Dynamics of Ferroelectric and Nematic Liquid Crystals Confined in Porous Matrices

1994 ◽  
Vol 366 ◽  
Author(s):  
Fouad M. Aliev
Keyword(s):  
Liquid Crystals ◽  
Heterogeneous Media ◽  
Volume Fraction ◽  
Low Frequency ◽  
Pore Wall ◽  
Polar Molecules ◽  
Average Pore ◽  
Surface Polarization ◽  
The Difference ◽  
Frequency Relaxation

ABSTRACTWe performed dielectric spectroscopy measurements to study dynamics of collective modes of ferroelectric (FLC) and molecular motion of nematic (NLC) liquid crystals with polar molecules confined in silica macroporous and microporous glasses with average pore sizes of 1000 Å (volume fraction of pores 40%) and 100 Å (27%) respectively. For FLC the Goldstone and the soft modes are found in macropores. The rotational viscosity associated with the soft mode is about 10 times higher in pores than in the bulk. These modes are not detected in micropores although low frequency relaxation is present. The last one probably is not connected with the nature of liquid crystal but is associated with surface polarization effects typical for two component heterogeneous media. The difference between the dynamics of orientational motion of the polar molecules of NLC in confined geometries and in the bulk is qualitatively determined by the total energy Fs of the interaction between molecules and the surface of the pore wall, which is found Fs ≈ 102erg/cm2.

Dielectric Spectroscopy of Nematic Liquid Crystal Confined in Random Porous Matrices

1995 ◽  
Vol 411 ◽  
Author(s):  
F. M. Aliev ◽  
G. P. Sinha
Keyword(s):  
Relaxation Time ◽  
Liquid Crystal ◽  
Melting Point ◽  
Dielectric Spectroscopy ◽  
Volume Fraction ◽  
Dielectric Behavior ◽  
Isotropic Phase ◽  
Polar Molecules ◽  
Average Pore ◽  
Porous Matrices

ABSTRACTWe performed dielectric spectroscopy measurements to study dynamics of dielectrically active modes of nematic liquid crystals (LC) 5CB with polar molecules confined in silica porous glasses with average pore sizes of 1000 Å (volume fraction of pores 40%) and 100 Å (27%). In the nematic phase of bulk 5CB there is only one mechanism of dielectric relaxation: rotation of polar molecules around short molecular axis with relaxation time τ ∼10−8s. The spatial confinement and the existence of a highly developed interphase have a strong influence on dielectric properties of LC. We found that at temperatures about 30°C below the bulk melting point in both porous matrices the dielectric behavior of confined liquid crystal is very different from the behavior expected for solid state. The dielectric modes were not frozen in both porous matrices and we observed four well defined relaxational processes with relaxation times τ1 ∼ 10−1s, τ2 ∼10−5s, τ3 ∼10−8s (bulk like) and τ3 ∼10−9s. These four processes do not vanish even at temperatures corresponding to deep bulk isotropic phase. The relaxation time of the first process (slow) shows glass-like behavior in a wide temperature range below bulk melting point.

Application of Broad-Band Dielectric Spectroscopy for Investigations of Liquid Crystal - Porous Media Microcomposites

1997 ◽  
Vol 500 ◽  
Author(s):  
G. P. Sinha ◽  
B. Batalla ◽  
F. M. Aliev
Keyword(s):  
Dielectric Properties ◽  
Dielectric Spectroscopy ◽  
Volume Fraction ◽  
Broad Band ◽  
Interfacial Polarization ◽  
Pore Wall ◽  
Slow Process ◽  
Frequency Range ◽  
Average Pore ◽  
Hindered Rotation

ABSTRACTWe applied ultra broad-band dielectric spectroscopy in the frequency range from 10–3 Hz to 109 Hz to investigate the effect of size, shape and volume fraction of the pores in the porous matrices on the dielectric properties of liquid crystals (LC) dispersed in these matrices. Measurements in such a broad frequency range make it possible to obtain detailed information on the important aspects of the electrical behavior of heterogeneous materials such as: conductivity, surface polarization, and influence of confinement on dynamics of molecular motion of polar molecules forming LC. We investigated alkylcyanobiphenyls in the isotropie, nematic and smectic phases dispersed in porous glasses (average pore sizes - 100 Å and 1000 Å) which have randomly oriented, interconnected pores, and anopore membranes (pore diameters - 200 Å and 2000 Å) with parallel cylindrical pores. Dispersion of LC resulted in qualitative changes of their dielectric properties. Analysis of broad-band dielectric spectra shows that in organic (LC) - inorganic (porous matrix) heterogeneous composites conductivity plays an important role at F <1 Hz. We observe the appearance of new dielectric modes: a very slow process with characteristic frequency ≃ (1 – 10) Hz and a second process in frequency range about (103 - 106) Hz. The slow process arises due to the relaxation of interfacial polarization at pore wall - LC interface. The origin of this could be due to absorption of ions at the interface. Another possibility is the preferential orientation of the permanent dipoles at pore surface. The second new mode is due to the hindered rotation of the molecules near the interface. Additionally we observed two bulk like modes due to the rotation of the molecules around their short and long axii which are modified.

Heterogeneous Nanocomposite Materials Based on Liquid Crystals and Porous Media

1996 ◽  
Vol 457 ◽  
Author(s):  
G. P. Sinha ◽  
F. M. Aliev
Keyword(s):  
Porous Media ◽  
Liquid Crystals ◽  
Relaxation Times ◽  
Low Frequency ◽  
Porous Matrix ◽  
Nanocomposite Materials ◽  
Smectic Liquid Crystals ◽  
Porous Matrices ◽  
Frequency Relaxation ◽  
Quantitative Changes

ABSTRACTAn effective way of preparing a variety of liquid crystal based nanocomposite materials is to disperse LC in porous media with different porous matrix structure, pore size and shape. We present the results of investigations of quasiequilibrium and dynamical properties of nematic and smectic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores (porous glasses) and parallel cylindrical pores (Anopore membranes) by light scattering, photon correlation and dielectric spectroscopies. Confining LC to nanoscale level leads to quantitative changes in physical properties and appearance of new behavior which does not exist in either of the components. Relaxation of director fluctuations which is characterized by single relaxation time in the bulk LC are transformed to a process with a spectrum of relaxation times in pores, which includes extremely slow dynamics typical for glass formers. Existence of developed interface in these materials leads to new dielectric properties such as an appearance of a low frequency relaxation of the polarization and modification of dipole rotation.

Nanoparticles dispersed in liquid crystals: impact on conductivity, low-frequency relaxation and electro-optical performance

2016 ◽  
Vol 4 (16) ◽  
pp. 3485-3491 ◽  
Author(s):  
Martin Urbanski ◽  
Jan P. F. Lagerwall
Keyword(s):  
Liquid Crystals ◽  
Dielectric Response ◽  
Low Frequency ◽  
Charge Carriers ◽  
Host Material ◽  
Optical Performance ◽  
Mobile Charge ◽  
Electro Optic ◽  
Frequency Relaxation

We show how the contamination with mobile charge carriers caused by nanoparticle doping affects the dielectric response of a nematic host material and deteriorates its electro-optic performance.

Collective and Molecular Dielectric Relaxation in Confined Liquid Crystals: Effect of Different Layer Thicknesses and Boundary Conditions

2003 ◽  
Vol 790 ◽  
Author(s):  
Fouad M. Aliev ◽  
Manuel Rivera Bengoechea
Keyword(s):  
Liquid Crystal ◽  
Boundary Conditions ◽  
Layer Thickness ◽  
High Frequency ◽  
Low Frequency ◽  
Pore Wall ◽  
Relaxation Processes ◽  
Short Axis ◽  
Librational Mode ◽  
Frequency Relaxation

ABSTRACTBroadband dielectric spectroscopy was used to study the influence of boundary conditions and layer thickness of liquid crystal (LC) confined to cylindrical pores on low frequency and high frequency relaxation processes. Low frequency measurements provided information on the relaxation of surface polarization that arose at LC – pore wall interface. The dynamics of molecular reorientations were investigated in high frequency experiments. In samples with axial orientation of molecules, the dielectric mode due to reorientation of molecules around their short axis was investigated. The homeotropic alignment of molecules facilitated the investigation of the librational mode. The behavior of this mode was different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. Broadening of the dielectric spectra was observed in confined LC. The broadening increases with decreasing liquid crystal layer thickness.

Application of Stereological Relations for the Characterization of Porous Materials via Microscopic Image Analysis

2015 ◽  
Vol 647 ◽  
pp. 180-187 ◽  
Author(s):  
Tereza Uhlířová ◽  
Eva Gregorová ◽  
Willi Pabst
Keyword(s):  
Image Analysis ◽  
Pore Size ◽  
Volume Fraction ◽  
Total Porosity ◽  
Pore Wall ◽  
Average Pore ◽  
Curvature Integral ◽  
Ethanol Addition ◽  
The Mean

In this work we demonstrate the application of stereology-based image analysis for the characterization of highly porous cellular ceramics (alumina foams) prepared by biological foaming with yeast and subsequent drying (80-105 °C) and firing (1570 °C). It is shown that the ceramics prepared usually have total porosities in the range 78-84 % and that the porosities made up by large pores (volume fraction of foam bubbles) are usually in the range 58-75 %. Further it is shown that the mean chord length and the Jeffries size, i.e. pore size measures related to the interface density and the mean curvature integral density, respectively, are relatively close to each other (usually 0.8-1.4 and 0.8-1.2 mm) with a ratio close to unity (0.9-1.3) and that the mean surface-to-surface distance of pores gives a realistic picture of the average pore wall thickness (usually 0.46-0.69 mm). Using a special processing variant (excess ethanol addition) it is possible to obtain microstructures with lower porosity (total porosity 68-70 %, foam bubble volume fractions 50-56 %) and smaller pore size (approx. 0.5 mm). Absolute errors are calculated using normalized deviations corresponding to 95 % reliability in the Student distribution and the standard errors for the quantities in question (both observed and estimated). Relative errors are found to be below 12 % when the number of measurements is of order 400-1000.

Dielectric Relaxation in Filled Nematic Liquid Crystals

1999 ◽  
Vol 559 ◽  
Author(s):  
G.P. Sinha ◽  
M. Kreuzer ◽  
F.M. Aliev
Keyword(s):  
Liquid Crystal ◽  
Liquid Crystals ◽  
Nematic Liquid Crystals ◽  
Broad Band ◽  
Low Frequency ◽  
Slow Process ◽  
Hydrophobic Particles ◽  
Total Polarization ◽  
Tumbling Motion ◽  
Frequency Relaxation

ABSTRACTNematic liquid crystals filled with Aerosil particles are new heterogeneous materials important for different optoelectronic applications. These materials are suspensions of small silica particles, about 10-17 nm in diameter, dispersed in nematic liquid crystals. The particles are known to form a network structure dividing liquid crystal into domains with linear size approximately 250 nm. We used both hydrophilic and hydrophobic particles, filling them with the nematic liquid crystal-5CB.Broad band dielectric spectroscopy (1 mHz - 1.5 GHz) was applied for the investigation of these materials. Two bulk-like modes due to the rotation of molecules around the short axis and the tumbling motion were observed in filled 5CB. Additionally, a low frequency relaxation process and the dispersion of dielectric permittivity due to conductivity were also observed. The modification of the surface of the particles has stronger influence on the slow process and is less important for the molecular modes. The contribution of the slow process for the hydrophilic sample to the total polarization is greater than for the hydrophobic sample. In addition, the corresponding characteristic frequencies are lower for the case of hydrophilic samples. These facts suggest that the low frequency relaxation is an Aerosil particle-liquid crystal interface related phenomena and the origin of this process maybe explained on the basis of surface induced polarization.

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