Evidence for de Vries structure in a smectic-Aliquid crystal observed by polarized Raman scattering

Abstract

The second- and fourth-order apparent orientational order parameters of the core part of the molecule P2 (app) and P4 (app) , have been measured by polarized vibrational Raman spectroscopy for a homogeneously aligned ferroelectric smectic liquid crystal with three dimethyl siloxane groups in the achiral terminal chain, which shows de Vries-type phenomena, i.e., very large electroclinic effect in the smectic- A (Sm-A) phase and a negligible layer contraction at the phase transition between the Sm-A and Sm- C(*) phases. The apparent orientational order parameters of the rigid core part of the molecule are extremely small both with and without the external electric field in Sm-A . These results provide evidence for the existence of the de Vries Sm-A phase, where the local molecular director is tilted at a large angle.

Birefringence suppression near the smectic A–smectic C phase transition in de Vries-type liquid crystals

“…While known SmA-SmC (or −SmC * ) transitions are not too far from a tricritical point, all these materials have phase transitions particularly close to or at a tricritical point. The fluoroether tail (8422, 8422[2F3], and others), the MHPOBC tail (and related compounds) and the siloxane tail (TSiKN65 and others) in these materials have been found by polarised infrared and Raman spectroscopy to assume unusual conformations in the SmC (or SmC * ) phase [43][44][45][46] where the two tails tilt in opposite directions relative to the layer normal to form a sickle-like shape. It has been hypothesised that this property may suppress tail out-of-layer fluctuations, which could result in smaller layer compressibility.…”

A study of a ferroelectric organosiloxane liquid crystal with a high spontaneous polarisation

“…If the phase transition is not purely of the second kind, but is of the first kind or some intermediate type, then the pyroelectric Chynoweth technique [16,17] does not work correctly. Another reason for the unusual behaviour of the pyroelectric coefficient may be the extremely low order parameter in this material, even when a rather large electric field is applied across the cell [21]. In another organosiloxane material DSiKN65, Selinger et al [22] have found the analogous results for the phase transition temperatures being contradicted by different methods.…”

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

“…They attributed these low order parameters to the segregation of siloxane segments within each layer. Hayashi et al [34] using Raman spectroscopy found a low order parameter for TSiKN65 both with and without field. They attributed this to a large tilt (∼30 • ) of the mesogen (treated as a rigid core) from the long molecular axis.…”