de Vries Behavior in Smectics near a Biaxiality Induced Smectic A – Smectic C Tricritical Point

Abstract

We show that a generalized Landau theory for the smectic A and C phases exhibits a biaxiality induced AC tricritical point. Proximity to this tricritical point depends on the degree of orientational order in the system; for sufficiently large orientational order the AC transition is 3D XY like, while for sufficiently small orientational order, it is either tricritical or 1st order. We investigate each of the three types of AC transitions near tricriticality and show that for each type of transition, small orientational order implies de Vries behavior in the layer spacing, an unusually small layer contraction. This result is consistent with, and can be understood in terms of, the ''diffuse cone'' model of de Vries. Additionally, we show that birefrin gence grows upon entry to the C phase. For a continuous transition, this growth is more rapid the closer the transition is to tricriticality. Our model also predicts the possibility of a nonmontonic temperature dependence of birefringence.

Ionic Liquid Crystals: Versatile Materials

“…This phenomenon could be explained by de Vries via his “diffuse cone” model, that is, lack of correlation in the tilt direction within the smectic layers (the “noncorrelation” model that proposes lack of correlation between the smectic layers is generally discarded nowadays). ,− During the transition, the azimuthal distribution of the molecular tilt directions, which is degenerate in the SmA phase, becomes biased (ordered) toward a certain direction in the azimuthal plane in which the macroscopic tilt of the SmC director n then appears. The interested reader is referred to the work of (among others) Giesselmann, Lagerwall, Lemieux, Clark, and Walba for more information about de Vries phase transitions and de Vries materials. ,− …”

1H NMR Relaxometric Study of Molecular Dynamics in a “de Vries” Liquid Crystal

“…Structural investigations on these systems were performed by several research teams, and the main techniques available to investigate the fundamental issues on this topic are X-ray diffraction and scattering, ,, nuclear magnetic resonance (NMR), − infrared (IR), and Raman spectroscopies. Moreover, several theoretical studies − ,− were published trying to explain the experimental results in terms of supramolecular structure, distribution of orientations within the smectic layers and orientational ordering properties.…”

“…An alternative model, proposed by Diele and referred to as conformational change model , justifies the absence of a layer shrinkage at the SmA–SmC* transition by a molecular conformational change. A third model was contemporaneously proposed by Saunders et al, Gorkunov et al, and Osipov et al, and it is referred to as interdigitated model . This model is particularly suited for nanosegregated systems, such as fluorinate or organosiloxane derivatives, which have small nematic and high smectic orientational order. , According to this model, molecules are not tilted in the SmA phase, but the order is extremely low and molecules are interdigitated.…”