Study of Formation,Phase Behavior,and Microdroplet Size of a Polyurethane-based Polymer-dispersed Liquid Crystal

Using calorimetry and scanning electron microscopy, we have investigated the formation process and phase behavior of a polyurethane-based polymer-dispersed liquid crystal system. We have measured the kinetics and energetics of the cure process during which liquid crystal microdroplets form by phase separation from the matrix as it cross-links. The greatest degree of cure occurs for samples cured at 375 K. For a given cure temperature, the heat of cure decreases more or less linearly with increasing liquid crystal concentration due to a dilution effect. The time constant for the cure process decreases rapidly with increasing temperature but is much less sensitive to liquid crystal content.

Samples cured below 375 K are apparently not fully phase separated, but subsequent treatment at higher temperatures evidently increases the degree of cure. The highest nematic-isotropic transition temperatures were achieved for liquid crystal concentrations above 40 volume percent. The nematic isotropic transition enthalpy, ΔH_NI, is a measure of the amount of liquid crystal contained in the microdroplets. A model has been developed which explains the linear increase of ΔH_NI with increasing liquid crystal concentration.

Optimum microdroplet formation occurs at 375 K, but only for liquid crystal concentrations below about 53 volume percent. At higher concentrations a reversed phase (“polymer ball”) morphology is seen. For the lower concentrations droplet size increases linearly with LC content. Droplet number density decreases with increasing droplet size in rough agreement with a simple model.     

Investigation of the Solubility of Pleochroic Dyes in Liquid Crystals

A precise, inexpensive and relatively fast method for determining the solubility of pleochroic dyes in liquid cyrstals is described. The method has been used to investigate the solubility of two anthraquinone dyes in liquid crystals of different molecular structure. The solubility of the dye is found to be strongly dependent on the molecular structure of the dye and that of the host liquid crystal. Also the temperature dependence of the solubility in the nematic phase has been found to be different from that in the isotropic phase of the liquid crystals. At the nematic isotropic phase transition temperature a discontinuity in the solubility curve has been observed.     

Aggregation of Anthraquinone Dye Molecules in a Nematic Liquid Crystal

We report an experimental evidence that molecules of anthraquinone dye aggregate in an anisotropic solvent, a nematic liquid crystal (NLC). We observe strong nonlinear dependencies of the dielectric permittivity and anisotropy of light absorption on the dye concentration c. Moreover, the data suggest that the aggregates are linear, but, in contrast to the isotropic case, their formation in a NLC undergoes two different concentration regimes with a well pronounced crossover. These effects are explained by the interplay between the aggregates’ lengths and their orientational ordering due to the interaction with the nematic director.     

The Use of Azo Dyes in Guest-Host Displays

Azo dye molecules are cylindrically shaped, and therefore, due to their resemblance to liquid crystal molecules, would seem to be suitable for use in guest-host mixtures. The dichroic ratio of a dye in a given nematic host depends on the molecular length of the dye molecules, the order parameter of the liquid crystal host and its molecular dimensions. It is possible to obtain a guest-host mixture with an order parameter which is higher than that of the pure host. Lateral substitution of mono-azo dyes lowers the order parameter, whilst lateral substitution of the corresponding bis-and tris-azo molecules results in only a slight change in their order parameter.

The stability of these compounds to light is poor, and depends on the wavelength for maximum absorption, and on the host material. Whilst lateral substitution increases the fastness of azo compounds when used for textile dying, this is not the case when they are employed with liquid crystal mixtures. The kinetics of degradation are zero order in dye concentration.

It has been found that azo dyes are not in general suited to applications where prolonged exposure to sunlight may be encountered, e.g. watch applications.     

Measurement of the An isotropic Refractive Indices of Polybenzylglutamate Liquid Crystals. Molecular Factors and Dispersion

The ordinary and extraordinary refractive indices have been measured for lyotropic liquid crystal solutions of poly-γ-benzyl-L-glutamate. The onset of the liquid crystal formation is noted as an abrupt jump in the sample birefringence in the biphasic region. The concentration dependence is otherwise unremarkable. The polymer is found to add a small, anisotropic increment to the refractivity of the supporting solvent. Both n _o and n _e can thus be measured in an ordinary Abbé refractometer for the low refractive solvents common to these liquid crystal solutions. The optical birefringence does not depend significantly on polymer molecular weight. Data is also included for the wavelength dependence (dispersion) of n _o, n _e, and Δn.     

Solubilization in Non-Aqueous Lyotropic Liquid Crystals

The solubilization of a hydrocarbon and two alcohols in a non-aqueous lamellar liquid crystal of lecithin and ethylene glycol was investigated determining the phase region of the liquid crystal and the geometrical dimension of the latter by low angle X-ray diffractometry.

The results indicated the solubilized molecules to be located both in the polar solvent between the lecithin layers in the liquid crystal and between the individual molecules. The hydrocarbon was mainly localized between the hydrophobic end surfaces of the amphiphile and the methanol showed a balanced partition. The solubilization of long chain alcohols caused a reduction of the interlayer distance presumably due to enhanced penetration of the ethylene glycol after solubilization of the alcohol.     

Polarisation‐independent liquid crystal devices

Liquid crystal (LC) devices can be used as amplitude modulators and phase modulators. Most LC devices are polarisation‐dependent and require at least one polariser. As a result, the optical efficiency is greatly reduced. In this paper, we review some of our recently developed polarisation‐independent LC devices. For amplitude modulation, we report two polariser‐free devices which combine light scattering with dye absorption: dye‐doped polymer‐dispersed liquid crystal (PDLC) and dye‐doped LC gels. For phase modulation, we also present two examples: residual phase type, e.g., voltage‐biased PDLC, Polymer‐Stabilised Cholesteric Texture (PSCT), and homeotropic LC gels; and double‐layered structure, such as thin polymer film‐separated orthogonal LC layers, and double‐layered LC gels. Potential applications of these polarisation‐independent LC devices for displays, laser beam steering and adaptive optics are emphasised.     

Orientational order of dissolved molecules in the nematic,smectic A and smectic B phases of 4-n-hexyloxybenzylidene-4′-n-hexylaniline

The orientational order S of a dichroic dye dissolved in a liquid crystal has been determined in the nematic, smectic A and smectic B phase of 4-n-hexyloxybenzylidene-4′-n-hexylaniline. The used method is based on the absorption measurement of unpolarized light in a homeotropic oriented liquid crystalline layer and in an isotropic layer. The orientational order of the dissolved molecules was compared with the order parameter of the liquid crystalline solvent.     

PHOTOISOMERISING EFFECTS IN NITROAZO FERROELECTRIC DYE GUEST HOST SYSTEMS

Abstract

The effect of a photoisomerising dye in a ferroelectric liquid crystal host has been investigated. A nitroazo dye incorporating the same three-siloxane unit as an organosiloxane ferroelectric host has been used to insure good miscibility and high dye concentrations. The effect of photoisomerisation by 300–400 nm UV of up to 430 mW. cm^?2 has been investigated in two mixtures with 25% and 50% molar weight of dye. The 25% mixture has an I-SmC*-crystal phase sequence, while the 50% mixture has I-SmA*-SmC*-crystal. Transition temperatures of both the mixtures are reduced by increased UV illumination. For the first mixture at a constant temperature, the spontaneous polarisation in the ferroelectric phase is reduced by UV illumination. For the second mixture both the spontaneous polarisation and tilt angle of the SmC* phase are reduced by UV illumination and the phase is changed from SmC* to SmA* at temperatures close to the transition. The change of tilt angle and observations of micrograph textures indicate that this is a macroscopic change of the material rather than any phase separation of the dye and host materials.     

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.     

Induced Crystal‐G Phase through Intermolecular Hydrogen Bonding Part XIV: Influence of Alkyl Carbons of p‐Hydroxybenzoates on Phase Distribution of Induced Phase

Two novel series of intermolecular hydrogen‐bonded liquid crystal complexes resulting from equimolar combinations of p‐n‐alkoxybenzoic acids (nABA) (where n represents alkoxy carbon numbers 3 to10 and 12) and non‐mesogens viz. ethyl‐p‐hydroxybenzoate (EHB) and propyl‐p‐hydroxybenzoate (PHB) have been synthesized. The formation and stabilization of the hydrogen bonding is confirmed by infrared spectral study, which reveals the terminal functionalities, ‐COOH and ‐OH moieties are complementary to each other by acting as both proton donor and acceptor. The results of comparative thermal and phase analyses of both the series are discussed in conjunction with free‐ p‐n‐alkoxybenzoic acids. The induction of crystal‐G phase with wide thermal spans is observed in the present complexes.     

Optic Effects of Smectic-Cholesteric Mixed Liquid-Crystals

Light transmission characteristics in optic effects of smectic-cholesteric mixed liquid-crystals are experimentally investigated. Cholesteric addition to a smectic liquid crystal changes greatly transmission characteristics in a cholesteric phase. Although a smectic liquid crystal alone is transparent, the mixed liquid crystals become opaque. However, this opaque state isconverted to the transparent state with applied fields. After the fields are removed, the transparent state is stored in case of then n-type mixture, e.g., BBBA:CN = 90:10 in weight, but returns to the original opaque state in case of thep-type mixture, e.g., C0B:CN = 90:10 in weight. In the smectic phase, the n-type mixture hardly responds to applied fields. but thep-type mixtureeasilyresponds. The response is dependent on a cell temperature: higher field is required in lower temperature. This temperature characteristic can be used for display application of liquid-crystal light valve.     

Synergy of organic dyes for KDP crystal growth

Single potassium dihydrogen phosphate (KDP) crystals were grown in a supersaturated solution containing an organic dye (sunset yellow FCF, brilliant blue FCF, and sky blue). The growth rate, morphology, and impurity dye distribution of faces, (100) and (101) in a KDP crystal were measured as dye concentration and the supersatutation of KDP were changed. Complete expressions for the effect of dye on all aspects of the growth of KDP crystals were discussed. The growth rates of (100) and (101) faces were well correlated by the empirical equation, and resulted in good estimation of the morphology. The distribution of dye in a KDP crystal was represented by the distribution model containing the minimum growth rate for coloring. The growth rate equation and distribution equation were expressed by functions of the supersaturation and dye concentration, and they could effectively provide the operational conditions with coloring the KDP crystal. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bilayered Super-Structures of Antiferroelectric Mesogens

Abstract

Crystal structures of two antiferroelectric mesogens, TFMHPBC and MHPBC-10, were analysed by an X-ray diffraction method. In both crystals, mesogen molecules formed a herringbone structure which was essentially the same as that proposed for the antiferroelectric liquid crystal phase. Because of the crystallographic 2₁-symmetry along the b-axis, only the b-axis component of the dipole moment remains in a smectic layer. Since the dipole moment in the next layer has the same magnitude but the opposite direction, both crystals show no dipole moment as a whole. These structural features observed in their crystal states seem to be conserved in their antiferroelectric liquid crystal phases which are just above their crystal phases.     

Melting and Crystallization Processes of EBBA III. Microscopic Observation

Melting and crystallization processes of EBBA (N-p-ethoxybenzylidene-p'-butyl aniline) have been studied by the method of microscopic observation. A solid sample obtained by rapid cooling (cooling rate: 40 K/min) from nematic to 5°C (sample A) shows granular texture. A band like crystal grows in sample A at room temperature. A solid sample obtained by quenching (cooling rate: 13 K/s) from nematic to about - 100°C (sample B) shows cloudy texture. In the heating process sample B transforms to a more stable crystalline form which shows mosaic texture. A needle crystal grows slowly in a supercooled nematic state, then smaller needle crystals grow rapidly around the needle crystal. The formation and transformation processes of solid modifications are explained on the basis of the idea of instability of them, and schematic phase diagram of those solid forms is presented.

Keywords: EBBA, liquid crystals, nematic, solids, crystal growth, microscopic observation     

Phase Equilibrium of the Ga–Al–Sb system

Employing the method of liquid-phase epitaxy (LPE) solid-solutions of Ga_1–xAl_xAl_xSb (0 ≦ x ≦ 0.8) have been obtained. The dependence of Sb solubility on Al concentration in the liquid phase at 403°C, 452°C, 500°C has been established. The dependence of AlSb concentration in the solid phase on the composition of the liquid phase has been investigated at 452°C. Using the chemical constants equilibrium method, the phase equilibrium of the Ga–Al–Sb system in the region of liquid phase composition near the Ga-rich corner of the phase diagram has been calculated. The comparison of experimental and calculated data for the liquid and solid phases shows their agreement within the limits of experimental error.     

Investigations of the crystal growth of PbMoO4 by the Czochralski method

The thermal conditions at the liquid/solid interface are characterized by temperature measurements in the melt and on the growing crystal with 〈001〉-seed orientation. The thermal boundary layer was determined, from which the relation of effective thermal conductivity of the liquid and solid phase was found to be λ_l,eff/λ_s,eff = 0.25. The liquid/solid interface is extremly convex towards the melt and has a conical shape. When the crystal diameter reaches a certain value the cone was truncated with the formation of a facet in the centre. This typical interface shape is mainly the result of a difference between the effective thermal conductivity of melt and crystal.     

Acid mediated tunability of stimulated laser emission from dye doped chiral microdroplets

Abstract

In the last decade, the possibility to use liquid crystal droplets as optical micro-cavities and lasers has attracted much attention since it paves the way for many applications in the field of sensors or tunable photonics. Several techniques can be used to obtain small micro-resonators as, for example, dispersing a cholesteric liquid crystal inside an immiscible isotropic fluid to create an emulsion. Since liquid crystals are extremely sensitive to external factors as temperature or external fields, laser tuning can be easily achieved. Here, we report on the possibility to tune the laser emission from dye doped cholesteric liquid crystals microdroplets dispersed in a glycerol matrix in presence of nitric acid molecules in the emulsion. Using a fluorescent dye with pH dependent optical properties, the emitted laser wavelength can be tuned in a range of 60?nm. This effect could find applications for the development of spectroscopy based sensors.     

Dielectric relaxation and electro-optical switching behavior of nematic liquid crystal dispersed in poly(methyl methacrylate)

Polymer dispersed liquid crystal composite films were prepared from poly(methyl methacrylate) and nematic liquid crystal E44 by solvent induced phase separation method. In the present investigation we report effect of liquid crystal concentration on the electro-optical and dielectric properties of the composite films. The results were interpreted in terms of phase separation of liquid crystal and polymer, shape and size of liquid crystal droplet, interfacial charge layer effect, liquid crystal loading and miscibility of liquid crystal in the polymer matrix. The miscibility between two phases at interface was investigated by employing Fourier‐Transform Infrared Spectroscopy and differential scanning calorimetry. Morphological study showed that liquid crystal phase is embedded in a spongy poly(methyl methacrylate) matrix and homogeneous distribution increased with increasing E44 content. Electro optical behavior of these composite films under the condition of an externally applied AC electric field (0–200 Vp-p, 50–1000 Hz) and wide range of temperature was determined using He–Ne laser (wave length 632.8 nm) as a light source. It was found that Poly(methyl methacrylate)/E44 (30/70) wt.% composite has more significant properties than the other concentrations. The performance of all composites showed variations with respect to applied voltage as well as temperatures. Dielectric measurement of polymer dispersed liquid crystals has been carried out in the frequency range from 20 Hz to 20 MHz and over the temperature range from 24 °C to 100 °C. The Maxwell–Wagner effect due to interfacial charge accumulation between boundaries of liquid crystal droplets and surrounding of polymer matrix has been observed.     

Liquid Crystal Auto-Induction and Amplification Function for Circularly Polarized Luminescence (CPL) with High gem-Value,and Dynamically Controllable CPL Devices

Optically active phenylenevinylene derivatives with fluorescence are employed for preparation of a cholesteric liquid crystal based circularly polarized light (CPL) emission device. The device shows intense CPL with a quite large value of the g_em-factor (= 0.6) and quick CPL light switching driven by cholesteric-nematic transition with a homeotropic alignment. This research develops the auto-induction of fluorescent chiral inducers in host liquid crystal for formation of a helical structure with CPL amplification. The present research applies a classical LC light-scattering system using chiral technology as a new method for obtaining CPL dynamic control with an intense g_em-factor.