Mesomorphic Properties of an Homologous Series of Alkoxy-Terminated Enamine-Ketone Containing Liquid Crystals

An homologous series of enamine-ketone compounds has been synthesized and their thermal properties investigated. The bis [3- p-n-alkoxyanilino)-2-butenoyl] benzenes exhibited nematic mesophases when the terminal alkoxy substituents were short and both nematic and smectic mesophases when the terminal alkoxy substituents contained twelve or more carbons. The thermal data indicate that although crystalline order is largely determined by the packing of the hydrocarbon tails, there is a low degree of interaction by the tails in the nematic mesophase.     

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.     

Dielectric Relaxation in Liquid Crystals

We present recent progress in the study of the liquid crystals by dielectric relaxation. First, we discuss the possible mechanism controlling the rotation of the molecules around a short axis: rotational diffusion or free volume effect. In the case of phase transition between Sm A phases, dielectric relaxation can bring information about the short range order and also the long range order when a dipole ordered structure is established.     

The Orientational Optical Non-Linearity of Liquid Crystals

Abstract

The cubic optical non-linearity of a cholesteric liquid crystal (CLC) having a planar helical texture is considered. A study was made of an effect which has a minimal establishment time and is due to the change in the director distribution along the helix under the orientational influence of the light field. Polarisation peculiarities of the non-linearity, when the light wave propagates along the helical axis, are discussed. Self-focusing should not occur for circulary polarised light, but has the constant ?₂ ～ 8.10^?8 cm³/erg for linearly polarised light. The additional rotation of the axes of elliptical polarisation is predicted to be proportional to the light intensity and determined by the same constant ?₂; this rotation is identical for both right-and left-handed CLC. Non-linear optical activity is absent in the approximation under consideration.

Analogous effects for light propagating through an oriented nematic are 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.     

Modulated Structures in Dielectric Liquid Crystals

Review of theoretical and experimental results concerned to formation of space-modulated orientational structures in liquid crystals is given. Properties of ideal liquid crystal dielectrics are considered. Origion conditions and properties of flexoelectric and flexoelectromagnetic structures in nematics, dielectric instabilities in cholesterics and smectics, ferroelectric structures in chiral smectics are discussed.

It is well known that an electric field induces a modulation of the orientational structure of liquid crystals. There are two types of such modulated structures (MS) having different physical natures. The dissipative one is connected with the transport of electric charges and mass of the substance. The thermodynamic MS are not accompanied by the irreversible transport processes. Such MS can exist in ideal liquid crystal dielectrics. In fact there are always some ion impurities and as a result two types of MS are possible in liquid crystals. However it is possible to change the thresholds of these orientational instabilities in dependence on the concentration of impurities, anisotropy of transport parameters, boundary conditions and frequency of an external field. Thus one can exclude or induce the effects of first and second types. Below we shall consider the thermodynamic orientational transformations and conditions of their observation.     

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.     

Electrical and Optical Properties of Ferroelectric Liquid Crystals and Influence of Applied Pressure

Much larger dielectric constants and spontaneous polarizations are evaluated in the smectic-C phase of p-alkoxybenzylidene-p'-amino-2-chloropropyl-cinnamate (HOBACPC, OOBACPC and DOBACPC) which have dipole moments at the chiral part compared with those of p-alkoxybenzylidene-p'-amino-2-methylbutyl-cinnamate. In the mixture of D-DOBAMBC and L-DOBACPC with inverse chirality, the pitch of the helical structure is longer compared with those of original compounds. On the other hand, the dielectric constant and the spontaneous polarization of the compounds are nearly linearly dependent on the concentration of L-DOBACPC.

The phase transition temperature from smectic-A to chiral smectic-C phases in DOBAMBC decreases remarkably with the mixing of organic molecules (PAA, eicosane, etc.) but increases with applied pressure.

The threshold field of electro-optical effects (transmission increase, colour switching, memory effects) in DOBACPC and HOBACPC etc. is lower than that in DOBAMBC etc. in accordance with the increase of the spontaneous polarization in the former.     

钒酸盐系列激光晶体制备和性能研究

用提拉法生长了Nd∶YVO4, Nd∶GdVO4,Nd∶GdxLa1-xVO4(x=0.8, 0.6, 0.45)系列晶体,对影响晶体质量的因素进行了分析,测量了几种晶体的结构和晶胞常数;测量了Nd∶YVO4, Nd∶GdVO4, Nd∶Gd0.8La0.2VO4和Nd∶Gd0.6La0.4VO4晶体的室温吸收谱和荧光谱,用LD泵浦Nd∶YVO4, Nd∶GdVO4, Nd∶Gd0.8La0.2VO4晶体,实现了1.06μm和1.34μm的激光输出.     

Optical Characterization of Lyotropic Chromonic Liquid Crystals

We report the results of our studies on the optical and thermal properties of binary mixture of compounds, viz., Berberine (BBE) and Poly-ethylene glycol (PEG). The mixture shows very interesting coexistent biphasic regions N + I and N + C phases respectively at different concentrations of BBE sequentially when the specimen is cooled from its isotropic liquid phase. The temperature variations of optical anisotropy and optical textures have also been discussed. Formation of above molecular orientations has been confirmed by X-ray studies.     

Some Novel Nematic Liquid Crystals of Negative Dielectric Anisotropy

A number of ether and ester derivatives of 1-(trans-4-n-alkylcyclohexyl)-2-(4-hydroxyphenyl)ethane bearing lateral substituents on the phenyl ring are reported. Several of the novel compounds exhibit enantiotropic nematic phases at room temperature. All of the liquid crystals described are of negative dielectric anisotropy (-0.5 -3.5). The effect of lateral substituents on the liquid crystal transition temperatures of these ethanes has been studied.     

Synthesis and Liquid Crystal Behavior of a Novel Ester Homologous Series

A novel ester homologous series of 4-[4’-n-alkoxy cinnamoyloxy] benzyl benzoates has been synthesized. The series consists of 12 homologues. Liquid crystal properties commence from the sixth member to the last member of the series with the exhibition of an enantiotropic nematic phase without the exhibition of any smectogenic mesophase. The remaining homologues do not exhibit liquid crystal behavior. The texture of the nematic phase is of the threaded or Schlieren type. The solid-nematic or isotropic transition curve adopts a zigzag path and the nematic-isotropic transition curve steeply rises and then falls in the phase diagram and behaves in normal manner. An odd–even effect is absent in the nematic-isotropic transition curve with an alteration of transition temperatures. The average thermal stability for the nematic mesophase is 153.1°C, and the nematogenic temperature ranges vary from 12°C to 50°C. Analytical data confirm the molecular structures of the homologues. The Liquid Crystal properties and transition temperatures were observed through an optical polarizing microscope, equipped with a heating stage. The mesomorphic characteristics of the novel ester series are compared with other, known and structurally similar series. The novel series is predominantly nematogenic with the absence of any smectogenic character, with relatively short mesophase temperature ranges and of the middle ordered melting type.     

Comparative Dielectric Investigations on Nematic Liquid Crystals with Aerosil

Dielectric measurements on a nematic room temperature mixture and the mixture with 20% aerosil were carried out. Addition of aerosil results in a small increase of the clearing temperature and in increasing relaxation frequency for the reorientation around the short molecular axis. The results are interpreted as orientation effect of the nematic sample at the fractal aerosil.     

Dielectric Behavior of Nematic Liquid Crystals under DC Electric Field

Dielectric behavior of nematic liquid crystals of p-methoxybenzylidene-p-n-butylaniline and p-ethoxybenzylidene-p-n-butylaniline have been investigated under DC electric field in relation to their optical behavior. Both dielectric permittivity and transmitted light intensity decrease at a threshold voltage of external DC electric field, and the latter reaches a constant low value at higher field, while dielectric permittivity increases via minimum to approach a constant value at higher field strength of about 2 × 10⁴ V/cm. Such a change of dielectric permittivity under DC electric field is characteristic of nematic and supercooled states, and exhibits large dependence on both temperature and sample thickness. This dielectrical observation is discussed in terms of dielectric anisotropy of the nematic liquids oriented to the field, and the disturbance of molecular orientation due to the dynamic scattering at higher DC electric field.     

The Dielectric Anomaly Near the Transition from the Smectic A* to Smectic C* Phase and Visco-Elastic Properties of Ferroelectric Liquid Crystals

A pyro-electric technique is developed which allows the measurement of the dielectric response near the A*-C* phase transition in ferroelectric liquid crystals. The temperature dependence of the elastic modulus K _θ(T) corresponding to the molecular tilt in smectic layers is calculated from the experimental data. Direct pyro-electric measurements of the relaxation time for the spontaneous polarization P _s and the data on K _θ(T) allow us to calculate the temperature behaviour of the twist-viscosity γ₁(T) for the smectic C* phase. The curves γ₁(T) are compared for the smectic C* and the nematic phase of the same compound and the dependence of the twist viscosity on the molecular tilt angle in the C* phase is investigated. The results of the dielectic measurements are discussed in terms of the mean-field approximation.     

Stable Liquid Crystals with Large Negative Dielectric Anisotropy - III

The synthesis of 4-n-pentyl and 4-n-butoxy derivatives of 1-(2,3-dicyano phenyl)-2-(4-(4-trans-n-pentyl cyclohexyl)phenyl) ethane and 1-(2,3-dicyano phenyl)-2-(4-trans-n-pentyl-4-trans-bicyclohexyl) ethane as well as of 1-(4-n-butoxy-2,3-dicyano phenyl) -2-(4-n-pentyl-4-biphenyl) ethane is described. These ethane derivatives as well as 4-butoxy-2,3-dicyano-4-n-pentyl p-terphenyl were all high melting and no enantiotropic or monotropic mesophases could be observed. However, they can be used as additives for photochemi-cally stable LC mixtures with low viscosity and large negative Δσ. The mesomorphic properties of some intermediate thiophene and furan derivatives are described.     

On the Electrical Switching Properties of Polymeric Liquid Crystals

The properties of side chain polymeric liquid crystals in an electric field are described. The polymers investigated are (A) a polysiloxan-copolymer with Cl- and OCH₃-end groups in the mesogenic side chains and (B) a polyacrylic ester homopolymer with a CN-group in the mesogenic side chain.

Applying the Fréedericksz-effect, the rise times and the passive decay times are determined. With the two-frequency technique, the active decay time is also obtained. The order of magnitude for the active decay time is about one second. The active decay time is approximately constant in a temperature region about 15 K below the clearing point.     

Liquid Crystalline Phases in the Homologous Series of Terephthalylidene-BIS-[4-n-Alkylanilines]

The liquid crystalline polymorphism in the homologous series of terphthalylidene-bis-4-n-alkylanilines was investigated by miscibility experiments. The variants of polymorphism found in the members with carbon atoms n = 3 to n = 10 in the alkyl chains can be derived from a variant HGFICA Nthrough the absence of one or several phases. Beside the known phases H, G, F, C, A, and N, the new phase type 1 was found. The results are discussed in connection with investigations on structure.