11/19/2012
Molecular shape is an important factor in determining the material properties of thermotropic liquid crystals (LCs). We synthesized and investigated several LC compounds formed by asymmetrically bent molecules with a rigid fourring core in the shape of the letter 'L'. We measured the temperature dependencies of dielectric permittivities, birefringence, splay K1 and bend K3 elastic constants, splay viscosity eta(splay) and flow viscosities eta(parallel to) and eta(perpendicular to). The bendsplay anisotropy delta K31  K3  K1 is negative, similar to the case of nematic LCs formed by symmetrically bent molecules of Vshape. The dielectric anisotropy Delta epsilon and birefringence are positive in the entire nematic range. The splay viscosity eta(splay) and the flow viscosities eta(parallel to) and eta(perpendicular to) are smaller than the viscosities measured for the symmetric Vshaped bentcore materials at similar temperatures. The ratio Gamma = eta(splay)/eta(parallel to,perpendicular to) is in the range 54 that is typical for rodlike LCs. The reported Lshaped bentcore nematic LCs combine the useful features of bentcore LCs (such as a negative delta K31, suitable for formulation of broadrange blue phases) with the relatively low viscosities, a property typical for rodlike LCs and beneficial for electrooptic switching applications.
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09/25/2002
Monte Carlo and theoretical studies of thin 3D films of biaxial and uniaxial nematics with tangential boundary conditions show distinct differences in structure and evolution of topological defects. In the uniaxial films, defects of strength k=+/1 are point defects that bear no bulk singularity and disappear by annihilation with each other. In the biaxial films, k=+/1 defects are true singular bulk disclinations that split into pairs of k=+/1/2 lines; the latter disappear by annihilation processes of the type +1/21/2=0. These observed differences are of relevance for the current debate on the existence of biaxial phases.
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11/10/2003
We demonstrate theoretically and experimentally a fastswitching nematic optical retarder capable to switch a few microns of optical retardation in less than 1 ms. For example, a nematic cell of thickness 14.5 mum switches 0.3 mum of retardation within 0.15 ms and 2.5 mum within 0.5 ms for single passage of beam. The corresponding figure of merit is two orders of magnitude higher than the one known for the best nematic materials synthesized so far. The fit is achieved by employing a dualfrequency nematic liquid crystal in highpretilt angle cells and a special addressing scheme that features amplitude and frequency modulated voltage. The scheme can be used in spatial light modulators, retarders, beam deflectors, polarization rotator, and displays.
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06/13/2008
The dielectric dispersion in the uniaxial nematic liquid crystals affects the switching dynamics of the director, as the dielectric torque is determined by not only the present values of the electric field and director but also by their past values. We demonstrate that this “dielectric memory” leads to an unusual contribution to the dielectric torque that is linear in the present field and thus polarity sensitive. This torque can be used to accelerate the “switchoff” phase of director dynamics.
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10/09/2008
We study the dynamics of the isotropicnematic phase transition caused by an applied electric field at the time scales of dielectric relaxation. In the classic LandauKhalatnikov theory of the phase transition dynamics, the nematic (nonpolar) order parameter is an instantaneous function of the applied field. We demonstrate that, when the field is changing faster than the time of dielectric relaxation, the induced polar order dynamics influences the dynamics of the nonpolar order parameter. We develop a model based on the Langevin equation to describe the simultaneous dynamics of both polar and nonpolar order parameters; the model is supported by experiment.
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12/12/2007
The dielectric anisotropy of liquid crystals causes director reorientation in an applied electric field and is thus at the heart of electrooptic applications of these materials. The components of the dielectric tensor are frequency dependent. Until recently, this frequency dependence was not accounted for in a description of director dynamics in an electric field. We theoretically derive the reorienting dielectric torque acting on the director, taking into account the entire frequency spectrum of the dielectric tensor. The model allows one to include the effects of multiple relaxations in both parallel and perpendicular components of the dielectric tensor, thus generalizing a recent model [Y. Yin et al., Phys. Rev. Lett. 95, 087801 (2005)] limited by the singlerelaxation approach. The model predicts the "dielectric memory effect" (DME)i.e., dependence of the dielectric torque on both the "present" and "past" values of the electric field and the director. The model describes the experimentally observed director reorientation in the case when the rise time of the applied voltage is smaller than the dielectric relaxation time. In typical materials such as pentylcyanobiphenyl (5CB), in which the dielectric anisotropy is positive at low frequencies, the DME slows down the director reorientation in a sharply rising electric field, as the sharp front is perceived as a highfrequency excitation for which the dielectric anisotropy is small or even of a negative sign. In materials that are dielectrically negative, the DME speeds up the response when a sharp pulse is applied.
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10/15/1999
We analyze the highelectricfield technique designed by Yokoyama and van Sprang [J. Appl. Phys. 57, 4520 (1985)] to determine the polar anchoring coefficient W of a nematic liquid crystalsolid substrate. The technique implies simultaneous measurement of the optical phase retardation and capacitance as functions of the applied voltage well above the threshold of the Frederiks transition. We develop a generalized model that allows for the determination of W for tilted director orientation. Furthermore, the model results in a new highfield technique, (referred to as the RV technique), based on the measurement of retardation versus applied voltage. W is determined from a simple linear fit over a wellspecified voltage window. No capacitancemeasurements are needed to determine W when the dielectric constants of the liquid crystal are known. We analyze the validity of the Yokoyama–van Sprang (YvS) and RV techniques and show that experimental data in real cells often do not follow the theoretical curves. The reason is that the director distribution is inhomogeneous in the plane of the bounding plates, while the theory assumes that the director is not distorted in this plane. This discrepancy can greatly modify the fitted value of 1/W, and even change its sign, thus making the determination of W meaningless. We suggest a protocol that allows one to check if the cell can be used to measureW by the YvS or RV techniques. The protocol establishes new criteria that were absent in the original YvS procedure. The results are compared with other data on W, obtained by a thresholdfield technique for the same nematicsubstrate pair.
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10/01/2005
We report on the optical properties of the nematic (N) phase formed by lyotropic chromonic liquid crystals (LCLCs) in well aligned planar samples. LCLCs belong to a broad class of materials formed by onedimensional molecular selfassembly and are similar to other systems such as “living polymers” and “wormlike micelles.” We study three water soluble LCLC forming materials: disodium chromoglycate, a derivative of indanthrone called Blue 27, and a derivative of perylene called Violet 20. The individual molecules have a planklike shape and assemble into rodlike aggregates that form the N phase once the concentration exceeds about 0.1M. The uniform surface alignment of the N phase is achieved by buffed polyimide layers. According to the light absorption anisotropy data, the molecular planes are on average perpendicular to the aggregate axes and thus to the nematic director. We determined the birefringence of these materials in the N and biphasic Nisotropic (I) regions and found it to be negative and significantly lower in the absolute value as compared to the birefringence of typical thermotropic lowmolecularweight nematic materials. In the absorbing materials Blue 27 and Violet 20, the wavelength dependence of birefringence is nonmonotonic because of the effect of anomalous dispersion near the absorption bands. We describe positive and negative tactoids formed as the nuclei of the new phase in the biphasic NI region (which is wide in all three materials studied). Finally, we determined the scalar order parameter of the N phase of Blue 27 and found it to be relatively high, in the range 0.72–0.79, which puts the finding into the domain of general validity of the Onsager model. However, the observed temperature dependence of the scalar order parameter points to the importance of factors not accounted for in the athermal Onsager model, such as interaggregate interactions and the temperature dependence of the aggregate length.
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11/23/2004
We have studied isotropictonematic pretransitional fluctuations in an aqueous solution of disodium cromoglycate (cromolyn) by static and dynamic light scattering. Cromolyn is a representative of lyotropic chromonic liquid crystals with building units being elongated rods formed by aggregates of disklike molecules. By combining lightscattering and viscosity measurements we have determined the correlation length and relaxation time of the orientational orderparameter fluctuations and estimated the size of the cromolyn aggregates. The pretransitional behavior of light scattering does not completely follow the classic Landaude Gennes model. This feature is most probably associated with the variable length of cromolyn aggregates. We have observed a dramatic increase of the shear viscosity near the transition to the nematic phase, the fact which correlates with the idea of growing supramolecular aggregates. The steep temperature dependence of the viscosity is accompanied by a practically temperatureindependent translational diffusion coefficient.
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07/12/1999
The popular “highelectricfield” technique to determine the polar anchoring coefficient W of a nematic–substrate interface requires the simultaneous measurement of the capacitance and optical phase retardation of a liquid crystal cell as a function of applied voltage. We develop a generalized model that makes it possible to eliminate the capacitance measurement. The new technique, called the RV (retardation versus voltage) technique, requires only themeasurement of retardation as a function of applied voltage, and allows for the determination ofW by a linear fit over a prescribed voltage window. The technique is not sensitive to uniformity of the cell thickness, does not require patterned electrodes, and allows for the local probe of thesurface. The value of W obtained by the RV technique is the same as W obtained by the traditional technique.
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