in Ionic Liquids - American Chemical Society

---

pubs.acs.org/JPCL

Lower Critical Solution Temperature (LCST) Phase Behavior of Poly(ethylene oxide) in Ionic Liquids Hau-Nan Lee† and Timothy P. Lodge*,†,‡ †

Department of Chemistry and ‡Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455

ABSTRACT We describe the LCST-type phase behavior of poly(ethylene oxide) (PEO) (M = 5000 and 20000) dissolved in 1,3-dialkylimidazolium tetrafluoroborate ionic liquids. The phase transition temperatures were identified using cloud point measurements. In these PEO/ionic liquid systems, liquid-liquid phase separation is observed in the temperature range from 130 to 170 °C. We report unusual temperature-composition phase diagrams in which the cloud point curves are strongly asymmetric, with the critical composition (wc) shifted to high concentrations of PEO. The critical temperature (Tc) and the critical composition (wc) of these systems are not a strong function of the molecular weight of PEO. In addition, the values of the LCST increase as the length of the alkyl chain in the imidazolium cation increases. By using ionic liquid blends as solvents, the LCSTs can be tuned by varying the mixing ratio of two ionic liquids. SECTION Macromolecules, Soft Matter

oth poly(ethylene oxide) (PEO) and ionic liquids (ILs) are often considered within the framework of green chemistry. Due to their biocompatibility, low toxicity, and low cost, PEOs are widely employed in many applications from medical uses to food packaging.1 Their low volatility and relatively low melting points have led to their use in polymerbased electrolytes.2 Ionic liquids have been proposed as alternatives to volatile organic compounds used in industry. Because of their extremely low volatility and good thermal stability, replacement of organic solvents with nonvolatile ionic liquids would prevent the emission of volatile organic compounds, a major source of environmental pollution.3 In addition, the properties of ILs can be easily tuned for a given application by varying the chemical structures of the ions.4 Hybrid “green” systems comprising ILs and PEO are being considered for many potential applications, such as polymer electrolytes in lithium batteries2 and solar cells5 and bifunctional solvents for dissolution-extraction systems.6 Moreover, composites of ILs and block copolymers with a PEO block can form thermoreversible ion gels7-9 and micelle shuttle systems10-13 that are relevant to fuel cells, thin-film transistors,14-16 drug delivery, and molecular storage and transport. To realize these applications, understanding the mutual miscibility of ILs and PEO is essential. In contrast to aqueous PEO solutions that show both lower critical solution temperature (LCST)and upper critical solution temperature (UCST) phase behavior,17,18 it is generally believed that PEO is soluble in most imidazolium-based ILs. For example, experiments showed that PEO is soluble in 1-ethyl3-methylimidazolium bis{(trifluoromethyl) sulfonyl}amide ([EMIM][TFSA]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) from room temperature up to at

least 200 and 150 °C, respectively. Molecular dynamics simulations19 suggested that PEO is soluble in 1-alkyl-3-methylimidazolium hexafluorophosphate ([CnMIM][PF6]) up to 230 °C. Small-angle neutron scattering20 measurements showed that 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) is also a good solvent for PEO at ambient temperature. More recently, Harner and Hoagland reported that PEO is dissolved in 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][Et-SO4]) at temperatures higher than the melting temperature of PEO.21 On the other hand, Rodriguez et al.6 reported that 1-alkyl-3-methylimidazolium chloride ([CnMIM][Cl]) and PEOs were not miscible and formed biphasic liquid mixtures. Here, we report the LCST phase behavior of PEO in 1,3-dialkylimidazolium tetrafluoroborate ionic liquids. The transition temperatures were identified using cloud point measurements. We found that the LCST of this system can be altered by changing the molecular weight and the concentration of the PEO. The influence of IL structure on the phase behavior was also studied by varying the length of the alkyl chain in the imidazolium cation. Figure 1 shows the temperature dependence of the transmittance at 632.8 nm for PEO-20 (the number indicates the molecular weight in kg/mol) in [EMIM][BF4] with different concentrations. The transmittance of 100% indicates a single-phase solution, and a sharp decrease in transmittance indicates that the solution undergoes phase separation. At temperatures higher than the melting point of

B

r 2010 American Chemical Society

Received Date: May 12, 2010 Accepted Date: June 8, 2010 Published on Web Date: June 11, 2010

1962

DOI: 10.1021/jz100617t |J. Phys. Chem. Lett. 2010, 1, 1962–1966

pubs.acs.org/JPCL

Figure 1. Temperature dependence of transmittance at 632.8 nm for PEO-20 in [EMIM][BF4] measured at a heating rate of ∼1 °C/ min. The weight fractions of PEO in solutions are indicated.

Figure 3. LCST phase behavior of PEO-20 in IL blends. (a) Temperature dependence of transmittance at 632.8 nm for 2 wt % PEO-20 in [EMIM][BF4]/[BMIM][BF4] blends measured at a heating rate of ∼1 °C/min. The weight fractions of [BMIM][BF4] in the IL blends are indicated. (b) The LCSTs as a function of the weight fraction of [BMIM][BF4] in IL blends. The solid line is a linear fit to the data.

Figure 2. Temperature-composition phase diagram for PEO-5 and PE-20 in [EMIM][BF4]. The values of LCST were determined from the transmittance measurements shown in Figure 1.

blends that exhibit LCST-type phase behavior,22 although, as compared to other typical polymer solutions with LCSTs, the molecular weight dependence of Tc in the PEO/[EMIM][BF4] system is relatively weak. Even more surprisingly, unlike the typical LCST phase diagram of polymer solutions, the PEO/ [EMIM][BF4] phase diagram is strongly asymmetric, with the critical composition shifted to a high concentration of PEO. The critical compositions (wc) of both PEO-5 and PEO-20 are ∼80 wt %. To investigate how IL structure affects the phase behavior of PEO, we also performed transmittance measurements on 2 wt % PEO-20 in [BMIM][BF4], and the result is shown in Figure 3a. Similar LCST phase behavior was observed, and the LCST was determined to be 209 °C. The length of the alkyl chain in the imidazolium cation has a significant influence on the miscibility of a PEO/IL solution. With the same concentration, the LCSTof the PEO/[BMIM][BF4] solution is roughly 45 °C higher than that in [EMIM][BF4]. Also shown in Figure 3a are the transmittance measurements of 2 wt % PEO-20 in [EMIM][BF4]/[BMIM][BF4] blends with different mixing ratios. Not surprisingly, in all of these IL blends, PEO-20 showed LCST phase behavior. In Figure 3b, we plot the LCST values of PEO-20 in IL blends as a function of the weight fraction of [BMIM][BF4]. The LCST values increase almost linearly from 163 °C for [EMIM][BF4] to 209 °C for

PEO-20 (∼60 °C), PEO-20 and [EMIM][BF4] are completely miscible; a clear solution and ∼100% transmittance are obtained. As the temperature increases above the LCST, the solution becomes cloudy, and the transmittance quickly decreases to almost zero. We define the LCSTas the temperature at which the transmittance drops to 80%. In this figure, the LCSTs of 2, 20, 40, and 80% of PEO-20 in [EMIM][BF4] are determined as 163, 152.5, 140, and 130.5 °C, respectively. After the phase transition, the cloudy solution gradually separates into two liquid-liquid phases. In the stable biphasic liquid mixture, the upper and lower phases comprise PEO-rich and [EMIM][BF4]-rich solutions, respectively, since the densities of PEO and [EMIM][BF 4] are ∼1.12 and ∼1.28 g/cm 3, respectively. This phase transition is completely reversible. Figure 2 shows the temperature-composition phase diagrams for PEO-5 and PEO-20 in [EMIM][BF4]. The phase diagrams were constructed by plotting the cloud points determined by the transmittance measurements illustrated in Figure 1. Both curves are convex downward, and the critical temperatures (Tc) were determined to be 130 and 136 °C for PEO-20 and PEO-5, respectively. Tc increases by only 6 °C as the molecular weight decreases from 20000 to 5000. Such a trend is commonly observed in other polymer solutions and

r 2010 American Chemical Society

1963

DOI: 10.1021/jz100617t |J. Phys. Chem. Lett. 2010, 1, 1962–1966

pubs.acs.org/JPCL

[BMIM][BF4] as the weight fraction of [BMIM][BF4] increases. This result indicates that the value of LCST can be easily controlled by appropriately adjusting the mixing ratio of two ILs. LCST-type phase separation is observed in many polymer solutions characterized by strong interactions. Compared to most organic solvents, water is more likely to have strong interaction with certain classes of polymers, such as polyethers, polyamides, and poly(vinyl ether)s, due to its capacity to form hydrogen bonds. Hence, many polymers show LCST phase behavior in water, whereas fewer polymers show a LCST in organic solvents. In aqueous polymer solutions, the formation of hydrogen bonds between polymer chains and water molecules lowers the enthalpy of mixing (ΔHmix); however, the specific molecular orientations required by these bonds lead to negative changes in entropy (ΔSmix) and positive contributions to the free energy (ΔGmix). Above the LCST, the growing entropic contribution to the free energy is dominant and results in phase separation, as in aqueous solutions of PEO.17,18 In PEO/water, a negative ΔSmix is attributed to the formation of well-oriented solvation around the PEO chains. It is possible that many polymers can show LCST phase behavior in ILs since there are strong interactions, such as Coulombic attractions and hydrogen bonds, and significant solvent structure. However, only recently have there been reports of LCST phase behavior of polymers in ILs. Pioneering studies by Watanabe and co-workers23-27 showed that poly(benzyl methacrylate) (PBnMA) and its derivatives and a PBnMA analogous random copolymer, poly(styrene-comethyl methacrylate), exhibited LCST phase behavior in [CnMIM][TFSA]. They hypothesized that, similar to the formation of liquid clathrate structures due to the cationπ interactions between IL cations and aromatic compounds,28-30 the negative ΔSmix arises from the formation of ordered structures via the cation-π interactions31 between the IL cations and the aromatic groups in the polymers. Watanabe and co-workers also found that poly(ethyl glycidyl ether) (PEGE) showed LCST phase behavior in [EMIM][TFSA].32 1H NMR measurements indicated the formation of hydrogen bonds between the protons of the [EMIM] cations and the oxygen atoms of PEGE. They then proposed that the hydrogen bonds are the primary driving force of the negative ΔSmix and the LCST phase behavior. In the PEO/[CnMIM][BF4] system, we speculate that the negative ΔSmix is caused by the competition between formation of polymer-cation and cation-anion hydrogen bonds. Both NMR measurements combined with IR spectroscopy measurements33 and vibrational spectroscopy measurements combined with theoretical calculations34 showed evidence of hydrogen bonding between the fluoride atoms and the H atoms of both the imidazolium ring and the alkyl side chains. Molecular dynamics simulations19 of PEO in [CnMIM][PF6] showed that the distance between the oxygen atoms of PEO chains and the imidazolium cations is slightly shorter than the anion-cation distance, which is likely due to the formation of hydrogen bonds. On the basis of these experimental and computational results, it is reasonable to infer that in our PEO/ILs system, there are strong interactions between the components, including hydrogen bonding between the

r 2010 American Chemical Society

oxygen atoms of the PEO chain and the H atoms of the imidazolium cations, hydrogen bonding between the fluoride atoms and the H atoms of both the imidazolium ring and the alkyl side chains, and the strong Coulombic attractions between ions. Consequently, similar to aqueous PEO solutions,17 strong interactions result in well-oriented solvation around the PEO, thereby providing the negative ΔSmix that is essential for LCST phase behavior. It is likely that the H bond between the H atom on the C2 position of the imidazolium ring and the O atom of PEO plays the most important role in determining the LCST phase behavior of the PEO/ILs system, as suggested by Watanabe and co-workers.32 However, other H atoms, such as the H atoms in the side chain of the imidazolium ring, may also play a role in the formation of H bonds. Experiments using an ionic liquid with its acidic C2 proton replaced by a methyl group, such as [EMMIM][BF4] and [BMMIM][BF4], may be helpful to clarify this. Two other observations here are also consistent with the PBnMA/ILs systems.23 First, as shown in Figure 3, replacing the ethyl chain in the imidazolium ring with a butyl chain results in a 45 °C increase in the LCST. This result is consistent with the increase in LCST with the alkyl chain length in PBnMA/[CnMIM][TFSA].23 An argument provided by Watanabe and co-workers can be used to explain this observation.23 Simulations35 showed that imidazolium-based ILs exhibited ordered segregated structures, consisting of polar ionic domains and nonpolar alkyl chain domains. It is likely that increasing the alkyl chain length on the imidazolium cation can increase the extent of microphase segregation, which in turn lowers the entropy of the ILs. Therefore, the loss of mixing entropy induced by the formation of hydrogen bonds between PEO and ILs would be reduced, leading to an increase in the LCST. Second, as shown in Figure 3b, in [EMIM][BF4]/[BMIM][BF4] mixtures, the LCST changes almost linearly with the weight fraction of [BMIM][BF4]; a similar trend was also observed for PBnMA in [EMIM][TFSA]/[BMIM][TFSA] blends.23 This linearity suggests that the mixing of the two different ILs is a random process. It is likely that PEO can show LCST phase behavior in other imidazolium-based ionic liquids, such as [EMIM][TFSA] and [BMIM][PF6]. However, the LCSTs in those systems is almost too high to be determined experimentally (well above 200 °C). The observed linear relationship in Figure 3b can be used to estimate a LCST that is too high to be measured directly. On the other hand, phase diagrams such that as shown in Figure 2, with a high critical composition, have not been commonly observed in polymer/IL, water, or organic systems. Both the critical temperature (Tc) and critical composition (wc) are not strong functions of molecular weight. Lowering the molecular weight from 20000 to 5000 only resulted in ∼6 °C increase in Tc, and the values of wc for PEO-5 and PEO-20 in [EMIM][BF4] were essentially the same (∼ 80%). From Flory-Huggins theory, the critical temperature (Tc) can be obtained from the equation   R 1 1 2 þ pffiffiffiffi þ 1 χc ¼ þβ ¼ ð1Þ Tc 2 N N where χc is the interaction parameter at Tc, N is proportional to the degree of polymerization, R/Tc is the enthalpic part of χc,

1964

DOI: 10.1021/jz100617t |J. Phys. Chem. Lett. 2010, 1, 1962–1966

pubs.acs.org/JPCL

and β is the entropic part of χc. Rearranging eq 1 R  Tc ¼  1 1 2 þ pffiffiffiffi þ 1 - β 2 N N

roughly 1 °C/min. The temperature dependence of transmittance was monitored using a laser power detector (SPEX) while the solution was stirred. Above the LCST, liquid/liquid phase separation occurs after stopping the stirring. In a few selected experiments, after heating above the LCSTs of the solutions, we slowly cooled the solutions at a rate of roughly 1 °C/ min and measured the temperature dependence of transmittance. The LCSTs obtained from the cooling experiments are consistent with the values obtained from the heating experiments to within 1 °C.

ð2Þ

In a LCST system, R is negative, implying a net attractive interaction between the components, such as hydrogen bonding, whereas β is positive and is associated with the detailed intermolecular packing. The observations of a weak molecular weight dependence of Tc and wc suggest that β provides the dominant contribution to determine the phase behavior of PEO in [EMIM][BF4]. The LCST phase diagram shown in Figure 2 is strongly asymmetric, with the critical composition shifted to a high concentration of PEO. This unusual phase diagram may be due to the formation of a dense polymer phase, as suggested by de Gennes.36 He predicted that in certain polymer systems such as PEO/water, away from the standard θ point, there may be a tendency for separation into a dense polymer phase and a very dilute system of swollen chains. Clearly, it will be of interest to explore the nature of the PEO/IL LCST system in more detail. In conclusion, we report a novel LCST-type liquid-liquid phase behavior of PEO in [EMIM][BF4], [BMIM][BF4], and their blends. The length of the alkyl chain in the imidazolium cation has a significant influence on the miscibility of this polymer/IL system. The LCST of PEO/[BMIM][BF4] is roughly 45 °C higher than the LCST of PEO/[EMIM][BF4]. Using IL blends as solvents, the LCST could be easily adjusted by varying the mixing ratio of the two ILs. The exact reasons for the unusual temperature-composition phase diagram and the weak molecular weight dependence and the influence of the end-group hydrogen of PEO on the LCST behavior will be explored. The discovery of the LCST phase behavior of PEO in ILs is expected to have impact on the design of a new PEO/ IL-based polymer electrolyte, which has potential applications such as smart materials or solutions for dissolution/ extraction process.

SUPPORTING INFORMATION AVAILABLE Synthesis details. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: lodge@ umn.edu.

ACKNOWLEDGMENT This work was supported by the National Science Foundation (DMR-0804197).

REFERENCES (1)

Sheftel, V. O. Indirect Food Additives and Polymers: Migration and Toxicology; CRC Press: Boca Raton, FL, 2000; spp 1114-1116. (2) Shin, J. H.; Henderson, W. A.; Passerini, S. PEO-Based Polymer Electrolytes with Ionic Liquids and Their Use in Lithium Metal-Polymer Electrolyte Batteries. J. Electrochem. Soc. 2005, 152, A978–A983. (3) Rogers, R. D.; Seddon, K. R. Ionic Liquids ; Solvents of the Future? Science 2003, 302, 792–793. (4) Hough, W. L.; Smiglak, M.; Rodriguez, H.; Swatloski, R. P.; Spear, S. K.; Daly, D. T.; Pernak, J.; Grisel, J. E.; Carliss, R. D.; Soutullo, M. D.; et al. The Third Evolution of Ionic Liquids: Active Pharmaceutical Ingredients. New J. Chem. 2007, 31, 1429–1436. (5) Wang, M.; Xiao, X. R.; Zhou, X. W.; Li, X. P.; Lin, Y. Investigation of PEO-Imidazole Ionic Liquid Oligomer Electrolytes for Dye-Sensitized Solar Cells. Sol. Energy Mater. Sol. Cells 2007, 91, 785–790. (6) Rodriguez, H.; Francisco, M.; Rahman, M.; Sun, N.; Rogers, R. D. Biphasic Liquid Mixtures of Ionic Liquids and Polyethylene Glycols. Phys. Chem. Chem. Phys. 2009, 11, 10916– 10922. (7) He, Y. Y.; Lodge, T. P. Thermoreversible Ion Gels with Tunable Melting Temperatures from Triblock and Pentablock Copolymers. Macromolecules 2008, 41, 167–174. (8) He, Y. Y.; Boswell, P. G.; Buhlmann, P.; Lodge, T. P. Ion Gels by Self-Assembly of a Triblock Copolymer in an Ionic Liquid. J. Phys. Chem. B 2007, 111, 4645–4652. (9) He, Y. Y.; Lodge, T. P. A Thermoreversible Ion Gel by Triblock Copolymer Self-Assembly in an Ionic Liquid. Chem. Commun. 2007, 2732–2734. (10) Bai, Z. F.; He, Y. Y.; Lodge, T. P. Block Copolymer Micelle Shuttles with Tunable Transfer Temperatures between Ionic Liquids and Aqueous Solutions. Langmuir 2008, 24, 5284–5290. (11) Bai, Z. F.; He, Y. Y.; Young, N. P.; Lodge, T. P. A Thermoreversible Micellization-Transfer-Demicellization Shuttle between

EXPERIMENTAL SECTION Two hydroxy-terminated PEO homopolymers (Mn=20000, Mw/Mn = 1.14; Mn = 5000, Mw/Mn = 1.07) were purchased from Aldrich. [EMIM][BF4] and [BMIM][BF4] were synthesized via detailed procedures provided in Supporting Information. All ionic liquids were used after drying in a vacuum oven at 50 °C for 2 days; the mass became constant after ∼12 h of drying. A strict procedure was applied to prepare PEO/IL solutions to minimize water content. PEO/IL solutions were prepared by mixing the polymers and ionic liquids in ampules (diameter = 13 mm) and stirring at 80 °C under vacuum until complete dissolution. The solutions were continuously stirred under vacuum (