Chain Exchange Kinetics in Diblock Copolymer Micelles
in Ionic Liquids: The Role of χ
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Abstract
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