2 research outputs found
Unusual Lower Critical Solution Temperature Phase Behavior of Poly(ethylene oxide) in Ionic Liquids
We describe the LCST-type phase behavior of polyÂ(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 (<i>T</i><sub>c</sub>) increases slightly (∼10 °C).
However, a larger increase in <i>T</i><sub>c</sub> (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 C<sub>2</sub> position) with a methyl group lowers the <i>T</i><sub>c</sub> and changes the shape of the phase diagram
significantly, suggesting that the hydrogen bonds between the H atoms
on the C<sub>2</sub> position of the imidazolium ring and the O atoms
of PEO play an important role in determining the LCST phase behavior
of this system