Unusual Lower Critical Solution Temperature Phase Behavior of Poly(ethylene oxide) in Ionic Liquids

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 (T_c) increases slightly (∼10 °C). However, a larger increase in T_c (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 T_c 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.