Abstract
Results of dielectric studies in the nematic and isotropic liquid phases of pentylcyanobiphenyl (5CB), a model rod-like liquid crystalline material, are shown. They are based on the discussion of the evolution of dielectric constant ( ε ), its changes under the strong electric field (nonlinear dielectric effect, NDE), and finally, the primary relaxation time. It is shown that changes in ε T and NDE are entirely dominated by the impact of pretransitional fluctuations (pre-nematic and pre-isotropic, respectively) which are associated with the weakly discontinuous character of the isotropic–nematic phase transition. This influence also extends for the low-frequency, ionic species dominated, region. Notable, that the derivative-based and distortions sensitive analysis revealed the tricritical nature of the I-N transition. Although the glass transition in 5CB occurs in the deeply supercooled state at T g ≈ − 68 ° C , the dynamics (changes of the primary relaxation time) follow a previtreous pattern both in the isotropic and in the nematic phase. Finally, the discussion of the ’molecular’ vs. ‘quasi-critical’ characterizations of the isotropic and nematic phases is presented. It shows the evident prevalence of the ‘quasi-critical-picture’, which offers the consistent temperature parameterization in the total tested temperature range.
Highlights
The name ‘liquid crystals’ may appear self-contradictory, but it describes the real nature of unique states locating between the isotropic liquid and the solid crystalline state
As the most classical liquid crystalline material, one can indicate pentylcyanobiphenyl (5CB)—the rod-like compound with the permanent dipole moment (μ ≈ 5 D) approximately parallel to the long molecular axis [3]
The analysis explored the complex dielectric permittivity ε∗ = ε0 + iε00. They were carried out in pentylcyanobiphenyl (5CB), with the mesomorphism [2,3]: Solid Crystal—(Tm ≈ 280 K)–Nematic—(TC ≈ 308 K)–Isotropic liquid. 5CB molecules are terminated by the permanent dipole moment (μ ≈ 5 D), approximately parallel to the long molecular axis [3]
Summary
The name ‘liquid crystals’ may appear self-contradictory, but it describes the real nature of unique states locating between the isotropic liquid and the solid crystalline state. Liquid crystals are crucial for challenging chapters of physics, chemistry, and modern biology [1,2,3,4,5,6,7]. As the most classical liquid crystalline material, one can indicate pentylcyanobiphenyl (5CB)—the rod-like compound with the permanent dipole moment (μ ≈ 5 D) approximately parallel to the long molecular axis [3]. It exhibits only the nematic mesophase, with the uniaxial orientational ordering while the translational disorder remains
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