Article pubs.acs.org/Macromolecules
Effects of Solvent Quality and Degree of Polymerization on the Critical Micelle Temperature of Poly(ethylene oxide‑b‑n‑butyl methacrylate) in Ionic Liquids Megan L. Hoarfrost† and Timothy P. Lodge*,†,‡ †
Department of Chemistry and ‡Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
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-3methylimidazolium: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.
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INTRODUCTION The micellization of amphiphilic molecules in solution is important for a wide variety of materials applications. Micelles are exploited, for example, as detergents, emulsifiers/ compatibilizers, lubricants, viscosity modifiers, membranes, and molecular shuttles in biological systems and in many industries.1−3 Block copolymers are attractive building blocks for designing micelles that have increased complexity and functionality. Compared to traditional surfactant micelles, block copolymer micelles are larger and more robust, take on a variety of complex shapes, have significantly lower critical micelle concentrations, and can be designed to incorporate a wide variety of chemical functionality.4,5 Many research groups are exploring their use for potential new applications, such as nanoreactors,6 nanotemplating,7 and “smart” drug delivery agents.3 In order to fully realize the potential of block copolymer micelles for materials applications, it is important to understand the thermodynamics governing their formation. Accordingly, there is a large body of literature focused on this subject, and much has been learned regarding micelle morphology, kinetics of formation, and thermodynamic driving forces for selfassembly.8−14 However, there are key experimental limitations for studying block copolymer micelle thermodynamics. First, © XXXX American Chemical Society
equilibration is very slow as a result of the long copolymer chains. The energy barrier for extracting a single chain from a micelle into solution is approximately proportional to χN, where χ is the interaction parameter between the core-forming block of the block copolymer and the solvent and N is the degree of polymerization of the core-forming block.11,12,15 Thus, increasing the chain length compared to traditional surfactants dramatically increases the energetic barrier for equilibration. In fact, it has been shown that block copolymer micelle structures often depend on the route by which they are prepared16,17 and that they very rarely reach their true equilibrium structures.18,19 This problem is exacerbated by the volatility of water and organic solvents traditionally used; micellization can only be studied over a limited temperature range, and it is difficult to perform thermal annealing (i.e., to obtain equilibrium structures or to assess whether a structure is at equilibrium). Ionic liquids have been identified as one potential solution for overcoming these experimental restrictions. As they are composed completely of ions, and have negligible vapor Received: December 18, 2013 Revised: February 4, 2014
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dx.doi.org/10.1021/ma402598r | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
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macroinitiator, following a procedure adapted from the literature.25,26 Butyl methacrylate, N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA), and anhydrous anisole were used as received from Sigma-Aldrich. Copper(I) bromide (CuBr) was stored in a glovebox before use. PEO methyl ether (PEO-OH) having a number-averaged molecular weight, MN, of 19 kg/mol was obtained from Polymer Source, reprecipitated into hexanes, and dried under vacuum (
