Article pubs.acs.org/Macromolecules
Unusual Lower Critical Solution Temperature Phase Behavior of Poly(ethylene oxide) in Ionic Liquids Hau-Nan Lee,† Nakisha Newell,† Zhifeng Bai,† and Timothy P. Lodge*,†,‡ †
Department of Chemistry and ‡Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: We describe the LCST-type phase behavior of poly(ethylene oxide) (PEO) dissolved in imidazolium-based tetrafluoroborate ionic liquids (ILs). Phase diagrams were determined by a combination of small-angle neutron scattering (SANS) and cloud point (CP) measurements. Unlike typical LCST phase diagrams of polymer solutions, the PEO/IL phase diagram is either roughly symmetric with a critical composition near 50% polymer or asymmetric with a critical composition shifted to an even higher concentration of PEO. As the molecular weight decreases from 20 500 to 4200 g/mol, the critical temperature (Tc) increases slightly (∼10 °C). However, a larger increase in Tc (27 °C) was observed as the molecular weight decreases from 4200 to 2100 g/mol, likely due to the increasing importance of hydrogen bonds between the −OH end groups of PEO and the fluorine atoms of the anions. This inference is supported by the strong dependence of the phase diagram on the identity of the PEO end groups (hydroxy vs methoxy). Furthermore, replacing the most acidic proton of the imidazolium ring (in the C2 position) with a methyl group lowers the Tc and changes the shape of the phase diagram significantly, suggesting that the hydrogen bonds between the H atoms on the C2 position of the imidazolium ring and the O atoms of PEO play an important role in determining the LCST phase behavior of this system.
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INTRODUCTION Ionic liquids (ILs) are a class of solvents with appealing physical properties such as negligible vapor pressure, good chemical and thermal stability, and wide liquid temperature ranges. They have been called “designer solvents” due to the readily tunable solvation properties accessible by varying the chemical structure of the cation or anion.1 Combinations of ILs with polymers have been proposed for many applications such as polymer electrolytes2 in lithium batteries.3 Self-assembled systems such as ion gels,4 micelles,5,6 and vesicles7 can also be obtained by incorporating block copolymers into ILs. Possible applications include membranes for gas separations,8,9 gels for thin-film transistors,10−13 and micelles/vesicles for molecular storage and transport systems.14,15 In order to optimize materials for these applications, knowledge of the phase behavior of polymers in ILs over a wide range of temperature is essential.16,17 A full understanding of polymer solution phase behavior has proven to be challenging. The narrow liquid range and high vapor pressure of water or organic solvents limit the experimental temperature range. In contrast to water or organic solvents, the nonvolatility and good thermostability of ILs provide a great opportunity to investigate the phase behavior over a much wide range of temperature. Furthermore, in polymer/IL systems, both LCST and UCST phase behavior have been reported. For example, Ueki and Watanabe demonstrated the UCST phase behavior of poly(N-isopropylacrylamide) in 1-ethyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide ([EMIM][TFSA]).18 Later, © 2012 American Chemical Society
Watanabe and co-workers showed that poly(benzyl methacrylate) and its derivatives19−25 exhibit LCST phase behavior in 1alkyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide [CnMIM][TFSA]. They hypothesized that the formation of ordered structures via the cation−π interactions26 between the IL cations and the aromatic groups in the polymers leads to the LCST phase behavior. Watanabe and co-workers also discovered that poly(ethyl glycidyl ether) (PEGE) showed LCST phase behavior in [EMIM][TFSA].27 In this case they proposed that the hydrogen bonds between PEGE and [EMIM][TFSA] are the primary driving force of the LCST phase behavior. More recently, we reported the LCST-type phase behavior of poly(n-butyl methacrylate) (PnBMA) dissolved in [CnMIM][TFSA], and the LCST of PnBMA can be tuned over a wide range (>230 °C) by mixing two different alkylmethylimidazolium ILs, without modifying the chemical structure of the polymers.28 One common feature of these UCST or LCST polymer/IL systems is that they all exhibit asymmetric temperature−composition phase diagrams, with the critical composition shifted to low concentrations of polymer, qualitatively consistent with the predictions of Flory− Huggins theory. In a previous study using optical transmittance, we reported that PEO exhibits unusual temperature−composition phase Received: February 18, 2012 Revised: March 30, 2012 Published: April 10, 2012 3627
dx.doi.org/10.1021/ma300335p | Macromolecules 2012, 45, 3627−3633
Macromolecules
Article
with a thermocouple in contact with the heating block. A sample-todetector distance of 3 m, a monochromated neutron wavelength λ = 7 Å, and a wavelength spread of Δλ/λ = 0.115 were used to cover a scattering wave vector range q = 0.01−0.13 Å−1. The scattering vector is defined as q = (4π/λ) sin(θ/2), where λ and θ are the neutron wavelength and scattering angle, respectively. Using established procedures,30 the two-dimensional scattering spectra were reduced by azimuthal integration to yield the one-dimensional form of intensity I(q) vs q, corrected for background detector efficiency, empty cell scattering, and sample transmission and then converted to absolute intensity with direct beam flux measurements. Finally, incoherent scattering from the sample was subtracted.
diagrams in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), in which the critical composition inferred from cloud points (CP) shifts to high concentrations of PEO.29 The LCST of this system can be tuned by varying molecular weight of PEO or by modifying the chemical structures of the IL. The present work extends the previous study by experimentally examining the influence of hydrogen bonding between the components on the LCST phase behavior and by utilizing small-angle neutron scattering (SANS) to access the spinodal curves, thereby reinforcing the location of the critical point. Phase diagrams determined by both SANS and CP measurements are presented.
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RESULTS AND DISCUSSION Could Point Measurements of PEO in ILs. LCST phase behavior of PEOs in [EMIM][BF4] or [EMMIM][BF4] was investigated using CP measurements. Figure 1 shows the
EXPERIMENTAL SECTION
Materials. 1-Ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), 1-ethyl-2,3-dimethylimidazolium tetrafluoroborate ([EMMIM][BF4]), and deuterated 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM-D5][BF4]) were synthesized in house; the details of synthetic procedures can be found in the Supporting Information. The PEO homopolymers used in this study were purchased from Aldrich. The Mn and Mw/Mn of these PEOs were determined by size exclusion chromatography (SEC) and matrixassisted laser desorption/ionization time-of-flight (MALDI-TOF) measurements and are summarized in Table 1. The details of polymer characterization are described in the Supporting Information.
Table 1. Sample Characteristics samples
Mn (kg/mol)
Mw/Mn
PEO-20(−OH)a PEO-11(−OH)a PEO-4(−OH)a PEO-2(−OH)b PEO-2(−OCH3)b PEO-2(−OH, −OCH3)b
20.5 10.7 4.2 2.1 1.7 1.9
1.03 1.07 1.26 1.01 1.08 1.02
Figure 1. Temperature dependence of transmittance at 632.8 nm for PEO-20(−OH) in [EMIM][BF4] measured at a heating rate of ∼1 °C/min. Weight fractions of PEO-20(−OH) in solutions are indicated.
a
Mn and Mw/Mn of PEO-20(−OH), PEO-11(−OH), and PEO4(−OH) were determined by size exclusion chromatography (SEC) with THF containing 1 vol % of tetramethylethylenediamine (TMEDA) as the eluent. bMn and Mw/Mn of PEO-2(−OH), PEO2(−CH3) and PEO-2(−OH, −OCH3) were determined by MALDITOF mass spectrometry. The numbers indicate the molecular weight in kg/mol. The −OH or −OCH3 functional groups in parentheses indicate the types of PEO end groups.
temperature dependence of the transmittance at 632.8 nm for PEO-20(−OH) in [EMIM][BF4] with different concentrations. Transmittance of 100% indicates a single-phase solution, and a decrease in transmittance indicates that the solution undergoes liquid−liquid phase separation. As shown in Figure 1, at low temperatures, PEO-20(−OH) in [EMIM][BF4] systems are completely miscible. However, as the temperature increases above the LCST, the solution becomes cloudy and the transmittance rapidly decreases. The CPs of 1, 5, 10, 20, and 30 wt % of PEO-20(−OH) in [EMIM][BF4] are determined to be 129.9, 125.4, 123.8, 121.8, and 119.5 °C, respectively. The change in solubility with temperature is completely reversible; upon cooling, the solution becomes clear again. Figure 2 shows the temperature−composition phase diagrams for different molecular weight PEOs in [EMIM][BF4]. The phase diagrams were constructed by plotting the CPs determined by the transmittance measurements illustrated in Figure 1. All curves are convex downward, and the critical temperatures (Tc) are estimated to be 119, 121, 130, and 157 °C for PEO-20(−OH), PEO-11(−OH), PEO-4(−OH), and PEO-2(−OH), respectively. Tc slowly increases as the molecular weight decreases from 20 500 to 4200 g/mol; the molecular weight dependence of Tc in this molecular weight range is relatively weak. However, a more substantial increase in Tc (27 °C) was observed as the molecular weight decreases from 4200 to 2100 g/mol. In addition, unlike the typical LCST phase diagram of polymer solutions and blends,27,31 the PEO/
Preparation of PEO/IL Solutions. A procedure was utilized to prepare PEO/IL solutions for CP and SANS measurements to minimize water content. For CP measurements, PEO and IL were combined in ampules and heated to ∼80 °C, with stirring, to melt the PEO and make homogeneous PEO/IL solutions. The solutions were then dried under vacuum (
