Effect of Changing Interfacial Tension on Fragmentation Kinetics of Block Copolymer Micelles

Micelle fragmentation, one of the key mechanisms responsible for equilibration of kinetically trapped micelles, is investigated for block copolymer micelles in ionic liquids (ILs). In particular, the role of driving force for micelle fragmentation is studied by altering the solvent quality after micelle preparation, amounting to a jump in interfacial tension γ between solvent and the micelle core. Direct dissolution of a 1,2-polybutadiene-b-poly(ethylene oxide) (PB-b-PEO) copolymer (M_n = 17.5 kDa and f_PEO = 0.38) in the ionic liquid [C₂mim][TFSI] results in large micelles with average size ⟨R_h⟩_o ≈ 68 nm and dispersity Đ ≈ 1.27. The solution of the as-prepared micelles is then diluted by the careful addition of a second ionic liquid [C₁₀mim][TFSI] having lower γ with the micelle core, such that the micelles remain unaffected. The γ and hence the quality of the solvent mixture were controlled by the degree of dilution. The choice of the second solvent is based on the measurement of γ for a series of [C_xmim][TFSI] ILs with 1-2-polybutadiene homopolymer, carried out using a pendant drop test. Diluting the micelles by adding another ionic liquid with lower γ tends to decrease the equilibrium micelle size, which, in turn, enhances the driving force for fragmentation of the bigger as-prepared micelles, represented by increase in the ratio of aggregation numbers Q/Q_eq. Subjecting the diluted micellar solution to temperature-jump to 170 °C followed by thermal annealing leads to fragmentation of the as-prepared micelles to attain a near-equilibrium state. The micelles are characterized using an in situ dynamic light scattering (DLS) technique to observe the time evolution of average micelle size, from which the relaxation time is obtained. Additionally, small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (TEM) measurements were carried out to obtain the micelle core size and distribution in the micellar solutions before and after fragmentation. The enhancement in the driving force achieved by controlling the amount of low γ solvent resulted in faster fragmentation; the characteristic fragmentation time decreases monotonically on increasing the size ratio Q/Q_eq from 1.2 to 5.