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1
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Statistical Theory of Biaxial Nematic and Cholesteric Phases

100%
Kapanowski A.
Acta Physica Polonica A
|
2011
|
vol. 120
|
issue 3
351-367
EN
A statistical theory of biaxial nematic and cholesteric phases is presented. It is derived in the thermodynamic limit at small density and small distortions. The considered phases are composed of short rigid biaxial or less symmetric molecules. The expressions for various macroscopic parameters involve the one-particle distribution function and the potential energy of two-body short-range interactions. The cholesteric phase is regarded as a distorted form of the nematic phase. Exemplary calculations are shown for several systems of molecules interacting via Corner-type potential based on the Lennard-Jones 12-6 functional dependence. The temperature dependence of the order parameters, the elastic constants, the phase twists, the dielectric susceptibilities, and the flexoelectric coefficients are obtained. The flow properties and defects in nematics are also discussed.
2
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Induced Smectic A Phase through Intermolecular Hydrogen Bonding: Part XVIII: Influence of p-n-Alkyl Benzoic Acids on Thermal and Phase Behavior οf Hydrogen-Bonded Liquid Crystals

100%
Vijayalakshmi K., Sreehari Sastry S.
Acta Physica Polonica A
|
2009
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vol. 115
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issue 3
690-693
EN
A novel series of intermolecular hydrogen bonded liquid crystals was synthesized with the mesogens of p-n-alkyl benzoic acids (nBA where n =5 to 10) and p-(p'-octyloxy benzylidene)-cyano aniline (OBCA) moieties. The thermal and phase behaviors of these mesogens (nBA:OBCA) are studied by thermal polarizing optical microscopy and differential scanning calorimetry techniques. Induced smectic A phase with focal conic fan texture is observed in all the synthesized compounds. Moreover, the nematic phase present in all the pure p-n-alkyl benzoic acids (nBA where n =6 to 10) is quenched in all the hydrogen bonded compounds (nBA:OBCA) and smectic A phase is induced. But, in the compound 5BA:OBCA the nematic phase is also present along with the induced smectic A phase. The structural elucidation pertaining to the formation and stabilization of intermolecular hydrogen bonding is carried out by a detailed IR spectral investigation.
3
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Flexoelectric Effect in Biaxial Nematics

100%
Kapanowski A.
Acta Physica Polonica A
|
2012
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vol. 122
|
issue 1
146-151
EN
The flexoelectric effect provides a linear coupling between electric polarization and orientational deformation in liquid crystals. It influences many electrooptical phenomena and it is used in some bistable nematic devices. A statistical theory of dipole flexoelectric polarization in biaxial nematic liquid crystals is used to calculate temperature dependence of order parameters, elastic constants, and flexoelectric coefficients. The splitting of the two Meyer flexoelectric coefficients and the appearance of new flexoelectric coefficients is obtained at the uniaxial-biaxial nematic transition. The ordering of the split flexoelectric coefficients corresponds to the ordering of the split elastic constants.
4
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Theory of the Dielectric Susceptibility of Liquid Crystals with Bent-Core Molecules

100%
Kapanowski A.
Acta Physica Polonica A
|
2011
|
vol. 120
|
issue 3
473-479
EN
Statistical theory of the dielectric susceptibility of polar liquid crystals is proposed. The molecules are calamitic or bent-core but the permanent dipole moment is perpendicular to the molecule long axis. The ordering of the phase is described by means of the mean-field theory based on the Maier-Saupe approach. The theory is used to calculate the temperature dependence of the order parameters and the susceptibilities. The phase diagram with four phases is obtained: isotropic, uniaxial nematic, uniaxial ferroelectric, and biaxial ferroelectric. Four critical points are predicted.
5
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Content available

Theory of the Dielectric Susceptibility of Liquid Crystals with Polar Nonuniaxial Molecules

100%
Kapanowski A., Wietecha T.
Acta Physica Polonica A
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2004
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vol. 106
|
issue 1
51-68
EN
Statistical theory of the dielectric susceptibility of polar liquid crystals is proposed. The molecules are not uniaxial but similar to cones. It is assumed that the permanent dipole moment of a molecule is parallel to the axis of the rotational symmetry. The ordering of the phase is described by means of the mean field theory based on the Maier-Saupe approach. The theory was used to calculate the temperature dependence of the order parameters and the susceptibilities. Predictions of the model for different sets of parameters are investigated.
6
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Influence of Dipole-Dipole Interactions on Structural Properties of Gay-Berne Particles

100%
Gwóźdź E., Bródka A.
Acta Physica Polonica A
|
2000
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vol. 98
|
issue 5
645-649
EN
Molecular dynamics simulations were performed for a liquid crystal system composed of the Gay-Berne particles with terminal transverse dipole moments. When the dipole-dipole interactions are weak, the isotropic-smectic transition is observed and the transition point shifts towards lower densities as the dipole moment increases. For higher dipole moments the system shows isotropic structure in the whole range of density, and strong dipole-dipole interactions produce two types of dimers that cannot be accommodated into smectic layers.
7
Content available remote

Theoretical Study of Nematic to Isotropic Transition in Porous Media

80%
Govind A., Banerjee K.
Acta Physica Polonica A
|
2016
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vol. 130
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issue 3
748-750
EN
Experimental observations show that the N-I transition temperature (T_{NI}) for liquid crystals embedded in solid porous materials is lower compared to that of the bulk liquid crystals and T_{NI} is reduced linearly with the inverse pore diameter. To explain this, various theoretical studies have been proposed. We propose to use the mean field approach. We modify the Maier-Saupe mean field theory to include the disordering effects of porosity as a disordering surface potential. A molecule near the surface is assumed to feel the mean field potential (the Maier-Saupe type) and also the surface induced potential. We calculate the values of the nematic order parameter and hence find the T_{NI} for different pore diameters. The weighted average of the order parameter is calculated considering the cylindrical symmetry of the pores. Our calculations on the variation of T_{NI} with pore diameter agree with experimental data. Also, the calculated values of specific heat peak decrease with decrease in pore radius, in agreement with experimental trends.
8
Content available remote

Nematic ordering problem as the polymer problem of the excluded volume

61%
Yakunin A.
Open Physics
|
2003
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vol. 1
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issue 2
355-362
EN
Based on a solution of the polymer excluded volume problem, a technique is proposed to estimate some parameters at the isotropic-nematic liquid crystal phase transition (the product of the volume fraction of hard sticks and the ratio of the stick length, L, to its diameter, D; the maximum value of this ratio at which one cannot regard the stick as hard). The critical exponents are estimated. The transition of a swelling polymer coil to ideal is revealed as the polymerization degree of a macromolecule increases. The entanglement concentration obtained agrees with experimental data for polymers with flexible chains. The number of monomers between neighbor entanglements is assumed to be the ratio L/D. A comparison of the theory with other ones and recent experimental data is made.
9
Content available remote

A bicontinuous structure in some systems with cubic mesophases

51%
Yakunin A.
Open Physics
|
2004
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vol. 2
|
issue 3
535-555
EN
A cubic structure of polymer colloid complexes is studied. The technique of the research includes i) an analysis of well-known literature SAXS data; on this basis, ii) constructing a simple model to estimate geometric structure parameters and to obtain a simulated scattering curve; and iii) comparing the model with the real structure obtained from the SAXS data, using the reconstruction of electron density distribution. A bicontinuous structure in cubic mesophases is formed.
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