Phase behavior and characterization of heptamethyltrisiloxane-based de Vries smectic liquid crystal by electro-optics, x rays, and dielectric spectroscopy

Abstract

A heptamethyltrisiloxane liquid crystal (LC) exhibiting I-SmA^{*}-SmC^{*} phases has been characterized by calorimetry, polarizing microscopy, x-ray diffraction, electro-optics, and dielectric spectroscopy. Observations of a large electroclinic effect, a large increase in the birefringence (Δn) with electric field, a low shrinkage in the layer thickness (∼1.75%) at 20 °C below the SmA^{*}-SmC^{*} transition, and low values of the reduction factor (∼0.40) suggest that the SmA^{*} phase in this material is of the de Vries type. The reduction factor is a measure of the layer shrinkage in the SmC^{*} phase and it should be zero for an ideal de Vries. Moreover, a decrease in the magnitude of Δn with decreasing temperature indicates the presence of the temperature-dependent tilt angle in the SmA^{*} phase. The electro-optic behavior is explained by the generalized Langevin-Debye model as given by Shen et al. [Y. Shen et al., Phys. Rev. E 88, 062504 (2013)10.1103/PhysRevE.88.062504]. The soft-mode dielectric relaxation strength shows a critical behavior when the system goes from the SmA^{*} to the SmC^{*} phase.

Chiral smectic- A and smectic- C phases with de Vries characteristics

“…A plausible explanation for the discrepancy between the two is as follows: surfaces in a planar-aligned cell tend to distort the helical structure of the Sm-C * phase. Values of the tilt angle with the applied voltage obtained by us show striking similarities in magnitude and response to the tilt angles measured by the electro-optical method [26,27] for both samples. For W599, Shen et al [27] found the tilt angle saturated at an angle as large as ∼33…”

“…2. These compounds were resynthesized by the Stevenson group in Belfast; the synthesis of MSi 3 MR 11 in particular is described in [26]. MSi 3 MR 11 is made up of a biphenyl 2-chloro-3-methypentanoate core with a trisiloxane backbone while W599 has a tricarbosilane tail.…”

“…The angle in the lower limit min is extracted from the experimental birefringence measurements made in the absence of the electric field applied to the cell. The birefringence data are taken from the literature [26,27]. It can be observed from Figs.…”

On the orientational distribution functions in de Vries-type smectic liquid crystals

“…This apparent field-induced opening of the cone was attributed to the conventional electroclinic effect comprising a tilt of the rod-like molecules in addition to the azimuthal biasing on the cone [38]. Similar investigations were done by other groups as well [5,36,40] and all led to the conclusion that the ∆n(θ)-coupling is well described by the assumption of Langevin-like ordering of molecules distributed on a hollow cone. This certainly gives strong support to the hollow cone model.…”

Fast Ferroelectric Liquid Crystal Based Optical Switch: Simulation and Experiments

“…While cooling down from isotropic temperature, θ exhibits optimal angle at room temperature. The phase transition to the ferroelectric phase θ is considered as the primary order parameter, and it is the angle between the smectic layer and the principal director n. 20 30 The switching time of FLC material under investigation has been characterized as a function of an applied electric field, shown in Figure 6b. Fast response time is one of the important peculiarities of FLCs together with bi-stability when compared to conventional nematic LC switches.…”