence on <i>N</i><sub>B</sub>. These results were compared
to scaling theory, and were found to be only in partial agreement
with the star model proposed by Halperin et al. However, the mean-field
calculations of micellar dimensions by Nagarajan and Ganesh were in
excellent agreement with the data. This comprehensive experimental
study provides precise quantification of the <i>R</i><sub>c</sub> and <i>L</i><sub>corona</sub> dependence on core
block lengths, due to the use of seven different block copolymers
with identical corona block lengths
Exchange Kinetics for a Single Block Copolymer in Micelles of Two Different Sizes
The effect of micellar size on the
chain exchange kinetics in spherical
micelles consisting of poly(methyl methacrylate)-<i>block</i>-poly(<i>n</i>-butyl methacrylate) (PMMA-<i>b</i>-PnBMA) in a mixture of ionic liquids (1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide, [EMIM][TFSI], and 1-butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide, [BMIM][TFSI]) was investigated
using time-resolved small-angle neutron scattering (TR-SANS). Two
spherical micelles with different core sizes were prepared from a
single block copolymer by using different protocols. In one case the
micelles were formed in the presence of a cosolvent, while in the
other a polymer thin film was directly dissolved in the ionic liquid.
Interestingly, the micelle core size prepared from the latter method
is ∼30% larger than that obtained in the former case. TR-SANS
experiments reveal that the rate of single chain exchange in the micelles
with a larger core size is slowed down by ∼3 times compared
to the smaller core radius. This can be possibly attributed to the
smaller interfacial area per chain, and larger corona density, for
micelles with a larger core dimension. These geometrical factors can
potentially lead to changes in both the attempt time and activation
barrier for chain expulsion during the unimer exchange process. Our
results clearly suggest that, in addition to the molecular characteristics
of the block copolymer and solvent, the geometrical structure of the
micelle plays an important role in the unimer dynamic exchange processes
in block copolymer micelles