Results of the dielectric studies of nine members of the nBT (4-n-alkyl-4'-thiocyanatobiphenyl, n= 2-10) homologous series in the crystal E (E) and isotropic (I) phases are presented. The dependence of the static permittivity ∊s in the isotropic phase, the longitudinal relaxation times τis and τ∥, and the activation enthalpies ΔH is and ΔH ∥ on the number of carbon atoms in the alkyl chain n are analysed. A considerable increase in the retardation factor g= τ∥/τis with decreasing n is observed. The results are compared with those obtained for similar two-ring homologous series. The parameters characterizing the molecular rotations around the short axis in the E phase (τ∥ and ΔH ∥) indicate a hardening of this solid-like phase with shortening of the alkyl chain.

Dielectric studies of the 4-n-alkyl-4'-thiocyanatobiphenyl (nBT) homologous series (n=2-10) in the isotropic and E phases

The isotropic phase dynamics of a system of 4-n-hexyl-4′-cyano-biphenyl (6CB) molecules has been studied by molecular dynamics computer simulations. We have explored the range of 275–330K keeping the system isotropic, although supercooled under its nematic transition temperature. The weak rototranslational coupling allowed us to separately evaluate translational (TDOF) and orientational degrees of freedom (ODOF). Evidences of subdiffusive dynamics, more apparent at the lowest temperatures, are found in translational and orientational dynamics. Mean square displacement as well as self-intermediate center of mass and rotational scattering functions show a plateau, also visible in the orientational correlation function. According to the mode coupling theory (MCT), this plateau is the signature of the β-relaxation regime. Three-time intermediate scattering functions reveal that the plateau is related to a homogeneous dynamics, more extended in time for the orientational degrees of freedom (up to 1ns). The tim...

Subdiffusive dynamics of a liquid crystal in the isotropic phase

X-ray diffraction, DFT theoretical, and IR polarized spectroscopic studies of the crystal-like E-mesophase of 4′-hexyloxy-isothiocyanatotolane (6OTOLT)

Direct transition from isotropic to smectic-E phase

The temperature evolution of the primary relaxation time in the isotropic phase of 4-cyano-4'-tetradecylbiphenyl (14CB) above the isotropic-smectic A (I-SmA) transition is discussed. Based on the enthalpy space and distortion-sensitive analysis, the prevalence of the mode coupling theory (MCT) "critical" and "glassy" dynamics is shown. The obtained singular dependence is related to the MCT critical temperature located approximately 48 K below the clearing (I-SmA) temperature. However, a weak but detectable distortion in the immediate vicinity of the transition occurs. It is also shown that the value of the fragile strength coefficient D(T) is characteristic of a very fragile glassy liquid whereas the steepness index m is typical of a strong one. Both magnitudes anomalously change on approaching the I-SmA phase transition. The static permittivity shows the pretransitional effect linked to the temperature of the hypothetical continuous phase transition located approximately 10.2 K below the I-SmA transition.

Glassy dynamics in the isotropic phase of a smectogenic liquid crystalline compound.

Results are presented of studies of linear and nonlinear dielectric measurements in the isotropic, smectic E phase in 4,4-alkyl-4´-isothicyanatobiphenyls (4BT). The mean-field behaviour was valid up to the smectic E phase transition. In the smectic E phase the Vogel-Fulcher law describes temperature and pressure dependences of the relaxation time. An analysis of the higher temperature relaxation time data led to predictions of the mode coupling theory with the power law universal function = A (T - TC ) , where = -3.3, TC = 288 ± 15 K. Increase of pressure brings the ´´ curve closer to the Debye shape.

https://iopscience.iop.org/article/10.1088/0953-8984/12/8/310/pdf

W Burian

Papers

Linear and nonlinear dielectric studies in the isotropic and smectic E phases in 4,4-alkyl-4´-isothiocyanatobiphenyl