Effects of Solvent Quality and Degree of Polymerization on the Critical Micelle Temperature of Poly(ethylene oxide-b-n-butyl methacrylate) in Ionic Liquids

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-b-n-butyl methacrylate) (PEO-b-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-b-PnBMA. It is shown that the interaction parameter between the solvent and PnBMA at 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.