Thermoreversible Partitioning of Poly(ethylene oxide)s between Water

Jul 2, 2014 - (12, 13) This driving mechanism has been employed to develop shuttles of block copolymer micelles,(14-19) nanogels,(20) polymer-grafted ...
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Thermoreversible Partitioning of Poly(ethylene oxide)s between Water and a Hydrophobic Ionic Liquid Zhifeng Bai,†,§ Michael W. Nagy,† Bin Zhao,∥ and Timothy P. Lodge*,†,‡ †

Department of Chemistry and ‡Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States § The Dow Chemical Company, Midland, Michigan 48674, United States ∥ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States ABSTRACT: We describe a poly(ethylene oxide) (PEO) homopolymer “shuttle” between water and a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). PEO homopolymers with varying molecular weight transferred reversibly and quantitatively between water at room temperature and [EMIM][TFSI] at an elevated temperature. The temperature of the transfer from water to [EMIM][TFSI] shows a linear dependence on PEO molecular weight and a dependence on polymer concentration consistent with expectation based on Flory−Huggins theory. These results are also consistent with the previously observed lower critical solution temperature (LCST) behavior of PEO in water. Dynamic light scattering study of the concentration and temperature dependence of the swelling degree of PEO corona of polybutadiene (PB)−PEO block copolymer micelles indicates that the solvent quality of [EMIM][TFSI] for PEO remains essentially the same as a good solvent over the temperature range of the PEO shuttle. Fundamental understanding of the PEO shuttle is of significance in development of systems for phase transfer of reagents and reaction products between ionic liquids and water.

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INTRODUCTION There is great interest in phase transfer in biphasic systems for various applications, such as recycling of catalysts, 1−3 preparation of structured nanoparticles,4 transfer of nanoparticles from a preparation medium to an application medium,5−7 and transport across biological membranes.8−10 One intriguing, recently introduced class of phase transfer is the “micelle shuttle”a reversible, quantitative, and intact transfer of block polymer micelles between two immiscible liquids. This was first realized with poly((1,2-butadiene)-b-ethylene oxide) (PB−PEO) block copolymer micelles between water at room temperature and a hydrophobic ionic liquid at elevated temperatures.11 The transfer of the micelles, self-assembled from the solvophobic PB and solvophilic PEO in both water and the ionic liquid, is driven by the lower critical solution temperature (LCST) phase behavior, i.e., decreasing solubility upon heating, of the PEO corona in water.12,13 This driving mechanism has been employed to develop shuttles of block copolymer micelles,14−19 nanogels,20 polymer-grafted particles,21,22 and carbon nanotubes23 with PEO or PEO-like coronas/brushes to impart desired properties, such as controlled loading and release,14,20 diverse structure and size of self-assembly carriers,12,16 catalytic activity,23 and tunable transfer temperatures (Tt).12,21,22 Dissipative particle dynamics simulation was also used to study the shuttling behavior of micelle shuttles.24,25 As ionic liquids have shown promise in reactions,26,27 separations,28 and biphasic catalysis,29,30 the © 2014 American Chemical Society

micelle/particle shuttle systems may allow for solubilization/ encapsulation of active agents and facile delivery/separation of active agents and products in such applications. It is important to note that such systems are relatively rare, as the process requires that one polymer, in this case PEO, be soluble in two immiscible small molecule liquids, a strong exception to the basic “like prefers like” rubric of solubility. Besides its practical interest for the micelle/particle shuttles, the phase behavior of PEO in biphasic water/ionic liquid systems is also of scientific interest. Among the most studied of polymers in solution, the phase behavior of PEO has attracted considerable attention. Most of the studies have focused on aqueous systems, where PEO exhibits an LCST phase behavior and even a subsequent upper critical solution temperature (UCST) phase behavior upon heating. Experimental and theoretical studies both indicate that the strong hydrogenbonding interactions between PEO and water play an important role.31−33 For PEO/ionic liquid systems, it has been found that PEO is generally compatible with (hydrophobic) imidazolium-based ionic liquids.34−36 Recently, Lee and co-workers reported an unusual LCST phase behavior of PEO in imidazolium-based tetrafluoroborate ionic liquids, which are more hydrophilic.37 The driving force of the LCST Received: May 11, 2014 Revised: June 18, 2014 Published: July 2, 2014 8201

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atmosphere prior to DLS measurements. DLS measurements were conducted over a temperature range of 25−150 °C at four different scattering angles between 60° and 110°. The samples were equilibrated in the oil bath at each temperature for 30 min before data collection. Solution Preparation. Aqueous solutions of PEO and PB− PEO(8−20) with varying polymer concentrations were prepared by dissolving weighed amounts of PEO or PB−PEO(8−20) in water at room temperature. A 0.5 wt % PB−PEO(8−20) micelle solution in [EMIM][TFSI] was prepared as a stock solution using a cosolvent method. The block copolymer and [EMIM][TFSI] were mixed with a dichloromethane cosolvent until complete dissolution. Most of the dichloromethane was slowly removed by a gentle nitrogen purge, and the solution was dried at ∼60 °C in a vacuum oven until constant weight was achieved. The stock solution was then diluted to appropriate concentrations for DLS. PEO Transfer and Transfer Temperature Measurements. A sealed ampule containing equal volumes of 1 wt % aqueous PEO solution and [EMIM][TFSI] was heated in a temperature-controlled oil bath at a heating rate of ∼0.5 °C/min with moderate stirring. The transfer temperatures of PEO-10, PEO-19, PEO-100, and PEO-1000 in the biphasic water/[EMIM][TFSI] system were determined using optical transmittance measurements at 633 nm. The temperature dependence of the transmittance of the aqueous phase was collected using a laser power detector (SPEX). The transfer temperature values were defined where the transmittance drops to 80%. The transfer temperature of PEO-4.4 in the biphasic system was determined by visual observation of the cloud point of the aqueous phase by eye. The PEO transfer was slow at the transfer temperature. For example, in a particular case of our experiment the aqueous phase of the biphasic system containing PEO-100 was still cloudy after equilibration at the transfer temperature for 1 h. The transfer became more rapid at higher temperature; for example, the aqueous phase turned from cloudy to clear after equilibration at a temperature 5 °C above the transfer temperature for 1 h. After the transfer was complete, the aqueous phase remained clear upon further heating to higher temperature. The ionic liquid phase was clear during the PEO transfer process upon heating. The reverse PEO transfer from [EMIM][TFSI] to water was achieved by cooling the system that was equilibrated above the transfer temperature to room temperature. Both the aqueous and ionic liquid phases immediately turned cloudy due to the decreasing miscibility of water and [EMIM][TFSI] upon cooling (for example, the solubility of [EMIM][TFSI] in water decreases by about 20% from 45 to 25 °C).41 The two phases turned clear after equilibration at room temperature for 1 h. The round-trip transfer of PEO in the biphasic system was confirmed by 1H NMR analysis as described in the next section, consistent with observations in the micelle shuttle systems.12−15 PEO Partitioning in the Biphasic Water/[EMIM][TFSI] System. An equal volume of deuterated water (D2O) was added to an [EMIM][TFSI] solution of PEO-10 (5 wt %). After mixing the two phases and equilibration at room temperature for 2 h, a fraction of the solution was taken out separately from both phases for 1H NMR analysis. Another batch of the sample that had been equilibrated at room temperature was heated at 90 °C with moderate stirring for 1 h, during which the aqueous phase turned cloudy and then clear. 1H NMR measurements were performed on both phases again. Independent measurements of 5 wt % PEO-10 in [EMIM][TFSI] and D2O were also made as controls.

phase behavior was also demonstrated to be strongly influenced by hydrogen bonding interactions between PEO and the ionic liquids. The striking feature of the PEO/ionic liquid phase diagram is that the critical composition is shifted to high polymer concentrations, in stark contrast to the prediction of the Flory−Huggins theory. This result has been explained, at least qualitatively, by the high cohesive energy density of the solvent and the tendency of the solvent to cluster in the high polymer concentration side of the phase diagram.38 In analogous biphasic water/organic solvent systems, Spitzer et al. reported spontaneous one-way transfer of highly watersoluble PEO to dichloromethane and chloroform, which was also entropically driven by the release of water molecules hydrogen bonded with PEO upon the transfer.39 Herein, we describe the partitioning of PEO in a biphasic system of water and a common hydrophobic ionic liquid with low viscosity and high thermostability, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). Five PEO homopolymers with varying molecular weights were employed to demonstrate the themoresponsive shuttling behavior in the biphasic system. The polymer partitioning was quantified by 1H NMR spectroscopy, and the transfer temperature was identified by turbidity measurements. To study the driving force of the shuttling behavior in detail, the dependence of the transfer temperature on polymer molecular weight and concentration was determined and compared with that of the LCST of PEO in water. Moreover, the solvent quality of [EMIM][TFSI] to PEO was investigated by dynamic light scattering (DLS) analysis of the concentration and temperature dependence of the swelling degree of the PEO corona of micelles formed by a PB−PEO block copolymer. The PEO shuttle can thus be understood as due to a change in relative affinity of PEO for the two solvents upon heatingthe solvent quality of water for PEO decreases upon heating (LCST), while [EMIM][TFSI] remains a good solvent.

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EXPERIMENTAL SECTION

Materials. A PB−PEO block copolymer, PB−PEO(8−20), was previously prepared by Dr. Sangwoo Lee by sequential anionic polymerization. The polymer has an 8 kDa PB block and a 20 kDa PEO block, previously determined by 1H NMR spectroscopy, and a dispersity (Đ) of 1.02, previously determined by size exclusion chromatography (SEC).40 Four doubly hydroxyl-terminated PEOs (also called poly(ethylene glycol))PEO-4.4 (Mn = 4.4 kDa, Đ = 1.02), PEO-10 (Mn = 10 kDa, Đ = 1.02), PEO-100 (Mn = 100 kDa), and PEO-1000 (Mn = 1000 kDa)were obtained from Aldrich. One doubly hydroxyl-terminated PEO homopolymer, PEO-19 (Mn = 19 kDa, Đ = 1.04), was acquired from Fluka. SEC with light scattering detection was employed to determine the absolute molecular weight and dispersity of PEO-4.4, PEO-10, and PEO-19. The molecular weights of PEO-100 and PEO-1000 were provided by the vendor. The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) was synthesized by an ion-exchange protocol described previously.16 General Methods. 1H NMR spectra were obtained on a Varian Inova 500 MHz spectrometer at room temperature. SEC measurements were taken on a system equipped with a Wyatt DAWN DSP multiangle light scattering detector, a Wyatt OPTILAB DSP refractive index detector, and three Phenogel (Phenomenex) columns. Tetrahydrofuran (THF) and THF containing 1% tetramethylethylenediamine (TMEDA) by volume were used as the eluents for the measurements of the PB−PEO and PEO polymers, respectively. Dynamic light scattering was performed on a homemade photometer, as described previously.12 PB−PEO micelle solutions were filtered through 0.45 μm syringe filters (GHP, Pall) into glass sample tubes with an inner diameter of 0.38 in. and then sealed under an argon

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RESULTS AND DISCUSSION PEO Transfer in the Biphasic Water/[EMIM][TFSI] System. Partitioning of PEO homopolymers in the biphasic water/[EMIM][TFSI] system cannot be visualized directly, as solutions with the polymer chains well solvated are transparent. However, when PEO homopolymers begin to transfer from one phase to the other, the “departure” phase turns cloudy. This is illustrated by the experimental images of the biphasic water/ [EMIM][TFSI] system containing PEO at 22, 70, and 90 °C in Figure 1. At 22 °C, both the aqueous (PEO-laden) and ionic 8202

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there was no extra ionic liquid to replace water molecules to solvate the polymer and trigger its phase separation from water. The partitioning of PEO homopolymer in the biphasic water/[EMIM][TFSI] system was studied by 1H NMR spectroscopy. After mixing an [EMIM][TFSI] solution of PEO-10 with deuterated water (D2O) at 22 °C, more than 99.5% of the PEO transfers to the aqueous phase, as shown in Figure 2. Upon heating the biphasic system to 90 °C, more than 99.5% of the PEO transfers back to the ionic liquid phase. This quantitative transfer is consistent with the quantitative shuttles of micelles with PEO coronas between water and ionic liquids reported previously.13−15 Transfer Temperature (T t). The cloudiness of the “departure” phase in the PEO shuttle allows determination of the transfer temperature using transmittance. Transmittance measurement is a common method employed to locate liquid− liquid phase separation boundaries in polymer solutions.37 Figure 3 shows the transmittance of the aqueous phase during the transfer of PEO with various molecular weights from water to [EMIM][TFSI] upon heating. At low temperature, PEO is fully soluble in the aqueous phase and forms a transparent solution, consistent with the image shown in Figure 1. Upon heating to a certain temperature, the transmittance of the aqueous phase drops sharply, consistent with the cloudy aqueous phase during the PEO transfer in Figure 1. It appears that the higher the molecular weight of the polymer, the more the transmittance decreases, presumably as it is easier to form larger polymer droplets in water that scatter more light. Figure 3 also indicates that the polymer with a higher molecular weight transfers at a lower temperature. As shown in Figure 4, there is a linear relation between the transfer temperature and the inverse molecular weight. Interestingly, analysis of the reported LCSTs of doubly hydroxyl-terminated PEO with varying molecular weight in water43,44 also yields a linear dependence on inverse molecular weight (Figure 4). Hydrogen bonding has been theoretically

Figure 1. Images of PEO homopolymer (100 kDa) transfer from water (the upper phase) to [EMIM][TFSI] (the lower phase).

liquid (PEO-free) phases were clear, but the aqueous phase turned cloudy upon heating to 70 °C and then clear again at 90 °C, while the ionic liquid phase remained clear during the transfer. The cloudiness of the “departure” phase during the transfer was also observed in the micelle shuttles with PEO micellar coronas,13−15 which was attributed to the formation of micelle-concentrated ionic liquid droplets in the aqueous phase as water is replaced by the ionic liquid to solvate the PEO corona.13 Likewise, the cloudiness of the “departure” phase in the PEO shuttle can also be attributed to the replacement of solvating water by [EMIM][TFSI], leading to PEO-concentrated ionic liquid droplets in the aqueous phase. It should be noted that there is a small amount of [EMIM][TFSI] dissolved in the aqueous phase,41 which can assist the formation of the PEO-concentrated ionic liquid droplets. If the biphasic system was heated quiescently, the water/ionic liquid interface, where [EMIM][TFSI] is abundant, first turned cloudy, and then the cloudiness gradually extended throughout the aqueous phase, indicating excess (i.e., more than saturated) [EMIM][TFSI] was needed to diffuse from the interface to the aqueous phase to form the PEO-concentrated ionic liquid droplets. It should also be noted that the cloudiness of the aqueous phase in the PEO transfer does not arise from the depression of the LCST of PEO in water by the ionic liquid, i.e., salting-out.42 If the ionic liquid phase was removed from the biphasic system in Figure 1, the aqueous phase did not turn cloudy at 70 °C, as

Figure 2. 1H NMR spectra (500 MHz) of the aqueous phase and the ionic liquid phase in the biphasic D2O/[EMIM][TFSI] system containing 5 wt % PEO-10 at 22 and 90 °C. References are 5 wt % of the PEO in monophasic D2O and [EMIM][TFSI]. 8203

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between the polymer repeat units and water, which thus contributes to an increase in the LCST of PEO. Replacement of the hydroxyl end group of PEO with more hydrophobic groups, e.g., methyl groups, decreases the LCST of PEO in water, due to the disappearance of the favorable hydrogen bonding interaction between the hydroxyl groups and water.45 Moreover, a linear inverse molecular weight dependence of the second virial coefficient (B), a measure of solvent quality for polymer, of doubly hydroxyl-terminated PEO in water was predicted theoretically45 and also observed experimentally.46−48 Since the driving force of the micelle/PEO shuttles originates from the relative affinity of PEO to water and ionic liquid, the linear inverse molecular weight dependence of the transfer temperature of the PEO shuttle with an essentially the same slope as that of the LCST of PEO in water suggests that the solvent quality of [EMIM][TFSI] for PEO does not change significantly over the temperature range of the micelle/PEO shuttles. As shown in Figure 5, a linear relation between the transfer temperature of the PEO shuttle and the LCST of PEO in water

Figure 3. Temperature dependence of transmittance of the aqueous phase of the biphasic H2O/[EMIM][TFSI] system containing 1 wt % 10 kDa (+), 19 kDa (□), 100 kDa (×) or 1000 kDa (○) at 633 nm with a heating rate of ∼0.5 °C/min.

Figure 4. Inverse molecular weight (1/Mw) dependence of the transfer temperature (T t) of 1 wt % PEO homopolymers in the biphasic water/[EMIM][TFSI] system (triangles, Mw = 4.4, 10, 19, 100, and 1000 kDa) and of the LCST of 1 wt % PEO homopolymer in water (filled circles, Mw = 5, 8, 14, 21, and 1020 kDa). The values of the transfer temperature are taken as the point where the normalized transmittance of the aqueous phase drops to 80%, using the data in Figure 3. The transfer temperature for the lowest molecular weight PEO-4.4 was determined by visual observation of the cloud point of the aqueous phase, due to the relatively modest decrease in measured transmittance. The LCST values were from refs 42 and 43. The solid lines are linear fits (LCST: y = 96.54 + 211466x, R = 0.995; Tt: y = 62.68 + 172869x, R = 0.987).

Figure 5. Transfer temperature of 1 wt % PEO homopolymers in the biphasic water/[EMIM][TFSI] system versus their LCST in water. The values of the transfer temperature of PEO with different molecular weights are from Figure 3 and those of the LCST of PEO with corresponding molecular weight are estimated from the fitted results obtained from Figure 4. The solid line is a linear fit (y = −16.24 + 0.8175x, R = 0.996).

can then be generated from their linear dependences displayed in Figure 4. Similar linear relations have been observed between the transfer temperature of the shuttles of silica particles grafted with thermosensitive poly(methoxyoligo(ethylene glycol) methacrylate)21 or thermo- and pH-doubly sensitive poly(methoxyoligo(ethylene glycol) methacrylate-co-methacrylic acid)22 polymer brushes in the biphasic water/[EMIM][TFSI] system and the cloud points of corresponding free polymers in water. Therefore, the transfer temperature, which dictates the micelle/particle partitioning in the biphasic system and is hence an important parameter of the shuttle systems, can be tuned by adjusting the LCST of corona polymers in water through

and experimentally proven to be crucial in the dissolution of PEO in water.33,45 The hydroxyl end group of PEO interacts strongly with water molecules, which is favorable for the dissolution. This interaction becomes more significant with a decrease in molecular weight, as the contribution from the hydrogen bonding between the hydroxyl end groups and water becomes larger as compared to that from the interaction 8204

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changing their molecular weight and copolymer composition,21 tuning pH,22 and adding additives, e.g., salt and sugar, to the aqueous phase.12 Also, it was previously reported that the transfer temperature of PB−PEO block copolymer micelles can be tuned by using hydrophobic ionic liquids with different anions and cations.12 The polymer concentration dependence of the transfer temperature of the PEO shuttles was also studied. As shown in Figure 6, the transfer temperature−concentration curve of

molecular weights, 8 and 14 kDa, in water.44 The critical weight fraction of the transfer temperature−concentration curve was located at about 6.3 wt %, which is comparable with those of the LCST−concentration curves and consistent with the theoretical value, 6.7 wt %, predicted by the Flory−Huggins theory w2,c

ϕ2,c

1 = = 1+ N

ρ2 w2,c ρ2

w1,c

+

(1)

ρ1

where ϕ2,c is the critical volume fraction of PEO, N is the number of repeated units, w1,c and w2,c are the critical weight fraction of water and PEO, respectively, and ρ1 and ρ2 are the densities of water and PEO,49 respectively. Both the similarity of the temperature−concentration curves and the consistency of the critical weight fraction values indicate again that the PEO shuttle is driven by the LCST of PEO in water and its transfer temperature is directly correlated to the LCST. Temperature Dependence of Solvent Quality of [EMIM][TFSI] for PEO. For a micelle solution, the degree of swelling of the micelle corona by solvent is dictated by the quality of the solvent for the corona; micelles in a better solvent have a larger corona dimension.50 Assuming that the aggregation number of micelles and the density of the micelle core are independent of temperature, the radius of the micelle core is independent of temperature. This assumption is eminently reasonable for this system; the PB block is sufficiently insoluble in both water and the ionic liquid that no change in micellar aggregation number is possible on practical time scales; in short, these micelles are kinetically “frozen”.40,51,52 Then, the quality of solvent for the micelle corona can be correlated to the micelle size that can be measured by DLS. This is demonstrated by heating an aqueous PB−PEO(8−20) micelle solution from 25 to 75 °C, where the mean hydrodynamic radius (Rh) of the micelles decreases from 51 nm with a reduced second cumulant53 (μ2/Γ2) of 0.06 to 44 nm with a μ2/Γ2 of 0.05. The decrease in the micelle size upon heating is consistent with the deteriorating solvent quality of water for the PEO corona. Essentially no change in the micelle

Figure 6. Concentration dependence of the transfer temperature of PEO (10 kDa, filled circles) in the biphasic water/[EMIM][TFSI] system (wt % of PEO in the aqueous phase) and the LCST of two PEO homopolymers (8 kDa, squares; 14 kDa, triangles) in water. The values of transfer temperature were measured using the method shown in Figure 3, and those of LCST were from ref 44.

PEO-10 is concave-upward, resembling the reported LCST− concentration curves of two PEO homopolymers with similar

Table 1. Temperature and Concentration Dependence of the Mutual Diffusion Coefficient, Hydrodynamic Radius, and Size Distribution of the PB−PEO Micelles in [EMIM][TFSI] and Extracted Tracer Diffusion Coefficient and Hydrodynamic Radius in the Limit of Low Polymer Concentration and kd Dm × 1013(m2/s)

⟨Rh⟩ (nm)

temp (°C)

0.24 g/L

0.17 g/L

0.11 g/L

0.083 g/L

0.24 g/L

0.17 g/L

0.11 g/L

0.083 g/L

25 50 75 100 125 150

3.15 7.30 14.0 23.8 35.8 51.7

3.08 7.18 13.6 23.6 35.4 51.2

3.00 7.04 13.8 23.3 35.6 51.1

3.10 7.14 13.5 23.0 34.4 49.7

20.1 20.3 20.3 20.2 20.6 20.5

20.5 20.6 21.0 20.4 20.9 20.6

21.1 21.0 20.6 20.7 20.8 20.8

20.4 20.7 21.1 20.9 21.5 21.3

μ2/Γ2

a

temp (°C)

0.24 g/L

0.17 g/L

0.11 g/L

0.083 g/L

25 50 75 100 125 150

0.07 0.09 0.07 0.05 0.07 0.06

0.06 0.08 0.07 0.04 0.08 0.08

0.03 0.07 0.05 0.04 0.07 0.06

0.06 0.12 0.11 0.08 0.13 0.05

Dt,0 × 1013 (m2/s)a 3.0 7.0 13 23 34 49

(0.1) (0.1) (0.3) (0.2) (0.7) (0.8)

Rh,0 (nm) 21.1 21.3 21.4 21.3 21.6 21.6

kd (L/g)a 0.18 0.18 0.18 0.20 0.19 0.21

(0.16) (0.09) (0.13) (0.04) (0.12) (0.09)

The data in the parentheses are standard errors from the fitting in Figure 7. 8205

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size and size distribution (Rh = 49 nm and μ2/Γ2 = 0.07) at 25 °C before and after the heating was observed, which also supports the fixed micellar aggregation number.51,52 The temperature dependence of the solvent quality of [EMIM][TFSI] for PEO was studied by DLS analysis of PB− PEO micelles in [EMIM][TFSI]. A [EMIM][TFSI] solution of PB−PEO(8−20) micelles prepared by a cosolvent protocol was chosen due to the stable micelle size at high temperature and narrow size distribution, as previously reported.40 Micelle solutions with low weight fractions (wPB−PEO) of 0.000 16, 0.000 12, 0.000 074, and 0.000 054 were used to achieve the dilute micelle regime; micelle solutions with wPB−PEO lower than 0.000 054 do not provide strong enough light scattering for analysis using the current DLS setup. The hydrodynamic volume fraction of the micelles (ϕm) can be calculated from ⎛ R h ⎞3 ρ[EMIM][TFSI]wPB−PEOc PB ⎛ R h ⎞3 ϕm = ϕc⎜ ⎟ = ⎜ ⎟ ρPB ⎝ Rc ⎠ ⎝ Rc ⎠

polymer concentration (Dt,0) can be extracted from fitting of the polymer concentration dependence of Dm using Dm = Dt,0(1 + kdC PB−PEO)

(3)

where kd is a combination of thermodynamic and frictional “virial coefficients” (a positive kd corresponds to a good solvent and a negative kd to a poor solvent) and CPB−PEO is the concentration of PB−PEO in the [EMIM][TFSI] solutions. The hydrodynamic radius in the limit of low polymer concentration (Rh,0) can then be calculated as listed in Table 1. There is essentially no change in Rh,0 upon heating, indicating that the solvent quality of [EMIM][TFSI] for PEO remains the same. The extracted values of kd are positive, indicating [EMIM][TFSI] is a good solvent for PEO; furthermore, kd also remains essentially the same upon heating from 25 to 150 °C, which again indicates no significant change in the solvent quality of [EMIM][TFSI] for PEO, consistent with the transfer temperature and LCST results discussed above. Interestingly, the LCST phase behavior of PEO in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and their blends,37 and a PEO derivative, poly(ethyl glycidyl ether) in [EMIM][TFSI],56 has also been reported. The insignificant change in solvent quality of [EMIM][TFSI] for PEO upon heating over the temperature range observed here thus indicates a subtle dependence of mixing thermodynamics on the structures of polymer and ionic liquid.

(2)

where ϕc is the volume fraction of the micelle core, ρ[EMIM][TFSI] is the density of [EMIM][TFSI],54 cPB is the weight fraction of the PB block of the PB−PEO block copolymer, and Rc is the radius of the micelle core.55 For example, a ϕm of 0.02 is estimated for a micelle solution with a wPB−PEO of 0.000 16; the four micelle solutions are thus in the dilute micelle regime. Table 1 lists the measured values of the mutual diffusion coefficient (Dm), Rh, and μ2/Γ2 of the PB−PEO(8−20) micelles with different polymer concentrations in [EMIM][TFSI] at 25, 50, 75, 100, 125, and 150 °C. Upon heating, Dm of the micelles increases, consistent with the decrease of matrix viscosity. The Rh of the micelles is about 20−21 nm with narrow size distribution as indicated by μ2/Γ2, and there is no significant change in Rh and μ2/Γ2 with changes in temperature and polymer weight fraction. The size and size distribution of the micelles stay essentially the same after the heating, indicating that the micelle aggregation number is fixed during the heating and agreeing with the kinetically trapped micelles in [EMIM][TFSI] prepared by the cosolvent method.40 As shown in Figure 7, the tracer diffusion coefficient in the limit of low

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SUMMARY We describe a thermosensitive shuttle of PEO homopolymers in a biphasic system containing water and a hydrophobic ionic liquid, [EMIM][TFSI]. PEO homopolymers with varying molecular weight in the biphasic system exhibit the same shuttling behaviorquantitative and thermoreversible transfer between water and the ionic liquidas the micelle and nanoparticle shuttle systems. The round-trip transfer of PEO is quantitative as indicated by 1H NMR analysis. The transfer temperatures of the PEO shuttles, determined by cloud point measurement, show an inverse linear dependence on the PEO molecular weight, as do the LCSTs of PEO in water. Thus, the transfer temperatures are linearly correlated to the LCSTs, which could be attributed to the favorable hydrogen-bonding interaction between water and the hydroxyl end groups of PEO. Moreover, the LCSTs and the transfer temperatures both exhibit a concentration dependence in broad agreement with the Flory−Huggins theory. Therefore, the driving force of the PEO shuttles can be attributed to the LCST phase behavior of PEO in water. On the other hand, the solvent quality of [EMIM][TFSI] to PEO was extracted by DLS study of the concentration and temperature dependence of the swelling degree of PEO corona of PB−PEO micelles. These results indicate that [EMIM][TFSI] is a good solvent for PEO and that its solvent quality remains essentially the same over the temperature range of the PEO shuttle. The fundamental understanding of the PEO shuttle is of significance in development of systems for phase transfer of actives and biphasic catalysis.

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Figure 7. Concentration dependence of the mutual diffusion coefficient of the PB−PEO(8−20) block copolymer micelles in [EMIM][TFSI] at 25 (■), 50 (●), 75 (▲), 100 (▼), 125 (◀), and 150 °C (▶). The solid lines are linear fits to the data at each temperature.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.P.L.). 8206

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported primarily by National Science Foundation through awards DMR-0804197 and DMR1206459 and by a Doctoral Dissertation Fellowship of the University of Minnesota (Z.B.). B.Z. thanks NSF for the support (DMR-1206385).

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