Chain Exchange Kinetics in Diblock Copolymer Micelles in Ionic Liquids: The Role of χ

Chain exchange kinetics of diblock copolymer micelles with lower critical micellization temperature (LCMT) phase behavior were investigated using time-resolved small-angle neutron scattering (TR-SANS). Three nearly identical isotopically substituted pairs of poly­(methyl methacrylate)-<i>block</i>-poly­(<i>n</i>-butyl methacrylate) (PMMA-<i>b</i>-PnBMA) diblocks were used in mixtures of the room temperature ionic liquids 1-ethyl-3-methyl­imidazolium bis­(trifluoro­methyl­sulfonyl)­imide and 1-butyl-3-methyl­imidazolium bis­(trifluoro­methyl­sulfonyl)­imide. In this case, the <i>h-</i>PnBMA and <i>d</i>₉-PnBMA blocks form the micellar cores. The results are consistent with previous measurements in other systems, in that the barrier to chain extraction scales linearly with the core block length. By varying the ratio of the two homologous solvents in the mixture, the value of χ between the core block and the solvent is varied systematically. The results show that the solvent selectivity has a remarkable effect on the chain exchange rate, as anticipated by a previous theory. However, in contrast to an assumption in previous studies, we find that the barrier to chain exchange is not simply proportional to χ. Accordingly, we propose a more elaborate function of χ for the energy barrier, which is rationalized by a calculation in the spirit of Flory–Huggins theory. This modification can account for the chain exchange behavior when χ is relatively modest, i.e., in the vicinity of the critical micelle temperature

Poly(methyl methacrylate)-<i>block</i>-poly(<i>n</i>-butyl methacrylate) Diblock Copolymer Micelles in an Ionic Liquid: Scaling of Core and Corona Size with Core Block Length

The structure of poly­(methyl methacrylate)-<i>block</i>-poly­(<i>n</i>-butyl methacrylate) (PMMA-<i>b</i>-PnBMA) micelles in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([EMIM]­[TFSI]), a selective solvent for the PMMA block, has been studied using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS). A series of seven PMMA-<i>b</i>-PnBMA diblock copolymers were prepared by reversible addition–fragmentation chain-transfer (RAFT) polymerization, in which the degree of polymerization of the PMMA block was kept constant while the PnBMA block length was varied. All the polymers formed spherical micelles at ambient temperature in dilute solution; their hydrodynamic radius (<i>R</i>_h) and core radius (<i>R</i>_c) were obtained by DLS and SAXS, respectively. It was found that <i>R</i>_c and the degree of polymerization of the core block, <i>N</i>_B, followed a power law relationship in which <i>R</i>_c ∼ <i>N</i>_B^0.71±0.01. The corona thickness (<i>L</i>_corona), given by the difference of <i>R</i>_h and <i>R</i>_c, does not show any apparent dependence on <i>N</i>_B. These results were compared to scaling theory, and were found to be only in partial agreement with the star model proposed by Halperin et al. However, the mean-field calculations of micellar dimensions by Nagarajan and Ganesh were in excellent agreement with the data. This comprehensive experimental study provides precise quantification of the <i>R</i>_c and <i>L</i>_corona dependence on core block lengths, due to the use of seven different block copolymers with identical corona block lengths

Phase Behavior of Binary Polymer Blends Doped with Salt

We present cloud point measurements on low molecular weight binary polymer blends doped with salts that exhibit unusual phase behavior. These blends include poly­(ethylene-<i>alt</i>-propylene)/poly­(ethylene oxide) (PEP/PEO) doped with lithium bis­(trifluoro­methane)­sulfonimide (LiTFSI), NaTFSI, KTFSI, LiClO₄, and sodium iodide NaI. The addition of salt dramatically decreases the miscibility of the binary blends and results in an asymmetric cloud point profile. The phase behavior is found to be governed by the concentration of the salt, the size of the anion, and the composition of the polymer mixture. The experimental results are compared with a recent theory, which predicts the effect of ions on the polymer phase diagram by taking into account both ion-induced cross-linking and self-energy effects. Furthermore, the coexistence curve of salt-doped PEP/PEO blends is determined quantitatively by ¹H NMR spectroscopy when the volume fraction of PEO is maintained at 0.6. The coexistence curve does not coincide with the cloud point profile, which can be attributed to the effect of the redistribution of ions between the two coexisting phases. In the interest of generality, the cloud point profile of polystyrene/poly­(ethylene oxide) (PS/PEO) doped with LiTFSI is also mapped out, in which similar phenomena are observed

Interfacial Tension-Hindered Phase Transfer of Polystyrene‑<i>b</i>‑poly(ethylene oxide) Polymersomes from a Hydrophobic Ionic Liquid to Water

We examine the phase transfer of polystyrene-<i>b</i>-poly­(ethylene oxide) (PS–PEO) polymersomes from a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([EMIM]­[TFSI]), into water. The dependence of the phase transfer on the molecular weight and PEO volume fraction (<i>f</i>_PEO) of the PS–PEO polymersomes was systematically studied by varying the molecular weight of PS (10 000–27 000 g/mol) as well as by varying the volume fraction of PEO (<i>f</i>_PEO) between 0.1 and 0.3. We demonstrate a general boundary for the phase transfer in terms of a reduced tethering density for PEO (σ_PEO), which is independent of the molecular weight of the hydrophobic PS. The reduced PEO tethering density was controlled by changing the polymersome size (i.e., increased polymersome sizes increase σ_PEO), confirming that it is the driving force in the transfer of PS–PEO polymersomes at room temperature. The phase transfer dependence on σ_PEO was also analyzed in terms of the free energy of polymersomes in the biphasic system. The quality of the aqueous phase, which affects the interfacial tension of the PS membrane, influenced the phase transfer. We systematically reduced the interfacial tension by adding a water-selective solvent, THF, which has a similar effect to increasing σ_PEO. The results indicate that the interfacial tension between the membrane and water plays an important role in the phase transfer with the corona and that the phase transfer can be controlled either by the dimensions of the polymersomes or by the suitability of the solvent for the membrane. The interfacial tension-hindered phase transfer of polymersomes in the biphasic water–[EMIM]­[TFSI] system will inform the design of temperature-sensitive and reversible nanoreactors and the separation of polydisperse particles according to size by tuning the quality of the solvent

Coil Dimensions of Poly(ethylene oxide) in an Ionic Liquid by Small-Angle Neutron Scattering

The infinite dilution radii of gyration (<i>R</i>_g,0) for five different molecular weights (10–250 kg/mol) of perdeuterated poly­(ethylene oxide) (<i>d</i>-PEO) have been determined at 80 °C in the ionic liquid 1-butyl-3-methyl­imidazolium tetrafluoroborate ([BMIM]­[BF₄]) using small-angle neutron scattering (SANS). The results establish the dependence of <i>R</i>_g,0 on polymer molecular weight as <i>R</i>_g,0 ∼ <i>M</i>_w^0.55. An excluded volume exponent of ν ≈ 0.55 indicates that [BMIM]­[BF₄] is a moderately good solvent and that PEO remains a flexible random coil in this ionic liquid. These results clarify the uncertainty surrounding PEO coil dimensions in this ionic liquid, as computer simulations at various levels of complexity have led to conflicting results, and prior experimental results also do not present a completely consistent picture

Conformation of Methylcellulose as a Function of Poly(ethylene glycol) Graft Density

Low molecular weight thiol-terminated poly­(ethylene glycol) (PEG) (<i>M</i> ≈ 800) has been grafted onto a high molecular weight methylcellulose (MC, <i>M</i>_w ≈ 150000) by a facile thiol–ene click reaction; graft densities varied from 0.7% to 33% (grafts per anhydroglucose unit). Static and dynamic light scattering reveals that the overall radius of the chain increases systematically with graft density, in a manner in excellent agreement with theory. As the contour length remains unchanged, it is apparent that grafting leads to an increase in the persistence length of this semiflexible copolymer, by as much as a factor of 4. These results represent the first experimental verification of the excluded volume theory at low grafting densities, and demonstrate a promising synthetic platform for systematically increasing the persistence length of a model semiflexible, water-soluble polymer

Rate of Molecular Exchange through the Membranes of Ionic Liquid Filled Polymersomes Dispersed in Water

The permeation of 1-ethyl-3-methylimidazolium ([EMIM]), 1-butyl-3-methylimidazolium ([BMIM]), and 1-butylimidazole through the bilayer membranes of nanoemulsion-like polymersomes was investigated by nuclear magnetic resonance spectroscopy (NMR) techniques. 1,2-Polybutadiene-<i>b</i>-poly­(ethylene oxide) (PB–PEO) polymersomes in the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([EMIM]­[TFSI]) were prepared by a cosolvent method and then migrated to the aqueous phase, which is not miscible with the IL, at room temperature. In this way stable, nanoscopic domains of the IL (average diameter ca. 200 nm) were dispersed in water. Two similarly sized molecules, charged [EMIM] and neutral 1-butylimidazole, were employed as tracer molecules, and proton NMR (¹H NMR) and pulsed-field-gradient NMR (PFG-NMR) experiments were conducted. Furthermore, transient ¹H NMR was used with [BMIM] to estimate how rapidly the charged molecules can go through the hydrophobic membrane into the polymersome interior. The molecules in the nanoemulsion solution showed two distinct sets of peaks due to the magnetic susceptibility difference across the membrane. This difference in ¹H NMR gave direct evidence of permeation of the molecules and the relative populations within the polymersomes versus in the aqueous exterior. The escape and entry rates were evaluated by fitting the PEG-NMR echo decay curves with a two-site exchange model. The molecules could permeate through the hydrophobic PB membranes on a time scale of seconds, but the entry and escape rates for the charged molecule ([EMIM]) were approximately 10 times slower than the neutral molecule (1-butylimidazole). These results confirm that this system has the potential to serve as a nanoreactor, facilitating reactions with various kinds of molecules including both charged and neutral molecules. It combines the facile transport and mixing of a majority aqueous phase with the multiple advantages of IL as a reaction medium. The ability to shuttle the polymersomes reversibly between aqueous and ionic liquid phases offers a convenient route to product separation and catalyst recovery

Size Control and Fractionation of Ionic Liquid Filled Polymersomes with Glassy and Rubbery Bilayer Membranes

We demonstrate control over the size of ionic liquid (IL) filled polymeric vesicles (polymersomes) by three distinct methods: mechanical extrusion, cosolvent-based processing in an IL, and fractionation of polymersomes in a biphasic system of IL and water. For the representative ionic liquid (1-ethyl-3-methyl­imidazolium bis­(trifluoro­methyl­sulfonyl) imide ([EMIM]­[TFSI])), the size and dispersity of polymersomes formed from 1,2-polybutadiene-<i>b</i>-poly­(ethylene oxide) (PB–PEO) and polystyrene-<i>b</i>-poly­(ethylene oxide) (PS–PEO) diblock copolymers were shown to be sensitive to assembly conditions. During mechanical extrusion through a polycarbonate membrane, the relatively larger polymersomes were broken up and reorganized into vesicles with mean size comparable to the membrane pore (100 nm radius); the distribution width also decreased significantly after only a few passes. Other routes were studied using the solvent-switch or cosolvent (CS) method, whereby the initial content of the cosolvent and the PEO block length of PS–PEO were systemically changed. The nonvolatility of the ionic liquid directly led to the desired concentration of polymersomes in the ionic liquid using a single step, without the dialysis conventionally used in aqueous systems, and the mean vesicle size depended on the amount of cosolvent employed. Finally, selective phase transfer of PS–PEO polymersomes based on size was used to extract larger polymersomes from the IL to the aqueous phase via interfacial tension controlled phase transfer. The interfacial tension between the PS membrane and the aqueous phase was varied with the concentration of sodium chloride (NaCl) in the aqueous phase; then the larger polymersomes were selectively separated to the aqueous phase due to differences in shielding of the hydrophobic core (PS) coverage by the hydrophilic corona brush (PEO). This novel fractionation is a simple separation process without any special apparatus and can help to prepare monodisperse polymersomes and also separate unwanted morphologies (in this case, worm-like micelles)

Effects of Solvent Quality and Degree of Polymerization on the Critical Micelle Temperature of Poly(ethylene oxide‑<i>b</i>‑<i>n</i>‑butyl methacrylate) in Ionic Liquids

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-<i>b</i>-<i>n</i>-butyl methacrylate) (PEO-<i>b</i>-PnBMA) in mixtures of two ionic liquids: 1-butyl-3-methylimidazolium: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-<i>b</i>-PnBMA. It is shown that the interaction parameter between the solvent and PnBMA <i>at</i> 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

Chain Exchange Kinetics of Asymmetric B₁AB₂ Linear Triblock and AB₁B₂ Branched Triblock Copolymers

Equilibrium chain exchange of asymmetric B₁AB₂ and AB₁B₂ branched triblock copolymers in a B selective solvent has been studied via dissipative particle dynamics simulations. Hybridization simulations are performed using B₁AB₂ and AB₁B₂ branched triblock copolymers at varying levels of asymmetry and compared with equivalent AB diblock copolymers. It is found that B₁AB₂ triblocks exchange ∼1 order of magnitude faster than diblocks (persisting over various values of χ<i><i>N</i></i>_core and total corona length), while AB₁B₂ triblocks exchange ∼4 times faster than diblocks. The dependence on asymmetry is weak, with very asymmetric triblocks (<i>N</i>_B₁ ≪ <i>N</i>_B₂) exchanging only 2 or 3 times faster than symmetric triblocks. Two causes are found for this: (1) increases in the density of corona beads near the micelle core for triblocks, resulting in greater stretching penalties and lower aggregation numbers, and (2) looped core blocks (B₁AB₂) spending more time near the surface of a micelle core than unlooped core blocks (AB₁B₂ and AB), resulting in a lower energy benefit of insertion. Additionally, unlooped core blocks pull out bead-by-bead with multiple activations, whereas the looped core blocks tend to aggregate near the micelle surface and pull out as a single entity, potentially further reducing the energy penalty of pullout. Because of this difference in mechanism, the looped core triblocks pull out more rapidly than the unlooped core triblocks even at identical aggregation numbers