Observation of the de Vries behavior in SmA* phase of a liquid crystal using polarised Raman scattering and infrared spectroscopy

Abstract

Two approaches exist in the literature for describing the orientational distribution function (ODF) of the molecular directors in SmA* phase of liquid crystals, though several models are recently proposed in the literature for explaining the de Vries behaviour. These ODFs correspond to either the conventional unimodal arrangements of molecular directors arising from the mean field theory that leads to the broad or sugar-loaf like distribution or to the "diffuse-cone-shaped" type distribution proposed by de Vries. The hypothesis by de Vries provides for a realistic explanation as to how at a molecular level, a first-order SmA* to SmC* transition can occur where the uniform molecular director azimuthal distributions condense to values lying within a narrow range of angles; finally these condense to a single value while at the same time ensuring a little or no concomitant shrinkage in the layer spacing. The azimuthal distribution of the in-layer directors is probed using IR and polarized Raman spectroscopic techniques. The latter allows us to obtain the ODF and the various order parameters for the uniaxial and the biaxial phases. Based on the results of these measurements, we conclude that the "cone-shaped" (or volcano-shaped) de Vries type of distribution can most preferably describe SmA* where "a first-order phase transition from SmA* to SmC*" and a low layer shrinkage can both be easily explained.

Influence of Vibrational Strong Coupling on an Ordered Liquid Crystal

“…Substituting the experimentally observed Rabi splitting Ω R for the smectic phase in eq 10, we obtain γ sme∥ = 0.67 and γ sme⊥ = 0.37. The orientational distribution function of an ordered liquid crystal phase has been studied with various techniques such as polarized absorption spectroscopy, 52 depolarization of the Raman scattering, 53 and X-ray scattering. 45,54 Here we follow the simplest method based on the order parameter derived from FTIR absorbance.…”

“…The orientational distribution function of an ordered liquid crystal phase has been studied with various techniques such as polarized absorption spectroscopy, depolarization of the Raman scattering, and X-ray scattering. , Here we follow the simplest method based on the order parameter derived from FTIR absorbance. For the smectic ∥ configuration, we assume that the electric field polarization is parallel to the molecular director (i.e., θ = 0).…”

Molecular biaxiality determines the helical structure – infrared measurements of the molecular order in the nematic twist-bend phase of difluoro terphenyl dimer

“…This is due to the rearrangement of molecules in the N TB phase as a helical structure appears. The orientation of the director gradually deviates from the helix axis, which is macroscopically observed as a reduction of the order parameter similar to the SmC phase [43][44][45]. Using polarized IR spectroscopy we can directly deliver the temperature dependence of the absorbance components in the temperature range of the nematic and the N TB phases.…”

Observation of “de Vries-like” properties in bent-core molecules

“…1). 10 Reports show that a substantial amount of “de Vries” LC materials have been prepared but all of them consist of rod-like (or calamitic) molecules mainly with polysiloxane 11–16 and polysilane 17–20 terminal chains. The nanosegregation of domains due to the immiscible chemically different sub-units i.e.…”