Orientational Behaviour of the Guest-Host Systems in the Smectic A and Nematic Phases

The measurements of the polarized absorption and fluorescence components spectra for the guest-host mixtures have been used to study the long orientational order in the uniaxial phases of the liquid crystal. The temperature dependence of the order parameters, ⟨P$\text{}_{2}$⟩ and ⟨P$\text{}_{4}$⟩, has been investigated and the molecular distribution function has been determined. On the basis of the ⟨P$\text{}_{2}$⟩ and ⟨P$\text{}_{4}$⟩, values some conclusions about the molecular orientation in the smectic A and nematic phases of the liquid crystalline samples have been drawn. It has been also found that the fluorescence spectroscopy is a sensitive method for determination of the phase transition temperatures of the liquid crystal doped with the fluorescent dye

Photoreorientation in liquid crystalline side-group polysiloxane with azobenzene derivative admixture

The photoreorientation effect in thin films of a liquid crystalline side-group polysiloxane with an azobenzene derivative admixture of the pseudo-stilbene type is presented. The film is introductory homogenously aligned in a typical liquid crystal cell of 3 pm in thickness due to rubbed polyimide coatings. The molar concentration of the azobenzene derivative, calculated per monomeric mesogenic units of the polysiloxane, is equal to 0.021 M/M and corresponds to 2 wt%. The exposure with linearly polarized light results in a reorientation of the azobenzene derivative and the 4'-cyano biphenyl side groups of the liquid crystalline polymer perpendicular towards the electric field vector of the incident light. Caused due to the reorientation process the degree of order is decreased front 0.73 to 0.44

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

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