Results of studies of the static and dynamic dielectric properties in rod-like 4-n-octyloxy-4'-cyanobiphenyl (8OCB) with isotropic (I)-nematic (N)-smectic A (SmA)-crystal (Cr) mesomorphism, combined with measurements of the low-frequency nonlinear dielectric effect and heat capacity are presented. The analysis is supported by the derivative-based and distortion-sensitive transformation of experimental data. Evidence for the I-N and N-SmA pretransitional anomalies, indicating the influence of tricritical behavior, is shown. It has also been found that neither the N phase nor the SmA phase are uniform and hallmarks of fluid-fluid crossovers can be detected. The dynamics, tested via the evolution of the primary relaxation time, is clearly non-Arrhenius and described via τ(T) = τ(c)(T-T(C))(-φ). In the immediate vicinity of the I-N transition a novel anomaly has been found: Δτ is proportional to 1/(T - T*), where T* is the temperature of the virtual continuous transition and Δτ is the excess over the 'background behavior'. Experimental results are confronted with the comprehensive Landau-de Gennes theory based modeling.

https://iopscience.iop.org/article/10.1088/0953-8984/25/24/245105/pdf

Distortion-sensitive insight into the pretransitional behavior of 4-n-octyloxy-4′-cyanobiphenyl (8OCB)

The present work arises from the significant difference between the experimental Landau tricritical point (LTCP) in the Sm A-to-N phase transition in binary mixtures of butyloxybenzylidene octylaniline (4O.8) and hexyloxybenzylidene octylaniline (6O.8) predicted by Stine and Garland, and that arising from the thermodynamic assessment using the Oonk's Equal Gibbs Curve method. By use of specific heat measurements the 4O.8 + 6O.8, phase diagram has been determined anew. The results of the subsequent application of Oonk's thermodynamic analysis are discussed and their compatibility with the behaviour universally exhibited by other liquid crystal binary mixtures for which a LTCP in the SmA-to-N phase transition has been experimentally determined are analysed.

Tests of the tricritical point in the SmA-to-N phase transition of binary mixtures of butyloxybenzylidene octylaniline and hexyloxybenzylidene octylaniline

We propose a phenomenological model to describe the tricritical behavior of the nematic to smectic-A (N-SmA) phase transition in liquid crystal mixture. To describe the mesophase transitions in binary mixture, nematic and smectic order parameters have been coupled with the concentration. We show that a tricritical point on the N-SmA phase transition line can be achieved under certain conditions. The predictive capability of the present model for determining the tricritical point of a binary mixture displaying the N-SmA transition has been demonstrated by testing with reported phase diagrams sharing both phases.

Tricritical behavior of the nematic to smectic-A phase transition in the binary mixture of liquid crystal

Improved analysis of the effect of chain flexibility on the nematic to smectic-A phase transition

We present an improved analysis of the reentrant nematic and tricritical behavior of the nematic to smectic A phase transition in liquid crystal mixture within phenomenological theory To describe the nematic–smectic A–reentrant nematic phase sequence in binary mixture, nematic and smectic order parameters have been coupled with the concentration We show that reentrant nematic and the tricritical point on the N–SmA phase transition line can be achieved in the same phase diagram under certain conditions We predict two tricritical points One corresponds to nematic to smectic A phase transition and other corresponds to reentrant nematic to smectic A phase transition A comparison of the results with experimental phase diagrams is described in great details The predictions of the theory are in good agreement with available experimental data

Reentrant nematic and tricritical behavior of the nematic to smectic-A phase transition in liquid crystal mixture

We examine the effect of pressure on the nematic to smectic-A N–SmA) transition within the Landau phenomenological theory. The influence of pressure on the N–SmA transition is discussed by varying the coupling between the orientational and translational order parameters. The possibility of the tricritical point at the N–SmA transition is discussed in a phenomenological way. The theoretical predictions are found to be in good qualitative agreement with experimental results.

Soumya Sasmal

Papers

Evidence of pressure dependence tricritical point at the nematic–smectic-A transition