Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series (original) (raw)
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Thermochimica Acta, 2000
Solidi®cation (supercooled nematic-to-solid or supercooled smectic-A-to-solid), melting (solid-to-nematic or solid-tosmectic-A), smectic-A-to-nematic, and clearing (nematic-to-isotropic or smectic-A-to-isotropic) transition temperatures W s , W m , W AN , and W c , respectively, and enthalpies DH s , DH m , DH AN , and DH c , respectively, have been determined by differential scanning calorimetry (DSC) for members of the p-(n-alkyl)-p H -cyanobiphenyl (RCB) and the p-(n-alkoxy)-p H -cyanobiphenyl (ROCB) homologous series of liquid crystals, where`R' is the number of carbon atoms in the n-alkyl or n-alkyloxy tails. The study shows that the transition of the less ordered nematic phase to the more ordered smectic-A phase, in liquid crystals that exhibit both phases (those with R8 or 9), involves a transient phase of intermediate order. Broad or overlapping solidi®cation exotherms indicate that two mesogenic aggregations supercool simultaneously. Finally, there is evidence for a transition between two solid forms that occurs at temperatures preceding W m or concurrent with it depending on`R' and the heating rate. This refutes an earlier view that during melting the solid form with the higher melting point solidi®es. #
Density Measurements and Thermodynamic Parameters of a Nematic Liquid Crystal
Thermodynamic functions using the coefficient of volume expansion (a) of a system, a number of thermo acoustic and an harmonic parameters such as the isochoric temperature coefficient of internal pressure (X), isochoric temperature coefficient of volume expansivity (Xj), the reduced compressibility (b), the reduced volume (V), isothermal microscopic Gruneisen parameter (C), fractional free volume (f) and Sharma parameter (So) can be investigated. The temperature dependence of these parameters gives a basic understanding about the intermolecular interactions in general. These studies also can be applied to a solute in dilute solutions. In the present study these different parameters have been determined using volumetric measurements in the case of a liquid crystal, namely, 4,40–azoxy anisole (PAA) in dilute solutions at different temperatures. Variations in various parameters with respect to temperature are discussed in the light of the results obtained. The variation of the properties is analyzed and discussed in this article.
Journal of Physics: Conference Series, 2017
Liquid crystals are one of the most interesting materials for various applications. The article considers the influence of temperature to the phase change and the value of surface potential of 4'-n-octyl-4-p-cyanobiphenyl liquid crystal Langmuir monolayers. These parameters have a critical role in process of transfer monolayers to the solid substrate and for further using of them. It is shown that with increasing temperature of subphase leads to the monolayer of liquid crystal becomes isotropic and exhibits a gradual decrease in the surface potential in process of compression.
A new approach to study interaction parameters in cyanobiphenyl liquid crystal binary systems
The phase transition of heptylcyanobiphenyl 7CB and pentylcyanobiphenyl 5CB liquid crystals was investigated using the differential scanning calorimetry DSC technique. Then, the phase transition of different compositions of 7CB/5CB binary mixture was studied to determine the eutectic point. The phase diagram of mentioned binary system in 7CB mole fraction of 0.45 at T = 273.45 K is in good agreement with that of predicted from Schroder–van Laar equation. The thermodynamic excess functions and interaction parameters were calculated to describe the phase transition physically using the non-random mixing for the first time. The P randomicity parameter was used to describe the phase transitions of C–N and N–I in which a small amount of P shows a non-random identity of C–N phase transition. Contrarily, the P is greater in N–I phase transition showing a random mixing process.
The phase transition behavior of the liquid crystal dimer α,ω-bis(4,4-cyanobiphenyl)nonane (CBC9CB), which has been reported to exhibit a nematic-nematic phase transition, has been investigated by means of high-resolution adiabatic scanning calorimetry. This nematic-nematic phase transition is weakly first-order with a latent heat of 0.24 ± 0.01 kJ kg −1. Mixtures up to 40 wt % with 4-pentyl-4-cyanobiphenyl (5CB) liquid crystals have also been investigated, which also show this nematic to nematic phase transition. The transition stays weakly first-order with a decreasing latent heat with increasing concentration of 5CB. For mixtures with more than 40 wt % uniaxial nematic-unknown nematic phase transition was not observed.
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.