Apparent Critical Micelle Concentrations in Block Copolymer/Ionic Liquid Solutions: Remarkably Weak Dependence on Solvophobic Block Molecular Weight

Abstract

The effects of block copolymer molecular weight (MW) and composition on the critical micelle concentration (CMC) were studied using ionic liquids (ILs) as model solvents. Pyrene fluorescence was used to measure CMCs as a function of block MW for three polystyrene–poly(ethylene oxide) (PS–PEO) samples and three PS–poly(methyl methacrylate) (PS–PMMA) samples in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide. The CMC decreased by a modest factor of 1.5 in the PS–PEO series, in which the solvophobic PS block MW remained unchanged (20 000) while the PEO block MW was decreased from 13 000 to 5000. This result correlated reasonably well with calculations from self-consistent-field (SCF) theory. A greater decrease (factor of 5) was seen in the PS–PMMA series, where the solvophobic PS block MW was varied from 3000 to 11 000 while maintaining a constant overall MW (ca. 15 000). However, this decrease was much weaker than that predicted by SCF calculations. A compilation of literature CMC data for amphiphilic block copolymers in water generally reveals a strong dependence on solvophobic block degree of polymerization N for low N, but a much weaker dependence for longer solvophobic blocks. From master plots of the compiled data, a scaling parameter shift from CMC ∼ exp(−cN) to CMC ∼ exp(−cN 1/3) was found above a critical solvophobic block N. The parameter c correlates with the χ parameter between the solvophobic block and the solvent. The weaker N dependence was found to fit the IL data very well. While such a change in MW dependence has previously been attributed to the collapse of unimer solvophobic blocks, we also discuss the potential role of kinetic limitations.

Synthesis and micellization properties of triblock copolymers PDMAEMA-b-PCL-b-PDMAEMA and their applications in the fabrication of amphotericin B-loaded nanocontainers

“…Pyrene strongly fluoresces in a non-polar environment, while in a polar environment it shows weak fluorescence intensity. This technique has been widely used to determine CMC for block copolymers and low molecular weight surfactants [13,35]. Figure 4a shows the excitation spectra of pyrene monitored at 390 nm with different concentrations of copolymer P2D2.…”

Can ionic liquid additives be used to extend the scope of poly(styrene)-block-poly(methyl methacrylate) for directed self-assembly?

“…[43][44][45] More recently, there have been reports on more concentrated systems where it has been demonstrated that blending ILs with BCPs such as PS-b-poly(2-vinyl pyridine), [46][47][48] PS-b-poly (ethylene oxide) 49 [PEO], and PEO-b-poly(propylene oxide)-b-PEO 50 can lead to changes in morphology and d-spacing of the phase-separated domains. In this study, we blend 1-ethyl-3-methylimidazolium …”

Lyotropic liquid crystalline phases with a series of N-alkyl-N-methylpiperidinium bromides and water

“…For ILs with short alkyl chains, they are often used as nonaqueous solvent to construct various aggregates, including micelles [6][7][8][9][10], vesicles [8,11], microemulsions [12][13][14][15][16], and lyotropic liquid crystals (LLCs) [17]. Anderson et al investigated the micellar aggregation behavior of different surfactants in two imidazolium ILs with 1-butyl-3-methylimidazolium cation and anions of Cl À or PF À 6 (BmimCl and BmimPF 6 ) [6].…”