Prenematic Self-Assembling of Mesogenic Molecules in Isotropic Liquid and Orientational Order in Nematic Phase

Abstract

Dielectric relaxation study of nematogenic 4-n-alkyl-4 -cyanobiphenyls (nCB, n = 5, 6), 4-(trans-4 -n-alkylcyclohexyl)isothiocyanatobenzenes (nCHBT, n = 6, 8), 4-cyanophenyl-4 -n-alkylbenzoates (nCPB, n = 6, 8), 4-cyano-3-fluorophenyl-4 -n-octyloxybenzoate (8OCFPB), and 4-cyanophenyl-4 -n-octyloxy-3 -fluorobenzoate (8OCPFB) was performed in the frequency range from 50 kHz to 100 MHz in the nematic and isotropic phases. The static permittivity and the relaxation process related to the rotation of molecules around their short axis was analyzed. For some of these liquid crystals anomalous temperature dependence of static permittivity in the pretransitional region of the isotropic phase was observed. Based on the Meier-Saupe-Martin model of molecular diffusion in nematics, the orientational order parameter P 2 was determined from dielectric relaxation times and retardation factor. The values of P 2 calculated from the dielectric relaxation data were compared with the results obtained from measurements of polarized electronic absorption. Correlations between the magnitude of the dielectric pretransitional effect and the orientational order in the nematic phase were discussed.

‘Quasi-Tricritical’ and Glassy Dielectric Properties of a Nematic Liquid Crystalline Material

“…. and to more complex as 'cybotactic groups' appearing in the LC material as the substitute of pretransitional (prenematic fluctuations) [3,4,[56][57][58][59][60]. There are also claims that the observed weakly first-order nature of the N-I transition must be traced at the level of molecular interactions because just before the phase transition to the nematic phase the relaxation rate (linked to the relaxation time) strongly decreases from several 10 kHz down to a few kHz.…”

Change of the Prenematic Order in the Isotropic Phase Close to the Isotropic–Nematic Phase Transition with Increasing Aerosil Nanoparticles Density

“…The observed static permittivity anomalies can be divided to the two types with and without evident maximum. The anomalous behavior of the static permittivity, near the I-N phase transition, can be explained as a result of the decrease of the permanent dipole moment caused by an antiparallel association of the molecules and the formation of the prenematic domains in the isotropic phase near the I-N phase transition [10]. The magnitude of the anomaly of the static permittivity can be characterized by the temperature shift δT between the temperature T max in the maximum of the permittivity (if possible to define this maximum) and clearing temperature T c .…”

“…This anomaly of the static dielectric permittivity shows the following characteristic features: (i) strong bending down in the isotropic phase of nCB near the I-N phase transition, (ii) temperature dependence of the static dielectric permittivity with and without pronounced maximum in the isotropic phase near the I-N phase transition. The observed anomalies were related to the decrease of the effective dipole moment, caused by the antiparallel association of the molecules and the formation of the prenematic domains [10,11]. Moreover, the position (parallel or perpendicular to the long axis of the molecule) and values of the dipole moments play crucial role in the observed pretransitional anomalies [7,9,10].…”

“…The observed anomalies were related to the decrease of the effective dipole moment, caused by the antiparallel association of the molecules and the formation of the prenematic domains [10,11]. Moreover, the position (parallel or perpendicular to the long axis of the molecule) and values of the dipole moments play crucial role in the observed pretransitional anomalies [7,9,10].…”

“…Also, in the last years, the growing interest has been dedicated to investigate the origins of the pretransitional anomaly of the static dielectric permittivity in the isotropic phase of nematic liquid crystals near the I-N phase transition [7][8][9][10]. This anomaly of the static dielectric permittivity shows the following characteristic features: (i) strong bending down in the isotropic phase of nCB near the I-N phase transition, (ii) temperature dependence of the static dielectric permittivity with and without pronounced maximum in the isotropic phase near the I-N phase transition.…”