The Orientational Order in Cholesteric Liquid Crystals

The investigation of the orientational order in the mesophase of steroidal cholesterogens has been all but neglected by experimentalists, possibly because of the lack of an appropriate technique. However electron resonance spectroscopy can be used to determine the order parameter of a spin probe dissolved in a cholesteric mesophase. In addition the structure of the spin probe may be made to resemble that of the cholesteric solvent. Here this technique is employed to determine the orientational order for cholesteryl and cholestanyl benzoate, as a function of temperature. The results of these experiments yield order parameters which are comparable, or slightly smaller, than those typically observed for nematics. A comparison of these values with mean field theories suggests that the low order parameters of steroidal cholesterogens stems from deviation of the constituent molecules from cylindrical symmetry.     

Behaviour of Ultrasonic Velocity in Cholesteric Liquid Crystals

Ultrasonic velocities (V) have been determined, employing a fixed path double crystal interferometer, in three cholesteric liquid crystals, namely cholesteryl propionate, cholesteryl laurate and cholesteryl myristate in their isotropic and anisotropic phases including the region of the phase transition. The variation of specific volume (v) is also studied in the same temperature range by a special dilatometer constructed for this purpose. Anomalous behaviour of ultrasonic velocity is observed near the isotropic-cholesteric phase transition in all three compounds. In the two polymesomorphic liquid crystals, namely cholesteryl laurate and cholesteryl myristate, contrary to the ultrasonic behaviour of cholesteryl stearate, prominent velocity dips are observed at cholesteric-smectic transition temperatures. The parameters adiabatic compressibility (β_ad) and molar sound velocity (R) are estimated and they are found to exhibit sudden jumps at cholesteric-smectic and isotropic-cholesteric transitions. The thermal expansion and temperature co-efficient of compressibility are found to show abnormal increase near the phase transition indicating the existence of large-magnitude pre-transitional effects near the phase transition. A comparative study of the ultrasonic behaviour of six aliphatic esters of cholesterol has shown that the magnitude of the velocity dip observed at the isotropic-cholesteric transition increased with increase of molecular weight and only cholesteryl acetate shows deviation.     

The Failure of Asymmetric Synthesis in Cholesteric Liquid Crystals

A set of selected bimolecular reactions leading to asymmetric and dissymmetric products was carried out in cholesteric solvents with the aim of obtaining asymmetric syntheses.

In all cases, the products did not show significant optical rotation. The reasons for the lack of asymmetric induction are briefly discussed.     

Infrared Spectroscopic Studies of Homologous Cholesteric Liquid Crystals

Abstract

Infrared (500–2500 cm^?1) spectra of cholesteryl alkyl carbonates (methyl, ethyl and oleyl) have been examined in the solid (cholesteryl methyl carbonate and cholesteryl ethyl carbonate) and liquid crystalline (cholesteryl oleyl carbonate) states. An attempt has been made to correlate the variation in the intensities and frequency shifts with the molecular structures. The shift in the carbonyl frequency in the case of cholesteryl oleyl carbonate may be correlated with the affect on the coplanarity with benzene ring.     

Helical Sense and Pitch of Cholesteric Liquid Crystals

The rotatory sense of a circularly polarized light transmitted through a cholesteric liquid crystal cell was studied by measuring the retardation of a birefringent plate placed upon the cell. Furthermore, the dependence of helical pitch on composition is described for various binary cholesteric and nematic-cholesteric mixtures. The helical sense of eleven cholesterics was determined by the experimental results. As a result, it is shown, for example, that cholesteryl chloride has a right-handed helical structure and cholesteryl propionate has a left-handed helical structure.     

Field induced Increase of Pitch in Planar Cholesteric Liquid Crystals

In cholesteric liquid crystals the application of an electric field parallel to the helix axis can change the wavelength of selective reflection. For decreasing pitch of the studied mixtures, the colour shift becomes more stable against the field-induced transformation of the initial Grandjean structure into a confocal texture. In compounds with a low crossover frequency, a field induced increase of the pitch was observed at the switching between low and high frequency fields. The investigations allow new conclusions on the origin of the colour shift.     

Optical Properties of Cholesteric Liquid Crystals at Oblique Incidence

The optical properties of a periodic structure characterized by a uniform rotation of the dielectric tensor about a given axis are theoretically analyzed. The electromagnetic wave is described as a superposition of elementary modes having the form of Bloch waves. Each elementary mode is represented by a sum of plane waves elliptically polarized, whose wavevectors are the solutions of a characteristic equation. This equation, presented in a preceding paper, is furtherly analyzed, in order to obtain the wave vectors in terms of a power series of a small parameter δ, representing the anisotropy of the dielectric tensor. The coefficients of the series up to terms containing δ⁶ are explicitly given, and the corresponding truncation errors computed. The spectral composition and the polarization states of the Bloch waves are also analyzed and discussed for different values of the incidence angle in the frequency range containing the lower reflection bands. In particular it is shown that in the regions between the reflection bands both the wave functions and the wavevectors can be evaluated with very good approximation by simple analytic expressions, while for an accurate evaluation of the reflection properties of the structure more involved expressions are needed.     

Flexible Structures Based on a Short Pitch Cholesteric Liquid Crystals

We report on the realization and characterization of electro-responsive and pressure sensitive polydimethylsiloxane (PDMS) conductive photonic structures combined with the reconfigurable properties of short pitch cholesteric liquid crystals (aligned in Grandjean configuration). By combining ion-implantation process and surface chemistry functionalization, we have overcome the insulating properties of PDMS and induced long range organization of cholesteric liquid crystals, thus controlling both diffraction and selective Bragg reflection of light by means of external perturbations (electric field, pressure). We have characterized our devices in terms of morphological, optical and electro-optical properties.     

Electric-Field Enhancement of Fluorescence Intensity of Cholesteric Liquid Crystals Containing Aromatic Chromophores

Fluorescence from cholesteric liquid-crystalline mixture containing cholesteryl 3-(l-pyrenyl)propanoate was measured under dc and ac electric field applied parallel to the helix axis. The excimer fluorescence of the pyrenyl chromophore increased and decreased its intensity reversibly upon turning the electric field on and off. Similar electric field effects were observed for such chromophores as tetracene, perylene, and 9,10-diphenylanthracene doped in the cholesteric mixture. The electric-field effect was observed only for fluorescence transitions which show left-handed circularly polarized fluorescence, indicating that only transitions which are polarized perpendicular to the optic axis in a quasi-nematic layer exhibit the effect. The experimental results were found to be intimately connected to the onset of the Helfrich periodic distortion of a planar structure.     

Phase Transition Studies in Mixed Polymesomorphic Liquid Crystals with Cholesteric and Smectic A Phase

Cholesteric mesomorphism was induced in a polymesomorphic liquid crystal with nematic and smectic A phases by mixing it with a low pitch cholesteric. A variety of mixtures for a wide range of concentrations were studied. It was observed that the addition of cholesteric liquid crystal disrupts the smectic phase of the parent polymesomorphic liquid crystal. The measured valuses of ultrasound velocity and specific volume and their related parameters were found to exhibit anomalies near each of the phase transitions. The mean interaction parameter $ \bar \alpha $ was estimated for different mixtures and was found to decrease with increasing concentration of cholesteric liquid crystal in the mixture. It was observed that smectic A-cholesteric transition becomes second-order at a particular concentration in agreement with the theoretical predictions.     

Influence of the Orientational Order on the Optical Properties of Cholesteric Liquid Crystals

An orientational order parameter for cholesteric liquid crystals is defined in analogy to nematic mesophases. The influence of this parameter on the selective reflexion and the optical rotatory dispersion is described. Its relative change with temperature is evaluated from ORD measurements for an induced cholesteric soIution and is found to be similar to that known for nematic liquid crystals.     

Reflectance,Transmittance and Color Gamut of Superimposed Layers of Cholesteric Liquid Crystals

The spectral reflectance curves of superimposed layers of cholesteric liquid crystals (CLC) having the same handedness add as there is little absorption. The peak reflectance of the sum cannot exceed 0.5 and the transmittance cannot be less than 0.5, which is the theoretical limit. The shape of the reflection and transmission band resembles rectangles and thus can be compared with the spectral curves of optimum colors. The chromaticity loci of CLC colors and its luminous reflectance equals 0.5 of the value of the luminous reflectance attached to the optimum colors. The color gamut thus produced can be shown to be greater than that which has been obtained with pigments.

A significant improvement can be obtained using superimposed layers of CLC having opposite handedness. Their spectral curves add and the peak reflectance is not limited by the value of 0.5 but approaches 1.0. Use of CLC's of opposite handedness is not always practical as they have different chemical and physical properties. A similar result can be obtained using two CLC layers of the same handedness and a half-wave retardation plate interposed between them. This plate converts the transmitted through the upper layer right-handed (left-handed) component into a left-handed (right-handed) one, which is then reflected from the lower layer and contributes to the reflected left-handed (right-handed) component from the upper layer. The chromaticity loci and color gamut of such superimposed CLC pairs of coatings now approaches that of optimum colors of the same luminous reflectance.     

Lasing Spectra of Dye Doped Cholesteric Liquid Crystals Under Reversible Phototuning of the Frequency

We investigate the lasing frequency phototuning in a distributed feedback (DFB) laser based on cholesteric liquid crystals (CLC). To improve the CLC planar texture quality, we use new materials such as nematic ZhK-654 and an analog of cholesteryl oleate as a twisting additive. The selective excitation of the cis-form molecules of the azoxy-nematic is carried out with the use of a combination of an interference filter with maximum transmission at 436 nm and a cut-off filter. Using the orienting substrates with SnO₂ allows us to reduce the threshold pump power by more than an order of magnitude. The proposed combination of filters for the selective excitation enables us to align the ranges of lasing wavelengths in both directions.     

Cholesteric liquid Crystals: Physical Properties and Molecular-Statistical Theories

It seems certain now that cholesteric liquid crystals (cholesterics, or CLC, for short) have been the first examples of mesomorphic state of matter known to humankind. Apart from the well-known discovery of the two-stage melting of cholesteryl benzoate made by Reinitzer in 1888¹ and cited in almost all the textbooks, one may recall some other papers where the authors, quite unaware of the real significance of their observations, presented undeniable evidence of the liquid crystalline state of matter. E.g., Professor Planer from the Lvov University as early as in 1861 did observe, putting it in the presentday terms, selective reflection from the planar texture of cholesteryl chloride forming a monotropic cholesteric mesophase on cooling.² One should also note that the works of Fergason^3,4 outlining the prospects of practical applications for CLC, did, in fact, precede similar endeavors by Heilmeier in the field of nematics. From the viewpoint of organic chemistry, a number of mesogenic cholesterol derivatives had been already synthesized and thoroughly studied⁵ by the time the name of MBBA (p-methoxybenzylidene-p′-n-butylaniline), to say nothing about cyanobiphenyls, was not yet known to anybody. The mere listing of CLC prospective applications^6,7. seems so impressive that, keeping in mind the above-described historical background, one should expect to find cholesterics in a key position in the field of liquid crystal science.     

Correlation Between the Fiber Texture of Nickel Electrodeposits as Supports and the Reflected Light Band from Cholesteric Liquid Crystals

In previous works [Mol. Cryst. Liq. Cryst., 61, 31(1980); 95, 323 (1983)] the influence of the solid support sorptive properties on the color (reflected light band) of cholesteric liquid crystals was revealed. Qualitative and quantitative (visible diffuse reflectance spectroscopy) measurements at several temperatures on bare metal (Zn, Fe, Cu, Al) surfaces mechanically treated, on chemically and electrolytically prepared ZnO and Fe₃O₄, on electrolytically prepared γ₁,-and γ₂-Al₂O₃, on copper surfaces electroplated with copper and on electrolytically prepared γ₂-Al₂O₃ under several current densities, covered with a cholesteric mixture were carried out and correlated with the sorptive properties of the support. In the present work similar optical measurements at several temperatures with the same mixture of cholesterics as the one of the previous works on nickel electrodeposits with different fiber texture were carried out, compared each other and correlated with the deduced sorptive abilities of the deposits.     

Liquid Crystals in Education

Abstract

The advancement of liquid crystal science and technology has been a truly interdisciplinary effort which combines basic principles of physics, chemistry, and engineering. The success of liquid crystal materials in flat panel displays and electro-optic applications can be attributed to the collaborations of scientists in various fields and the unique interface between academia and industry. Many scientists and engineers engaged in liquid crystal research are also educators in their respective disciplines andlor area of expertise. Those in academia routinely teach undergraduate and graduate courses in physics, chemistry, and engineering, and those in industry often find themselves teaching technical short courses and seminars. Many of us have visited elementary and high schools in the USA to speak to students about careers in science and engineering, and to present some eye-catching demonstrations that excite students about science and technology.