Effects of Solvent Quality and Degree of Polymerization
on the Critical Micelle Temperature of Poly(ethylene oxide‑<i>b</i>‑<i>n</i>‑butyl methacrylate) in
Ionic Liquids
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Abstract
Block
copolymers are attractive building blocks for designing micelles
having complex shapes and functionality, but experimental investigations
into the detailed thermodynamics of block copolymer micellization
have been constrained. In this work, we take advantage of the favorable
solvent properties of ionic liquids to study the thermodynamics of
block copolymer micelle formation. Specifically, we investigate the
effects of solvent quality and degree of polymerization on the critical
micelle temperature (cmt) of poly(ethylene oxide-<i>b</i>-<i>n</i>-butyl methacrylate) (PEO-<i>b</i>-PnBMA)
in mixtures of two ionic liquids: 1-butyl-3-methylimidazolium:bis(trifluoromethylsulfonyl)imide
([BMIm][TFSI]) and 1-ethyl-3-methylimidazolium:TFSI ([EMIm][TFSI]).
The solvent quality for the core-forming block of the block copolymer,
PnBMA, is varied over a wide range by blending the two ionic liquids
in different ratios, resulting in a large variation in the cmt of
PEO-<i>b</i>-PnBMA. It is shown that the interaction parameter
between the solvent and PnBMA <i>at</i> the cmt is approximately
constant for all of the ionic liquid mixtures. The enthalpies and
entropies of micelle formation also do not vary with ionic liquid
composition, suggesting that the nature of the polymer/solvent and
solvent/solvent interactions do not change much as the ionic liquid
composition is varied despite the large change in solvent quality.
Furthermore, the cmt is shown to depend on the degree of polymerization
of PnBMA as predicted by theory