Xuerui Cheng

In Situ Observation of Multiple Phase Transitions in Low-Melting Ionic Liquid [BMIM][BF4] under High Pressure up to 30 GPa

In situ characterization of phase transitions and direct microscopic observations of a low-melting ionic liquid, 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM][BF(4)]), has been performed in detail by Raman spectroscopy. Compression of [BMIM][BF(4)] was measured under hydrostatic pressure up to ~30.0 GPa at room temperature by using a high-pressure diamond anvil cell. With pressure increasing, the characteristic bands of [BMIM][BF(4)] displayed nonmonotonic pressure-induced frequency shifts, and it is found to undergo four successive phase transitions at around 2.25, 6.10, 14.00, and 21.26 GPa. Especially, above a pressure of 21.26 GPa, luminescence of the sample occurs, which is connected with the most significant phase transition at around this pressure. It was indicated that the structure change under high pressure might be associated with a conformational change in the butyl chain. Upon releasing pressure, the spectrum was not recovered under a pressure up to 1.16 GPa, thereby indicating that this high-pressure phase remains stable over a large pressure range between 30 and 1.16 GPa in low-melting ionic liquid [BMIM][BF(4)]. Although the sample was kept under the normal pressure for 24 h, the spectrum was recovered, and it showed that the phase transition of [BMIM][BF(4)] was reversible. In other words, such a low-melting ionic liquid [BMIM][BF(4)] remains stable even after being treated under so a high pressure of up to 30 GPa.

In situ crystallization of ionic liquid [Emim][PF6] from methanol solution under high pressure.

The solubility of 1-ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]) in methanol under high pressure is newly measured quantitatively according to the correlation between the ratios of Raman intensity and the concentrations. In situ crystallization and cation conformation of [Emim][PF6] from methanol solution under high pressure have been investigated by using Raman spectroscopy in detail. Remarkably, crystal polymorphism was observed and two crystalline phases (phases I and II) coexisted under high pressure up to ∼ 1.4 GPa. However, only phase II was obtained by recrystallization at ∼ 2 GPa. Our findings may facilitate the development of an effective way for crystallization and purification of ionic liquids under high pressure.

Preparation and characterization of a novel ionic conducting foam-type polymeric gel based on polymer PVdF-HFP and ionic liquid [EMIM][TFSI]

Poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP)/ionic liquid (IL) (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI]) polymer gels have been prepared by solvent volatilization with and without ultrasound irradiation, respectively. The gel structure and electrochemical property are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR), and complex impedance spectroscopy (CIS). It is found that a novel foam-type polymer-ionic liquid gel is prepared with ultrasound irradiation. And, the ultrasound-induced polymer-ionic liquid gel has a higher crystallinity and more diverse crystal size polymer network, comparing with that prepared without ultrasound irradiation. The foam-type gel structure can be explained by the formation of pre-ordered aggregation of molecular chain during the ultrasound irradiation process. The ionic conductivity of the PVdF-HFP/[EMIM][TFSI] gel decline after ultrasound irradiation, which can be attributed to the high crystallinity and looser microstructure. Furthermore, it is found that the ultrasound irradiation can promote the crystalline transition of PVdF-HFP from β to α phase and improve its crystallinity.

In situ solidification of ionic liquid [EMIM][EtOSO3] from melt under high pressure

In situ solidification of 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM][EtOSO3] from melt under high pressure has been investigated by using Raman spectroscopy. The results indicate that [EMIM][EtOSO3] might experience a phase transition at about 2.4 GPa upon compression, which could be identified as solidification to a superpressurized glass by pressure broadening of the sharp ruby R1 fluorescence line. Upon cooling, it solidifies as a glassy state rather than crystallizes at low temperature down to 93 K. These facts are suggestive of a phase transition of liquid to a superpressurized glass induced by compression in [EMIM][EtOSO3], which is similar to the glassy state at low temperature.

High pressure and low-temperature polymorphism in cyclopentanol

The polymorphism of cyclopentanol (C5H10O) has been investigated as a function of temperature at ambient pressure and as a function of hydrostatic pressures to 3.7 GPa at room temperature. Differential scanning calorimetry (DSC) and Raman spectra reveal that two plastic phases and two fully ordered crystalline phases are formed during cooling. High pressure Raman and infrared spectra show that cyclopentanol undergoes two-phase transformations. At around 0.6 GPa, the liquid cyclopentanol transforms to a solid plastic structure. On further compression to 1.9 GPa, one fully ordered crystalline phase is observed. Based on pieces of evidence such as peak splitting and emergence of new peaks, it can be concluded that the ordered crystalline structure has a lower symmetry. In addition, the decrease in the wavenumber of the O–H stretching modes at low temperature and high pressure suggests the ordered crystalline phases are characterized by the formation of hydrogen-bonded molecular chains.

An In situ Study on the Orderly Crystal Growth of Pluronic F127 Block Copolymer Blended with and without Ionic Liquid during Isothermal Crystallization

Isothermal crystallization behavior of Pluronic F127 blended with and without an ionic liquid (IL) was investigated by in situ polarized optical microscopy (POM) and Fourier transform infrared spectroscopy (FTIR). For the pure F127, the POM and FTIR results showed that the spherulite size and crystallinity of F127 increased with the melting temperature increasing to 60, 80, and 135°C. This could be explained by the flexibility of the polymer chain at high melting temperatures. For the F127 blended with IL, the POM results showed that the morphology of F127 evolved from spherulite to dendritic segregation and fibrous crystal with the increasing IL content. FTIR results indicated that hydrogen bonds were formed between F127 and IL, and the intensity of the hydrogen bonds became strengthened gradually with increasing IL content. The effect of hydrogen bonds on the morphology evolution of F127/IL is discussed.