Analysis of the Dielectric Anisotropy of Typical Nematics with the Aid of the Maier-Meier Equations

2000 ◽  
Vol 55 (3-4) ◽  
pp. 449-456 ◽  
Author(s):  
Stanisław Urban ◽  
Jerzy Kędzierski ◽  
Roman Dąbrowski
Keyword(s):  
Experimental Data ◽  
Dipole Moment ◽  
Dielectric Properties ◽  
Dielectric Permittivity ◽  
Order Parameter ◽  
Nematic Phase ◽  
Liquid Crystalline ◽  
Molecular Axis ◽  
Dielectric Anisotropy

Abstract Using the experimental data on the dielectric anisotropy, density, polarizability, dipole moment, and order parameter in the nematic phase of seven typical liquid crystalline substances, the applicability of the Maier-Meier theory for the description of the dielectric properties of nematics is checked. Substanc-es with different polarity and different tendency to form associates in the nematic phase were studied. It is found that the Maier-Meier equations describe fairly well the dielectric permittivity components of nematics. The estimated values of the angle β formed by the dipole moment with the long molecular axis are compared with those obtained in other studies

Dielectric Studies of 4-w-Hexyloxy-4’-Cyanobiphenyl (60CB) at Elevated Pressure

1999 ◽  
Vol 54 (6-7) ◽  
pp. 365-369 ◽  
Author(s):  
S. Urban ◽  
M. Smoluchowski
Keyword(s):  
Dielectric Permittivity ◽  
Order Parameter ◽  
Nematic Phase ◽  
Activation Volume ◽  
Activation Enthalpy ◽  
Ambient Pressure ◽  
Relaxation Times ◽  
Elevated Pressure ◽  
Dielectric Anisotropy ◽  
Dielectric Studies

Abstract The principal dielectric permittivity components in the nematic phase of 4-rc-hexyloxy-4’-cyanobiph-enyl (60CB) were measured as functions of temperature at ambient pressure and as functions of pres-sure up to 100 MPa at several constant temperatures. The dielectric anisotropy is analized in the frame of the Maier-Meier equations. The pressure dependence of the order parameter is deduced. Preliminary results for the activation volume and activation enthalpy from the pressure and temperature dependenc-es of the longitudinal relaxation times are obtained.

scholarly journals Order Parameters, Orientational Distribution Functions and Heliconical Tilt Angles of Oligomeric Liquid Crystals

2019 ◽  
Author(s):  
Richard Mandle ◽  
John W. Goodby
Keyword(s):  
Phase Transition ◽  
Liquid Crystals ◽  
Tilt Angle ◽  
Order Parameter ◽  
Nematic Phase ◽  
Liquid Crystalline ◽  
Thermal Evolution ◽  
Order Parameters ◽  
Distribution Functions ◽  
Orientational Distribution

We compare the order parameters, orientational distribution functions (ODF) and heliconical tilt angles of the TB phase exhibited by a liquid-crystalline dimer (CB7CB) to a tetramer (O47) and hexamer (O67) by SAXS/WAXS. Following the N-TB phase transition we find that all order parameters decrease, and while 〈P2 〉 remains positive 〈P4 〉 becomes negative. For all three materials the order parameter 〈P6 〉 is near zero in both phases. The ODF is sugarloaf-like in the nematic phase and volcano-like in the TB phase, allowing us to estimate the heliconical tilt angle of each material and its thermal evolution. The heliconical tilt angle appears to be largely independent of the material studied despite the differing number of mesogenic units.

The Physical and Molecular Properties of New Low Melting Nematics with Negative Dielectric Anisotropy

1999 ◽  
Vol 54 (6-7) ◽  
pp. 448-452 ◽  
Author(s):  
R. Dąbrowski ◽  
J. Jadżyna ◽  
J. Dziaduszek ◽  
Z. Stolarz ◽  
G. Czechowski ◽  
...  
Keyword(s):  
Dipole Moment ◽  
Physical Properties ◽  
Temperature Dependence ◽  
Carboxylic Acids ◽  
Order Parameter ◽  
Room Temperature ◽  
Dielectric Anisotropy ◽  
Molecular Dipole ◽  
Nematic Order ◽  
Moment Vector

Abstract The paper presents some basic physical properties (static electric permittivities, refraction indices, density and viscosity) of 2-chloro-4-n’-alkylphenyl esters of 4-n-alkylbicyclo[2,2,2] octane-1-carboxylic acids (n’=7, n = 5 and 7) which are, at room temperature, nematics with a negative dielectric an-isotropy. On the basis of temperature dependence of the principal static permittivities ε‖(T) and e⊥(T) of the nematics, using the Maier-Meier equations, the angle between the dipole moment vector and the long axis of mesogenic molecules, the apparent molecular dipole moment square pl μ2app(T), and the nematic order parameter S(T) were determined.

Dielectric Behaviour of 1-(4-Isothiocyanatophenyl)-2-(4-hexyl- bicyclo[2,2,2]octane-1)ethane in Nematic and Isotropic Phases

1996 ◽  
Vol 51 (8) ◽  
pp. 933-938 ◽  
Author(s):  
J. Jadżyn ◽  
C. Legrand ◽  
P. Kędziora ◽  
B. Żywucki ◽  
G. Czechowski ◽  
...  
Keyword(s):  
Dipole Moment ◽  
Relaxation Time ◽  
Liquid Crystal ◽  
Order Parameter ◽  
Nematic Phase ◽  
Frequency Region ◽  
Relaxation Processes ◽  
Short Axis ◽  
Electric Permittivity ◽  
Orientation Axis

Abstract Static and dynamic investigations of the dielectric properties of the liquid crystal 1-(4-isothio-cyanatophenyl)-2-(4-hexyl-bicyclo[2,2,2]octane-1)ethane in the nematic and isotropic phases have been carried out in the frequency region from 1 kHz to 1 GHz. Two relaxation processes, described by the Debye functions, have been observed not only in the isotropic but also in the nematic phase, when the measurements of the electric permittivity vs. frequency have been made parallel to the orientation axis of liquid crystal. These processes are related to the rotation of the permanent dipole moment around two main molecular axes. The height of the potential barrier which hinders the rotation of the liquid crystal molecule around the short axis in the ordered nematic phase and the order parameter of the liquid crystal investigated have been estimated on the basis of the relaxation time values in the nematic and isotropic phases.

Light Absorption Properties of Anthraquinone Dyes and Guest-Host Liquid Crystal Displays

1999 ◽  
Vol 559 ◽  
Author(s):  
Shuangxi Wang ◽  
Sugat Abeygunaratne ◽  
Liang-Chy Chien
Keyword(s):  
Liquid Crystal ◽  
Light Absorption ◽  
Order Parameter ◽  
Liquid Crystalline ◽  
Dielectric Anisotropy ◽  
Zero Field ◽  
Absorption Properties ◽  
Anthraquinone Dye ◽  
Anthraquinone Dyes ◽  
Reverse Mode

ABSTRACTThe light absorption properties of anthraquinone dye in different hosts are compared in order to demonstrate the influence of the dye-liquid crystal interactions on dye order parameter. Surprisingly, the anthraquinone dye have a higher order parameter in liquid crystalline mixtures of positive dielectric anisotropy as well as in a liquid crystalline mixture of negative dielectric anisotropy (S = 0.71-0.82). We studied the Heilmeier displays use homogeneous alignment. The display is operated at the reverse mode, transparent at zero field. The switching voltage between the bright and dark states for these displays is 2-4 volts, depending on host liquid crystal and cell configuration.

Comparison of Dielectric Properties of Three Alkyl and Alkoxy Azoxybenzenes (nAOBs and nOAOBs, n = 5, 6, 7) in the Isotropic and Liquid Crystalline Phases

2004 ◽  
Vol 59 (10) ◽  
pp. 674-682 ◽  
Author(s):  
Stanisław Urban ◽  
Joanna Czub ◽  
Bo Gestblom
Keyword(s):  
Nematic Phase ◽  
Liquid Crystalline ◽  
Mean Field Theory ◽  
Relaxation Times ◽  
Mean Field ◽  
Dielectric Anisotropy ◽  
Smectic C ◽  
Smectic Phases ◽  
Liquid Crystalline Phases ◽  
Perpendicular Component

Results of dielectric studies of three 4,4’-di-n-alkyl-azoxybenzenes (nAOB) and 4,4’-di-n-alkoxyazoxybenzenes (nOAOB), n = 5, 6 and 7, are presented. The nAOB compounds exhibit positive, and the nOAOBs negative dielectric anisotropy in the nematic phase. In the smectic A phase of 7AOB the anisotropy changes its sign, whereas in the smectic C phase of 7OAOB it persists negative; moreover, in the latter case the perpendicular component of the permittivity increases with respect to the value of the nematic phase. This may be due to parallel dipole-dipole correlation of the perpendicular components involved by the layer arrangements in both smectic phases. For all substances a systematic decrease of the permittivity components with chain length was observed. Different types of smectic order in the 7th members of both series are discussed in relation to the mean-field theory developed recently by Govind and Madhusudana. The dielectric relaxation times and activation enthalpies characterizing the molecular rotation around the principal inertia axes are given.

Pressure Dependence of the Dielectric Anisotropy of 5CB and 6CB

1995 ◽  
Vol 50 (9) ◽  
pp. 826-830
Author(s):  
Stanislaw Urban
Keyword(s):  
Magnetic Field ◽  
High Pressure ◽  
Magnetic Materials ◽  
Pressure Dependence ◽  
Order Parameter ◽  
Nematic Phase ◽  
Correlation Factor ◽  
Dielectric Anisotropy ◽  
High Pressure Chamber ◽  
Electronic Polarization

Abstract The pressure-dependence of the dielectric anisotropy, Δε = ε0|| - ε0⊥, of two nematogens, n-pentyl-and n-hexyl-cyanobiphenyl (5CB and 6CB) has been measured at constant temperature. For this purpose a high pressure chamber made of non-magnetic materials was constructed, allowing for the orientation of the samples by a magnetic field. The pressure dependencies of ΔεT were analyzed in the frame of the Maier-Meier equations for the principal permittivity components in the nematic phase. It was found that the relation between Δε and the order parameter S is not properly described by these equations. The Kirkwood correlation factor calculated on the basis of the present results and the Maier-Meier equations is smaller than 0.5, whereas it increases to a more realistic value (~0.7) if the electronic polarization is incorporated in the equations.

Fluctuation-induced dielectric permittivity in the isotropic phase of cholesteric liquid crystals

2016 ◽  
Vol 30 (09) ◽  
pp. 1650053
Author(s):  
Prabir K. Mukherjee ◽  
Asok K. Das
Keyword(s):  
Experimental Data ◽  
Phase Transition ◽  
Dielectric Permittivity ◽  
Order Parameter ◽  
Orientational Order ◽  
Isotropic Phase ◽  
Orientational Order Parameter ◽  
Cholesteric Phase ◽  
Static Dielectric Permittivity ◽  
Temperature And Pressure

The temperature and pressure dependence of the static dielectric permittivity in the isotropic phase of the isotropic to cholesteric phase transition is calculated using Landau–de Gennes’s fluctuation theory, allowing spatial variation of the orientational order parameter. A comparison is made with experimental data available in the isotropic phase of the isotropic to cholesteric phase transition.

scholarly journals MEASUREMENT OF THE DIELECTRIC PROPERTIES OF LIQUID CRYSTAL MATERIAL FOR MICROWAVE APPLICATIONS

2019 ◽  
Author(s):  
Juan R. Sanchez ◽  
Vicente Nova ◽  
Carmen Bachiller ◽  
Belen Villacampa ◽  
Alberto De la Rua ◽  
...  
Keyword(s):  
Liquid Crystal ◽  
Dielectric Properties ◽  
Dielectric Permittivity ◽  
Loss Tangent ◽  
Dielectric Material ◽  
Orientational Order ◽  
Dielectric Anisotropy ◽  
Microwave Devices ◽  
Liquid Crystal Material ◽  
Microwave Applications

Liquid Crystal (LC) is an anisotropic liquid material which flows like a liquid, but at the same time its molecules have an orientational order like in the solid state [1]. Thus, LC is a promising dielectric material for designing reconfigurable devices at microwave frequencies. In order to optimize the design of reconfigurable microwave devices, accurate values of the dielectric permittivity and the loss tangent of LCs are needed. However, new LCs are not well characterized at these frequencies because of its recent use for microwave applications. Therefore, the characterization in this frequency range is required for its practical use within microwave components and devices [2].   In this work, the split-cylinder resonator method has been used for the characterization of LCs at two frequency points, i.e. 5 and 11 GHz. The method is based on the measurement of the resonance frequency and quality factor of the two states of the LC molecules for extracting the complex dielectric permittivity [3].   For achieving these two states, no electric or magnetic fields are needed, just the cell must be turned 90º inside the cavity. The dielectric properties (permittivity and loss tangent) of four different LC samples, GT3-23002 from Merck and QYPD193, QYPD142, and QYPD036 from Qingdao QY Liquid Crystal Co, have been obtained. The highest values of the dielectric anisotropy are presented for the samples QYPD036 and QYPD193, together with the highest values of the corresponding loss tangent parameters. Furthermore, it is observed for all the LCs that the loss tangent decreases and the dielectric anisotropy increases at higher frequencies, which must be taken into account in the development of reconfigurable microwave devices.

scholarly journals Study of Azo Dye-n TPEB Mixtures in the Nematic and Smectic B Phases by Means of Polarized Light Absorption

1998 ◽  
Vol 53 (10-11) ◽  
pp. 813-818
Author(s):  
Danuta Bauman ◽  
Ewa Chrzumnicka ◽  
Roman Dąbrowskia
Keyword(s):  
Phase Transition ◽  
Light Absorption ◽  
Azo Dye ◽  
Order Parameter ◽  
Nematic Phase ◽  
Liquid Crystalline ◽  
Polarized Light ◽  
Structural Form ◽  
Phase Transition Temperatures ◽  
Clearing Temperature

Abstract The influence of the dichroic azo dye on the phase transition temperatures of the mesogenic homolo-gous series of 1-[4-n-alkyl-biphenyl]-2-[4-isothio-cyanatophenyl] ethane (nTPEB) for n = 2 -10 has been examined. For the dye-nTPEB mixtures the polarized absorption spectra as a function of tem-perature have been recorded. From these spectra the order parameter of the dye dissolved in the liquid crystalline hosts in the whole region of the nematic phase has been determined. The order parameter has revealed the odd-even alternation, similarly as the clearing temperature. In the smectic B phase the mosaic texture has been observed and conclusions concerning structural form of this phase have been drawn.

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