Phase transitions and dynamics in ionic liquid crystals confined in nanopores.

Abstract

We investigate the phase-transition behavior of ionic liquid crystals, namely 1-methyl-3-alkylimidazolium tetrafluoroborate, [Cnmim]BF4, confined in cylindrical nanopores using differential scanning calorimetry, x-ray scattering, and dielectric relaxation spectroscopy. Here, n is the number of carbon atoms in the alkyl part of this ionic liquid crystal. For n = 10 and 12, 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 crystal confined in nanopores decrease linearly with the increase in the inverse pore diameter, except for the transitions between the smectic and isotropic phases. In the metastable phase, the relaxation rate of the α-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 effect of confinement 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, in the nanopores, the melting temperature of the smectic phase is higher than that of the crystalline phase. This suggests that the smectic phase can be thermodynamically stable, thanks to the confinement effect.