167 research outputs found
Synthesis and Properties of High Tilted Antiferroelectric Esters with Partially Fluorinated Alkoxyalkoxy Terminal Chains
Novel chiral esters with partially fluorinated alkoxyalkoxy terminal chains are described. Their phase transition temperatures, enthalpies, and electrooptical properties are reported. A helical pitch in pure compounds and their mixtures based on selective reflection of light is also characterized
Switching dynamics of surface stabilized ferroelectric liquid crystal cells: effects of anchoring energy asymmetry
We study both theoretically and experimentally switching dynamics in surface
stabilized ferroelectric liquid crystal cells with asymmetric boundary
conditions. In these cells the bounding surfaces are treated differently to
produce asymmetry in their anchoring properties. Our electro-optic measurements
of the switching voltage thresholds that are determined by the peaks of the
reversal polarization current reveal the frequency dependent shift of the
hysteresis loop. We examine the predictions of the uniform dynamical model with
the anchoring energy taken into account. It is found that the asymmetry effects
are dominated by the polar contribution to the anchoring energy. Frequency
dependence of the voltage thresholds is studied by analyzing the properties of
time-periodic solutions to the dynamical equation (cycles). For this purpose,
we apply the method that uses the parameterized half-period mappings for the
approximate model and relate the cycles to the fixed points of the composition
of two half-period mappings. The cycles are found to be unstable and can only
be formed when the driving frequency is lower than its critical value. The
polar anchoring parameter is estimated by making a comparison between the
results of modelling and the experimental data for the shift vs frequency
curve. For a double-well potential considered as a deformation of the
Rapini-Papoular potential, the branch of stable cycles emerges in the low
frequency region separated by the gap from the high frequency interval for
unstable cycles.Comment: 35 pages, 15 figure
Induced Anticlinic Ordering and Nanophase Segregation of Bow-Shaped Molecules in a Smectic Solvent
Recent experiments indicate that doping low concentrations of bent-core
molecules into calamitic smectic solvents can induce anticlinic and biaxial
smectic phases. We have carried out Monte Carlo (MC) simulations of mixtures of
rodlike molecules (hard spherocylinders with length/breadth ratio ) and bow- or banana-shaped molecules (hard spherocylinder dimers
with length/breadth ratio or 2.5 and opening angle ) to
probe the molecular-scale organization and phase behavior of rod/banana
mixtures. We find that a low concentration (3%) of dimers
induces anticlinic (SmC) ordering in an untilted smectic (SmA) phase for
. For smaller , half of each bow-shaped
molecule is nanophase segregated between smectic layers, and the smectic layers
are untilted. For , no tilted phases are induced. However,
with decreasing we observe a sharp transition from {\sl intralamellar}
nanophase segregation (bow-shaped molecules segregated within smectic layers)
to {\sl interlamellar} nanophase segregation (bow-shaped molecules concentrated
between smectic layers) near . These results demonstrate that
purely entropic effects can lead to surprisingly complex behavior in rod/banana
mixtures.Comment: 5 pages Revtex, 7 postscript figure
Fractionation of cellulose nanocrystals : enhancing liquid crystal ordering without promoting gelation
Colloids of electrically charged nanorods can spontaneously develop a fluid yet ordered liquid crystal phase, but this ordering competes with a tendency to form a gel of percolating rods. The threshold for ordering is reduced by increasing the rod aspect ratio, but the percolation threshold is also reduced with this change; hence, prediction of the outcome is nontrivial. Here, we show that by establishing the phase behavior of suspensions of cellulose nanocrystals (CNCs) fractionated according to length, an increased aspect ratio can strongly favor liquid crystallinity without necessarily influencing gelation. Gelation is instead triggered by increasing the counterion concentration until the CNCs lose colloidal stability, triggering linear aggregation, which promotes percolation regardless of the original rod aspect ratio. Our results shine new light on the competition between liquid crystal formation and gelation in nanoparticle suspensions and provide a path for enhanced control of CNC self-organization for applications in photonic crystal paper or advanced composites
Effect of Ischaemia-Reperfusion on Rabbit Kidney and Human Brain
Free radicals are produced in various organs at ischaemia-reperfusion. The final stage in radical
damage is lipid peroxidation. We have demonstrated previously that a lipid-soluble antioxidant
improves restoration of bioenergetics in rabbit kidneys after ischaemia, as reflected in 31P
spectrometry. Radical production in the brain during surgery for carotid artery stenosis can be
measured using an ex vivo spin trap method.
Aims of the present study: 1. To examine whether pretreatment with a combination of a lipidsoluble
and a water-soluble antioxidant causes improved restoration of bioenergetics in rabbit
kidneys after ischaemia compared to single treatment with a lipid soluble antioxidant. 2. To
examine whether pretreatment with allopurinol or acetylcysteine influences radical production in
conjunction with surgery for carotid artery stenosis. 3. To study the relationship between various
markers for arteriosclerosis and the production of free radicals in conjunction with surgery for
carotid artery stenosis.
Methods: New Zealand white rabbits were used for the NMR experiments. Volume-selective 31P
spectrometry was used to determine changes in bioenergetics during and after ischaemia
following various pretreatments. An ex vivo spin trap method was used to measure radical
production in the brain during carotid endarterectomy in control patients as well as patients
pretreated with allopurinol or acetylcysteine. ICAM-1, MCP-9, MMP-1 and oxLDL serum levels
were determined in the control patients.
Results: Pretreatment with a combination of a lipid-soluble and a water-soluble antioxidant
resulted in improved restoration in cell bioenergetics after ischaemia compared to single
treatment with a lipid-soluble antioxidant. Production of radicals can be measured reproducibly
using the ex vivo spin trap method. Pretreatment with allopurinol eliminated the strong
correlation between e.g. degree of stenosis and leucocyte counts and radical production, which
might indicate a beneficial effect of pretreatment with a xanthine oxidase inhibitor. Pretreatment
with acetylcysteine on the other hand appeared to increase radical production. High levels of
MMP-1 and low levels of ICAM-1 were associated with high radical production.
Conclusion: A combination of a lipid-soluble and a water-soluble antioxidant is most effective
in improving cell bioenergetics after ischaemia in rabbit kidneys. Allopurinol appears to have a
beneficial effect in conjunction with carotid endarterectomy while acetylcysteine appears to
increase radical production. MMP-1 is associated with increased radical production
Tilt plane orientation in antiferroelectric liquid crystal cells and the origin of the pretransitional effect
The optic, electro-optic, and dielectric properties of antiferroelectric liquid crystals AFLCs are analyzed and discussed in terms of the local tilt plane orientation. We show that the so-called pretransitional effect is a combination of two different electro-optic modes: the field-induced antiphase distortion of the antiferroelectric structure and the field-induced reorientation of the tilt plane. In the presence of a helix, the latter corresponds to a field-induced distortion of the helix. Both electro-optic modes are active only when the electric field has a component along the tilt plane. Thus, by assuring a horizontal surface-stabilized condition, where the helix is unwound by surface action and the tilt plane is everywhere parallel to the cell plates, the pretransitional effect should be suppressed. We also discuss the dielectrically active modes in AFLCs and under which circum- stances they contribute to the measured dielectric permittivity
Electrolyte effects on the nematic-isotropic phase transition in lyotropic liquid crystals
A phenomenological approach to the description of the electrolyte effect on the nematicâ
isotropic phase transition in lyotropic liquid crystals is proposed. The influence of the
electrolyte is discussed by varying the coupling between the concentration variables and the
orientational order parameter. The analysis shows that the discontinuity in the first order
nematicâisotropic phase transition as measured by TNI{T0 increases as a function of the NI
weight fraction of the electrolyte. Here TNI is the first order nematicâisotropic phase transition temperature and TN0I is the extrapolated supercooling limit. The electrolyte dependence of the CottonâMouton coefficient and the non-linear dielectric effect in the isotropic phase above the nematicâisotropic phase transition are calculated. The theoretical predictions are found to be in good agreement with experimental results
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