Unusual phase transition mechanism of poly(ethylene oxide) in an ionic liquid: opposite frequency shifts in C-H groups

The dynamic thermally reversible phase transition behaviour of poly(ethylene oxide) (PEO) in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) has been investigated by calorimetric measurements (DSC) and FT-IR, in combination with perturbation correlation moving window (PCMW) and two-dimensional correlation spectroscopy (2DCOS) for the first time. A relatively high lower critical solution temperature (LCST) was observed which is different from that of conventional thermo-sensitive polymers in aqueous solution. Four types of hydrogen bonds were found in this system, among which the type I (the hydrogen bonds between the unsaturated C-H on the imidazole ring and the oxygen atoms of PEO) and type II (the hydrogen bonds between the saturated C-H on the PEO backbone and the fluorine atoms of the IL anions) ones have a more predominant influence on the phase transition. Moreover, type I plays the predominant role in both the heating and cooling process. DSC revealed that the phase transition temperature of PEO-[EMIM][BF4] solutions decreases with the increasing concentration of PEO, as well as with a decreasing scanning rate. Through PCMW, the LCST was determined to be ca. 137 degrees C during heating and ca. 131 degrees C during cooling. Finally, 2DCOS was employed to elucidate the dynamic mechanism of PEO in [EMIM][BF4], which revealed hydrogen bond disruption, chain aggregation and exclusion of IL molecules. Solutions with a low PEO concentration tend to form loosely aggregated globules, compared to the more densely aggregated globules in samples with a high PEO concentration. Additionally, it is worth mentioning that several unusual and peculiar phenomena were also observed, such as opposite frequency shifts in C-H groups, a "tail-raising" phenomenon, critical PEO concentration for the phase transition temperature, and intersection of the frequency shift curves for samples with high concentrations of PEO during cooling.