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

Abstract

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 [Cxmim]­[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.

Role of Distance from Equilibrium in the Fragmentation Kinetics of Block Copolymer Micelles

“…10 The experimental data have been fitted with all three theoretical predictions for comparison, as displayed in Figure S17. Additionally, Q eq vs γ correlation is consistent with our previous results, 54 as compared in Figure S18. Note that in the previous study, γ was varied in mixtures of two ILs attained by varying the amount of low γ solvent added (and thus polymer concentration), whereas in the present study, γ is varied by the addition of a second IL chosen from a homologous series with different-length alkyl groups on the imidazolium cation at fixed polymer concentration.…”

“…The interfacial tension γ between a series of ILs and the core-forming polymer block PB was determined using pendant drop tensiometry on a KRÜSS DSA-30S drop shape analyzer as previously described using standard procedures. , A droplet of IL was suspended in a reservoir of PB homopolymer at 70 °C. Measurements were taken for multiple droplets for a given IL, and for each droplet, multiple frames or measurements were collected.…”

“…1.5−6. 54 The protocol involves changing the solvent quality after micelle preparation. Asprepared micelles in [C 2 mim][TFSI] were diluted with [C 10 mim][TFSI] to different degrees of dilution at room temperature, preserving the initial size.…”

“…Interfacial Tension Measurement. The interfacial tension γ between a series of ILs and the core-forming polymer block PB was determined using pendant drop tensiometry on a KRU ̈SS DSA-30S drop shape analyzer as previously described 54 using standard procedures. 58,59 A droplet of IL was suspended in a reservoir of PB homopolymer at 70 °C.…”

“…A recent study introduced an experimental methodology that enables variation of Q / Q eq over a broader range, ca. 1.5–6 . The protocol involves changing the solvent quality after micelle preparation.…”