Apparent critical micelle concentrations in block copolymer/ionic liquid solutions: Remarkably weak dependence on solvophobic block molecular weight

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 (20000) while the PEO block MW was decreased from 13000 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 11000 while maintaining a constant overall MW (ca. 15000). 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.