Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series (original) (raw)
Journal of Molecular Liquids, 2011
Different thermodynamic parameters are estimated from volume expansion coefficient, α for a number of N-(p-n-alkoxybenzylidene)-p-n-butyloxyanilines, nO.O4 compounds in isotropic phase (at T IN + 5°C) and in liquid crystalline phases. The variation of different thermodynamic parameters with the alkoxy chain length in nO.O4 series and their variation with the temperature in individual liquid crystal compounds are discussed. The parameters like available volume (V a), inter molecular free length (L f) and molecular radius (M r) are also computed from density, and refractive index for these nO.O4 compounds and the nature of molecular free length and molecular radius are analyzed and discussed.
Molecular Crystals and Liquid Crystals
2011
An electric field couples to the orientational order in liquid crystals through dielectric anisotropy and is conjugate to the order. However the ionic flow produced by the field causes a dissipation of energy in the medium rendering an experimental study of the effect of the field on the order difficult. We have devised a method of measuring the local temperature of the sample, and both the dielectric constant whose value depends on the orientation& order and the conductivity which is sensitive to the short range order in the medium. We report the electric field temperature phase diagrams of pentyl cyanobiphenyl(5CB) and octyloxy cyanobiphenyl(80CB). The results include (a) the field induced enhancement of the orientational order (b) the critical field beyond which the nematic paranematic transition is continuous and (c) the effect of the field on the nematic-smectic A transition in 80CB. The results are analysed using the appropriate Landau theories.
Optical Materials, 2018
The temperature dependent properties of three liquid crystals (LC) namely effective geometry parameter, order parameter and temperature gradients of refractive indices of two fluorinated isothiocyanato nematic liquid crystal (NLC) compounds 4′-pentylcyclohexyl-3,5-difluoro-4-isothiocyanatobiphenyl, 4'−butylcyclohexyl−3, 5− difluoro− 4− isothiocyanato-biphenyl of high clearing temperature (T C) and low T C LC compound 4 (trans-4 /-n-hexycyclohexyl) isothiocyanatobenzene (6CHBT) have been studied. In this paper, the effective geometry parameters of the three LC compounds are calculated. The order parameters of three LC compounds are also calculated from the effective geometry parameter. From the available data, it is found that the two fluorinated NLC compounds exhibit moderately high value of birefringence (∼0.24-0.26) and 6CHBT exhibits a lower value of birefringence (∼0.16). From the study, it is found that the order parameter is found to be higher in higher birefringence and higher T C LC compounds. The values of temperature gradient of ordinary refractive index (dn o /dT) are found to be showing negative to positive value while increasing temperature which gives the crossover temperature (T CO). The crossover temperature has been calculated from the temperature gradient at the value (dn o /dT) = 0. LC compounds with lower T C are found to have larger T CO value which gives narrower temperature between T CO and T C. The T CO value of compound having larger alkyl group is found to be smaller. The Lorentz field factors for both compound are also reported which can be used for order parameter calculation by different aspects.
LIQUID CRYSTALS TRANSITION TEMPERATURE OF 4-N-ALKOXY-2,3,5,6- TETRAMETHYLPHENYL-4 –NITROAZOBENZENE
The liquid crystal transition temperatures of the homologous series of 4-ny-alkoxy 2,3,5,6-tetramethyl phenenyl-4'-nitroazobenzenes, where n:2-B were studied. It was found that the crystal phase (Cr) changed to the smectic phase (S) and with increasing temperature the late phase hanged to the isotropic liquid phase (l). The classification of these different modification was done by the texture observations of the substance with the aid of hot stage polarizing microscope. The conductivity of these mesogens were measured, and it was found to be increased with increasingnumber of carbon atoms in the terminal alkyl group.
Liquid Crystals, 2012
In this work we study the electro-rheological behaviour of a series of four liquid crystal (LC) cyanobiphenyls with a number of carbon atoms in the alkyl group, ranging from five to eight (5CB-8CB). We present the flow curves for different temperatures and under the influence of an external electric field, ranging from 0 to 3 kV mm −1 , and the viscosity as a function of the temperature, for the same values of electric field, obtained for different shear rates. Theoretical interpretation of the observed behaviours is proposed in the framework of the continuum theory of Leslie-Ericksen for low molecular weight nematic LCs. In our analysis, the director alignment angle is only a function of the ratio between the shear rate and the square of the electric field -boundary conditions are neglected. By fitting the theoretical model to the experimental data, we are able to determine some viscosity coefficients and the dielectric anisotropy as a function of temperature. To interpret the behaviour of the flow curves near the nematic-isotropic transitions, we apply the continuum theory of Olmsted-Goldbart, which extends the theory of Leslie-Ericksen to the case where the degree of alignment of the LC molecules can also vary.
Thermal study of octylcyanobiphenyl liquid crystal confined to controlled-pore glass
Fluid Phase Equilibria, 2004
We study the phase behavior of octylcyanobiphenyl (8CB) liquid crystal confined to control porous glass (CPG) using high-resolution calorimetry. We used silanized and nonsilanized CPG matrices with characteristic void diameters ranging from 400 to 20 nm. With decreasing diameter the first-order isotropic to nematic (I-N) and second-order nematic to smectic A (N-SmA) phase transition temperatures monotonously decrease. The weakly first-order features of the I-N transition approaches the smeared tricritical behavior. The effective N-SmA critical exponent apparently decreases toward the 3D XY value for diameters above 100 nm, however, the N-SmA transition progressively broadens and becomes strongly suppressed for diameters below 100 nm. A simple phenomenological model is used for a qualitative explanation of results.
Calorimetric study of octylcyanobiphenyl liquid crystal confined to a controlled-pore glass
Physical review. E, Statistical, nonlinear, and soft matter physics, 2003
We present a calorimetric study of the phase behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG). We used CPG matrices with characteristic void diameters ranging from 400 to 20 nm. In bulk we obtain weakly first-order isotropic to nematic (I-N) phase transition and nearly continuous character of the nematic to smectic-A (N-SmA) phase transition. In all CPG matrices the I-N transition remains weakly first order, while the N-SmA one becomes progressively suppressed with decreasing CPG pore radius. With decreased pore diameters both phase transition temperatures monotonously decrease following similar trends, but increasing the stability range of the N phase. The heat-capacity response at the weakly first order I-N and continuous N-SmA phase transitions gradually approaches the tricritical-like and three-dimensional XY behavior, respectively. The main observed features were explained using a bicomponent single pore type phenomenological model.
2019
The 4-n-heptyloxy-4 ́-cyanobiphenyl (7OCB) is a member of 4-n-alkoxy-4 ́-cyanobiphenyl homologous series. This liquid crystal series is well known for its electro-optical properties. The 7OCB compound transforms from crystal to nematic at 53.5°C and nematic to isotropic phase at 75.0°C. Using Hartree-Fock and DFT methods with 6-31G(d,p) basis set, molecular geometry, conformational analysis, electronic structure, MEP, HOMO-LUMO surfaces and molecular parameters of 7OCB molecular system have been investigated. Efforts have been made to elucidate the physico-chemical properties of the 7OCB liquid crystal.