Abstract
The conformation of 36 kDa polyethylene oxide (PEO) dissolved in three glyme-Li(+) solvate ionic liquids (SILs) has been investigated by small angle neutron scattering (SANS) and rheology as a function of concentration and compared to a previously studied SIL. The solvent quality of a SIL for PEO can be tuned by changing the glyme length and anion type. Thermogravimetric analysis (TGA) reveals that PEO is dissolved in the SILs through Li(+)-PEO coordinate bonds. All SILs (lithium triglyme bis(trifluoromethanesulfonyl)imide ([Li(G3)]TFSI), lithium tetraglyme bis(pentafluoroethanesulfonyl)imide ([Li(G4)]BETI), lithium tetraglyme perchlorate ([Li(G4)]ClO4) and the recently published [Li(G4)]TFSI) are found to be moderately good solvents for PEO but solvent quality decreases in the order [Li(G4)]TFSI ∼ [Li(G4)]BETI > [Li(G4)]ClO4 > [Li(G3)]TFSI due to decreased availability of Li(+) for PEO coordination. For the same glyme length, the solvent qualities of SILs with TFSI(-) and BETI(-) anions ([Li(G4)]TFSI and [Li(G4)]BETI) are very similar because they weakly coordinate with Li(+), which facilitates Li(+)-PEO interactions. [Li(G4)]ClO4 presents a poorer solvent environment for PEO than [Li(G4)]BETI because ClO4(-) binds more strongly to Li(+) and thereby hinders interactions with PEO. [Li(G3)]TFSI is the poorest PEO solvent of these SILs because G3 binds more strongly to Li(+) than G4. Rheological and radius of gyration (Rg) data as a function of PEO concentration show that the PEO overlap concentrations, c* and c**, are similar in the three SILs.
Highlights
IntroductionIn Li+–glyme SILs, the glyme molecules bind to the Li+ ion to produce large complex cations
Ionic liquids (ILs) are salts with melting points below 100 1C
We examine the relationships between the chemical structure of the SIL and the solvent environment it provides for Poly(ethylene oxide) (PEO) using small angle neutron scattering (SANS), thermogravimetric analysis (TGA), and rheology
Summary
In Li+–glyme SILs, the glyme molecules bind to the Li+ ion to produce large complex cations. In a poor SIL, Li+–anion interactions are stronger than Li+–glyme interactions, and there can be up to 90% uncoordinated free glyme in the SIL.[14] Good SILs have properties similar to a conventional IL, whereas the properties of poor SILs are like those of concentrated salt solutions.[20] Henderson et al studied the ionic association of different glymes (G1 to G4) in a variety of lithium salts, illustrating the effects of ionic association strength of the salts and the coordinating glyme length on the formation of solvate ionic liquids.[21,22,23] In general, good SILs are more likely to form when the ionic association strength in the salt is relatively weak. The length of glyme has a more complex effect, it appears that the SILs with longer glyme are more stable, as they are more slowly exchangeable.[21]
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