Phase transitions and structural changes of ionic liquid crystals in nanopores

Ionic liquid crystals (ILCs) are ionic liquids including liquid crystal molecules, and hence ILCs show interesting phase transition behavior. In our previous study, 1-methyl-3-alkylimidazolium and tetrauoroborate ([C_nmim]BF₄), by using various experimental techniques. In this study, DSC, X-ray and dielectric measurements were performed to clarify phase transitions of ILCs confined in nanopores. For the heating process from the highly-ordered smectic phase, DSC together with X-ray scattering measurements on [C₁₂mim]BF₄ clearly show that the phase transition behavior in this system changes depending on the pore size as follows: For pore sizes larger than 160 nm, during the heating process from highly ordered smectic phase, ILC melts at ca. 3 deg C and immediately starts to crystallize. Then, they transform to the Sm phase at ca. 28 deg C after a transition to another crystalline phase. For pore sizes less than 40 nm, the phase transition near 3 deg C is shifted to ca. -3 deg C, and no thermal anomaly is observed near 28 deg C. The X-ray pattern also shows a change from the highly ordered smectic phase, where there is a broad peak at q = 2.2 nm^-1, into another phase at -3 deg C, then a wide-angle ordered pattern changes to only a broad amorphous halo at about 15 deg C. In the presentation, we will discuss in detail the structural changes associated with the phase transition in ILCs in nanopore, together with the results of dielectric measurements.