Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Effect of Ionic Liquid Components on the Coil Dimensions of PEO Aakriti Kharel† and Timothy P. Lodge*,‡,† †
Department of Chemistry and ‡Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455-0431, United States
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ABSTRACT: Small-angle neutron scattering is used to measure the infinite dilution radius of gyration (Rg,0) for a range of molecular weights (10−250 kg/mol) of perdeuterated poly(ethylene oxide) (d-PEO) in various imidazolium-based ILs at 80 °C. The Flory exponents (ν) evaluated in the ILs studied (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]), 1butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), and 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM][BF4])) lie between 0.58 and 0.60, indicating good solvent behavior. The coil dimensions of d-PEO increase on increasing the alkyl chain length of the cation and on decreasing the anion basicity. A more pronounced effect on coil dimensions is observed upon altering the anion, compared to the dependence on cation alkyl chain length. The temperature dependence of Rg,0 is found to be moderate in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), where the coil size decreases upon increasing the temperature from 80 to 135 °C. Addition of up to 10 wt % of a salt, lithium bis(trifluoromethylsulfonyl)imide, to mixtures of d-PEO and [BMIM][TFSI] has no significant effect on Rg,0. These results are discussed in terms of the relative interactions among PEO, the cation, and the anion.
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On the experimental side, several studies16−21 have assessed the chain dimensions of moderate chain lengths of PEO in ionic liquids using SANS. However, experiments that evaluate coil dimensions for a range of PEO molecular weights are desirable so that ν can be obtained. Recently, we used SANS to provide the first experimental evidence22 establishing the dependence of the infinite dilution radius of gyration (Rg,0) on polymer molecular weight for deuterated PEO (d-PEO) in [BMIM][BF4] at 80 °C. The Flory exponent ν = 0.55 ± 0.02, suggesting that PEO is a slightly swollen flexible coil in [BMIM][BF4]. More recently, this work has also successfully guided the development of new simulation models that accurately predicted PEO chain conformation for modest chain lengths, obtaining ν ≈ 0.56.23 Both the phase behavior and chain conformations are dictated primarily by polymer−solvent interactions, which can be affected by varying solvent, temperature, and the addition of salt. In the case of ILs as solvents, interactions can be readily tuned by altering the cation or anion. For example, the mixing behavior of polymers in ILs varies significantly with the ion identity in many cases.1,10 The LCST of PEO in [BMIM][BF4] with a butyl chain at the imidazolium cation is 45 °C higher than its ethyl counterpart, [EMIM][BF4].11,12 An even more pronounced effect is observed when the anion BF4− is replaced by bis(trifluoromethylsulfonyl)imide (TFSI) or hexafluorophosphate (PF6−), as no phase separation has been observed in the experimental temperature window (up to 200 °C) for PEO in these ILs. It is now well established that
INTRODUCTION Ionic liquids (ILs) offer unique combinations of physicochemical properties such as nonvolatility, nonflammability, and high thermal and chemical stabilities, making them interesting solvents for polymers.1 Moreover, the selection of an appropriate cation or anion can alter the properties of ILs and establish a platform for a wide range of tunable solvation properties. Polymer and IL mixtures have also been investigated as promising materials for various applications,2,3 including printable electronics,4,5 gate dielectrics in organic thin-film transistors,6,7 lithium batteries,8 and gas separation membranes,9 but the effective design of materials and success of these applications are dependent on the fundamental understanding of the solvation behavior of polymers in ILs. The phase behavior of polymers in imidazolium-based ILs is interesting, as a lower critical solution temperature (LCST) is common, presumably due in part to the structure-forming tendency of the IL.10 One well-studied system is poly(ethylene oxide) (PEO) in imidazolium-based ILs, which exhibits an unusual LCST-type phase separation. Lee and Lodge11,12 used cloud point measurements and small-angle neutron scattering (SANS) to determine the phase diagram for several PEOs in 1ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) and found that the critical concentration is in the range of 40− 80 wt % PEO, unexpectedly shifted toward polymer-rich compositions. The conformational behavior of PEO in 1-butyl3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) also raises questions. Computer simulations13−15 yield conflicting chain dimensions of PEO in [BMIM][BF4], using both atomistic and coarse-grained models at various temperatures, with the discrepancies attributable to the limitation of these models to account for the complex interactions of PEO in ILs. © XXXX American Chemical Society
Received: February 18, 2019 Revised: March 29, 2019
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DOI: 10.1021/acs.macromol.9b00354 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
mTorr) for 48 h at 60 °C. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) was purchased from 3M, vacuum dried at 100 °C for 96 h, and stored in an argon-filled glovebox. Solution Preparation. Dilute d-PEO/IL solutions were prepared using a cosolvent, as described elsewhere.22 In short, the polymer and the solvent were combined gravimetrically, followed by the addition of dichloromethane (DCM) to facilitate dissolution. The solutions were stirred under N2 purge to evaporate DCM overnight. Finally, the mixtures were dried under dynamic vacuum (
