On the Nature of the Smectic A-to-Nematic Phase Transition of 8CB

Abstract

Density as a function of temperature was measured in the smectic A and nematic phases in the immediate vicinity of the smectic A-to-nematic transition temperature (T AN′ ) of p-n-octyl-p′-cyanobiphenyl (8CB). The very small volume change obtained at the smectic A-to-nematic (A-N′) phase transition of "pure" 8CB confirms that it is weakly first-order. There is also evidence that the A-N′ phase transition of pure 8CB takes place in two steps; the first is a very weak first-order transition, involving an extremely small volume change, to a phase of "intermediate" order which extends over about 0.07 K and then undergoes a second transition, which may or may not be second-order, to the nematic phase.

Probing of Reorganization Dynamics within the Different Phases of Themotropic Liquid Crystals

“…This is due to the more number of mesogens (smectic and nematic phases) are involved in a liquid crystalline phase of 8CB as compared to the 5CB (in 5CB only nematic phase is present at 20 °C to 34 °C) ,. Smectic phase appears at relatively low temperature region (∼22 °C to 33 °C), while nematic phase emerges at high temperature region (33 °C to 41 °C) within the liquid crystalline phase of 8CB ,. Smectic phase is more rigid towards preserving its orientational order and more viscous as compared to the nematic phase of the same substrate.…”

“…At very low temperature (∼10 °C), 5CB and 8CB form a highly anisotropic pure crystalline phase . In a crystalline phase average solvation time (∼3.6 ns and ∼4.6 ns for 5CB and 8CB) is found to be slowest among all other solvation times measured in other phases (liquid crystalline phase and isotropic phase).…”

“…In the liquid crystalline phase, solvation dynamics is found to be faster when compared with solvation time in a pure crystalline phase, but slower when compared to the solvation time in an isotropic liquid phase. For example, crystalline phase of 5CB is converted to liquid crystalline nematic phase above 18 °C, where average solvation time is accelerated to a much faster time scale to ∼1.5 ns (at 25 °C); nearly 2 times faster as compared to the solvation time (∼3.5 ns) in crystalline phase (at 10 °C) . At a high temperature (above liquid crystalline‐to‐liquid phase transition temperature) solvent phase turns to an isotropic liquid phase, where average solvation time is found to be fastest among all other solvation times obtained from different phases.…”

“…Low solvent viscosity at high temperature facilitates the fast rearrangement of the LCs molecules around a solute dipole and thereby shortens the average solvation time. For 5CB, isotropic liquid phase can be attained above ∼35 °C . Average solvation time in liquid phase of 5CB is shortened by ∼1.7 times, from ∼640 ps to ∼385 ps as temperature is raised from ∼40 °C to∼60 °C.A concomitant decrease (∼2.5 times) in viscosity of 5CB, from ∼25 cP to ∼10 cP was noted for similar change in temperature.…”

Comment on On the Nature of the Smectic A-to-Nematic Phase Transition of 8CB

“…The second problem is the existence of the intermediate phase 1 . It seems to be just an artifact of the lines guiding the eye drawn in Figure 1 of ref 1.…”