Dynamics characterization of the glass formation of twist-bend liquid crystal dimers through dielectric studies
Broadband dielectric spectroscopy was used to study the molecular orientational dynamics of the glass-forming, thioether-linked cyanobiphenyl liquid crystal dimers. As was expected theoretically, two different relaxation processes that contributed to the dielectric permittivity of dimers were observed. The low-frequency relaxation mode, m1, was attributed to an end-over-end rotation of the dipolar groups parallel to the director. The high-frequency relaxation mode, m2, was associated with the precessional motions of the dipolar groups around the director. The relaxation times for both modes were analyzed over a wide temperature range down to near the glass transition temperature. The different analytic functions used to characterize the temperature dependence of the relaxation times of the two modes are discussed. Notably, the critical-like description via the dynamic scaling model gave good numerical fittings and a consistent physical picture of the orientational dynamics on approaching the glass transition. Compared to the IR spectroscopy finding, in the higher temperature region of the twist-bend phase, where the longitudinal correlations of dipoles grew, the m1 mode experienced a sudden increase in enthalpy. At the same time, m2 changed continuously, which is described by the critical mode coupling behavior. Both types of molecular motion appear to strongly cooperate at a low-temperature range of the twist-bend phase but change in a coordinated manner as the temperature of the material approaches the glass transition point. As was expected, it was found that both molecular motions determined the glass dynamics at the same glass transition temperature.