Phase transitions and dynamcis in ionic liquid crystals confined in nanopores

We investigated the phase transition behavior of ionic liquid crystals, 1-methyl-3-alkylimidazolium tetrafluoroborate, [C_n mim]BF₄, confined in cylindrical nanopores using differential scanning calorimetry, X-ray scattering, and dielectric relaxation spectroscopy. For n=12 and 10, the isotropic liquid phase changes to the smectic phase and then to a metastable phase for the cooling process. During the subsequent heating process, the metastable phase changes to the isotropic phase via crystalline phases. The transition temperatures for this ionic liquid confined in nanopores decrease linearly with the increase in the inverse of the pore diameter, except for the transitions between the smectic and isotropic phases. In the metastable phase, the relaxation rate of theα-process shows the Vogel-Fulcher-Tammann type of temperature dependence for some temperature ranges. The glass transition temperature evaluated from the dynamics of the α-process decreases with the decrease in the pore diameter and increases with the increase in the carbon number n. The confinement effect on the chain dynamics can clearly be observed for this ionic liquid crystal. For n=10, the melting temperature of the crystalline phase is slightly higher than that of the smectic phase for the bulk, while that of the smectic phase increases more significantly with the decrease in pore diameter than that of the crystalline phase. This suggests that the smectic phase can be a thermodynamically stable phase due to the confinement effect.