Liquid Crystal Substances Research Articles

Liquid crystalline substances doped with carbon nanotubes as a sensitive medium of the CO2 sensor

The creation of highly sensitive optical CO2 sensors in air is an urgent problem, as it allows monitoring of the content of CO2 both indoors and in ambient air, where its level is usually 600…1000 ppm. As a sensitive element of the CO2 sensor, it is proposed to use a liquid crystal substance (cholesteric-nematic mixture based on 5CB with cholesteric impurities), supplemented with multi-layered carbon nanotubes. The spectral characteristics of the liquid crystal mixture in the range of 400…600 nm were studied. The content of carbon nanotubes ranged from 0.1 to 0.5 wt.%. It was established that with an increase in concentration in the range of 10…100 mg/m3, the transmission minimum shifts to the long-wave region of the spectrum. The maximum spectral sensitivity in this range is 6 nm ⁄ mg ⁄ m3. The researched material can be used as a sensitive element of an optical CO2 sensor. Further research should be conducted in the direction of investigating the interaction of such a composite with other gases, as well as finding opportunities to improve the properties of the composite. Perhaps the use of multi-walled carbon nanotubes.

Liquid crystals purity assay using nonaqueous capillary electrokinetic chromatography.

A method for purity control of newly synthesized lactic acid-based liquid crystals has been developed. The electrokinetic chromatography proved to be suitable for the separation of these electroneutral substances from their impurities. The separations were performed in an acidic acetonitrile-based background electrolyte (BGE) with a pseudostationary phase formed by a cationic surfactant. During the optimization step, appropriate concentrations of cetyltrimethylammonium bromide, acetic acid, and water were seeked. In the optimized method, separations were carried out in acetonitrile with 1-mol/L acetic acid, 80-mmol/L cetyltrimethylammonium bromide, and 6% (v/v) water. Interesting positive effects of a small water content in the BGE on electroosmotic flow and resolution of liquid crystal substances from their impurities were observed and discussed. Samples of five liquid crystal substances, both pure and containing impurities from synthesis, were analyzed. The identification of analytes was based on a comparison of relative migration times related to the migration time of mesityl oxide. For all five samples, impurities were separated from the liquid crystals and the method thus showed its viability. To the best of our knowledge, this method is used for the first time for the purity control of newly synthesized liquid crystals. This method can be used to confirm or complement the results obtained by commonly used high-performance liquid chromatography and supercritical fluid chromatography methods. Furthermore, the electrokinetic chromatography method requires very small amounts of sample, solvents, and buffer constituents. Overall, its operational costs are significantly lower.

Mesomorphism dependence on terminal polar group in the nonlinear novel azoester series

Synthesis of a novel azoester homologous series is carried out with a view to understand and establish the effect of molecular structure on liquid crystal (LC) behaviors of a substance. Novel series consists of ten LC substances. All the homologs are enantiotropically nematogenic without exhibition of smectic property. Transition and melting temperatures, textures of LC are determined by an optical polarizing microscopy equipped with a heating stage. Textures of nematic phase are threaded or schlieren. Transition curves of a phase diagram behaved in normal manner. Nematic–isotropic transition curve exhibited odd–even effect. Analytical and spectral data supported and confirmed the structures of homologues.

Induced smectic phases of stoichiometric liquid crystal mixtures.

We revealed the detailed structures of induced smectic liquid crystal (LC) phases composed of a binary mixture of charge-transfer (CT) LC substances. Although neither of the constituents had highly ordered smectic phases, the mixture exhibited smectic-E (SmE) or smectic-B (SmB) phases when mixed at ratios of 1 : 1 and 2 : 3, respectively. The results of polarized optical microscopy, differential scanning calorimetry, X-ray diffraction, and infrared spectroscopy indicated that the induced smectic phases were stabilized by an exquisite balance between the CT interactions, dipolar interactions, and excluded volume effects. We proposed a possible model for the molecular arrangements in the SmE and SmB phases, which consistently explained the experimental results including the stoichiometric ratios.

Effect of the liquid crystal solute on the rotator phase transitions of n-alkanes

Recent experimental studies have shown that the liquid crystal substance plays an important role in determining the structures and the phase transitions of the different rotator phases in binary mixtures of n-alkane and liquid crystals.

A Study of Liquid Crystalline Properties and Their Relation to the Molecular Structure of Novel Ethylene Derivatives

A novel homologous series α-4-[4′-n-Alkoxy benzoyloxy] benzoyl β-4″-methoxy phenyl ethylenes of 12 homologue substances is presented and comprises of 5 compounds (heptyl to tetradecyl) that exhibit both a smectic and a nematic phases. The pentyl, hexyl, and hexadecyl homologues only exhibit the nematic without exhibition of any smectogenic property. The methyl to butyl homologues are not liquid crystalline. All novel liquid crystal (LC) substances of novel series are enantiotropically mesomorphic. The textures of nematogenic homologues are threaded or Schlieren and that of the smectogenic homologues are typical of the smectic A or C. Analytical and spectral data support the molecular structures. Solid-mesomorphic or isotropic, Sm-N, and N-isotropic transition curves of a phase diagram behave in a normal manner. An odd–even effect is exhibited by the Sm-N and N-isotropic transition curves. Thermal stabilities for the smectic and nematic mesophases are 118.0°C and 157.0°C, respectively. The Sm-N mesophase length is between 13°C and 21°C, whereas the N-I mesophase length varies between 20°C and 46°C. The liquid crystal properties of the novel series are compared with known structurally similar homologous series.

On the long-time behavior of some mathematical models for nematic liquid crystals

A model describing the evolution of a liquid crystal substance in the nematic phase is investigated in terms of two basic state variables: the velocity field u and the director field d, representing the preferred orientation of molecules in a neighborhood of any point in a reference domain. After recalling a known existence result, we investigate the long-time behavior of weak solutions. In particular, we show that any solution trajectory admits a non-empty ω-limit set containing only stationary solutions. Moreover, we give a number of sufficient conditions in order that the ω-limit set contains a single point. Our approach improves and generalizes existing results on the same problem.

On a non-isothermal model for nematic liquid crystals

A model describing the evolution of a liquid crystal substance in the nematic phase is investigated in terms of three basic state variables: the absolute temperature ϑ, the velocity field u and the director field d, representing preferred orientation of molecules in a neighbourhood of any point of a reference domain. The time evolution of the velocity field is governed by the incompressible Navier–Stokes system, with a non-isotropic stress tensor depending on the gradients of the velocity and of the director field d, where the transport (viscosity) coefficients vary with temperature. The dynamics of d is described by means of a parabolic equation of Ginzburg–Landau type, with a suitable penalization term to relax the constraint |d| = 1. The system is supplemented by a heat equation, where the heat flux is given by a variant of Fourier's law, depending also on the director field d. The proposed model is shown to be compatible with first and second laws of thermodynamics, and the existence of global-in-time weak solutions for the resulting PDE system is established, without any essential restriction on the size of the data.

Linearly polarized IR-spectroscopy of partially oriented solids as a colloid suspension in nematic host: a tool for spectroscopic and structural elucidation of the embedded chemicals

Colloid suspensions in nematic liquid crystal were employed for the first time in 2004 as a tool for the partial orientation of solids, to be examined by linearly polarized IR-(IR-LD) spectroscopy. It has been found is found that a partial orientation (15–20%) of suspended particles, is adequate for the recording of reasonable linearly polarized IR-spectra is achieved when: 5 ± 1% by weight of the given solid compound (organic, inorganic, transition metal complex or glass) with particle size within the limits 0.3–0.9 μm is mixed with a nematic liquid crystal substance (ZLI 1695, ZLI 1538 or MLC 6815) suitable for IR spectroscopy and the slightly viscous suspension obtained is phase pressed between two KBr-plates. These latter are roughened in one direction prior to use with fine sandpaper (C800) (size 5 μm). Then, the KBr-plattes and pressed suspension are moved repeatedly with 3 μm/s for 100 times. The optimal cell thickness is 100 μm. If mathematical procedures are used for polarized IR-spectra interpretation, then it is possible to perform structural elucidation of the embedded compounds, independently of their melting point, crystalline or amorphous state, and the quality of the monocrystals or polycrystalline of the e sample. The method permits the study of organic and inorganic compounds, transition metal complexes and glasses. Here we will discuss the fundamental questions concerning the above state such as the morphology of the suspended particles, the particle size, the influence of the physical chemistry properties of liquid crystal medium on the degree of orientation of suspended particles; the velocity of the shearing of the suspension, the degree of the roughening of the KBr-plates and their effects on the degree of orientation, the influence of the space group on the orientation parameter, the nature and balance of the forces acting on the suspended particles; their degree of orientation, the mathematical model used. Conventional and linearly polarized IR-spectroscopy and electron microscopy are used for elucidation of these points. Statistical approaches have also been applied in order to estimate the impact of the experimental parameters (size, velocity, thickness) on the IR-signal for each of the 13 systems studied. An experimental design of the type involving full factorial design on two levels of variation of the input factors is presented.

Correlation Between Order Parameter of Liquid Crystal Mixture and Dichroic Ratio of its Guest-Host System

Good correlation was found between the order parameter S of liquid crystal (LC) host-mixture and the dichroic ratio of its guest-host system. We investigated temperature dependence of the S value of several LC substances, focusing on its index β. The index β was determined by measuring the temperature dependence of refractive index. LC substances with a cyano terminal group have smaller β values than those with an alkoxy or a fluoro-substituted terminal. This means that the S value is expected to be larger for the cyano substances than the alkoxy or fluoro substances at a fixed reduced-temperature. There exists good correlation between the calculated S value of a host LC mixture and the measured dichroic ratio of its guest-host system. This enables us to design highly ordered LC mixtures for high contrast guest-host LC displays based on β values of LC substances.

Activation energies for metastable to stable phase transition of 4,4'-di-n-heptyl- oxyazoxybenzene

The paper studies metastable phases of 4,4'-di-n-heptyloxyazoxybenzene (liquid crystal substance) using adiabatic calorimetry. The process of transformation between metastable and stable phases is described quantitatively. The conclusion concerns activation energies for metastable phase to stable phase transition.

MOLECULAR DYNAMICS SIMULATIONS OF SMECTIC C PHASE APPEARING IN LANGMUIR MONOLAYERS

We perform molecular dynamics simulations using a simple model system of liquid crystal substance on air-water interface. A system of soft spherocylinders, which do not show smectic C phase in bulk, exhibits a tilted smectic phase when interacting with a model water plane. We observe not only macroscopic properties such as the pressure-surface area curve, but also correlation functions and diffusions as well.

Computer modelling of the optical response of a liquid-crystal display at high control voltages Part 2 How the parameters of the liquid-crystal material affect the dynamics of the display?s optical response

Computer modelling is used to investigate the optical-response dynamics of a liquid-crystal (LC) display at high control voltages as a function of the physical parameters of the substance. The character of the influence of the main physical parameters of the LC substance on the memory time is explained for an LC display in which the interference effect of the optical modes is used in a structure with a twist angle of 180°.

IONIC CONDUCTION IN NEMATIC AND SMECTIC A LIQUID CRYSTALS

Permittivity dispersion measurements in a very low frequency range were made on n-pentyl (5CB) and n-octyl 8CB cyano biphenyls, providing characterization of mobile ions by the diffusion coefficient, or the mobility, and their temperature dependence in the nematic phase and/or the smectic A phase. Ionic conduction was confirmed in the nematic and the smectic A phases from the continuity of these temperature-dependent properties in the isotropic, the nematic, and the smectic A phases with reasonable changes at the transition temperatures. The Walden rule was found to be applicable to the ionic conduction in the nematic phase of 5CB without a pre-transitional phenomemon near the nematic-smectic phase transition, when appropriate mean values are taken for both the anisotropic mobility of ions and the anisotropic viscosity of the liquid crystal substance. It was also pointed out that care should be taken for the applicability of the Walden rule to liquid crystals in the case when the anisotropy of ionic conduction is argued, especially for liquid crystal materials containing mobile ions with a far less anisotropic shape than the liquid crystal molecules.

Highly-anisotropic liquid-crystal mixtures for tunable microwave devices

Progress in the microwave dielectric properties of novel liquid crystal (LC) mixtures is reported, specially designed for voltage-tunable microwave devices. These advanced mixtures possess much higher microwave dielectric anisotropy and much lower loss tangent than standard LC substances such as K15 of Merck. Moreover, with Δn=0.37 at 26 GHz those novel LC mixtures exhibit by far the highest microwave anisotropy at room temperature, known from literature. Compared to a former approach with K15, the figure-of-merit of a tunable microstrip-phase shifter device has been increased significantly from 20°/dB up to 110°/dB, both measured at 24 GHz. To the authors knowledge, this is the best value demonstrated for a tunable phase shifter based on nonlinear dielectrics above 20 GHz, up to now. This substantial progress opens up totally new low-cost LC applications beyond optics.

Liquid‐crystal mixtures having tolane substances for field‐sequential‐color TN‐LCDs

Abstract—Novel liquid‐crystal (LC) mixtures featuring high optical anisotropy Δn) and small rotational viscosity (γ1) were developed for field‐sequential‐color TN‐LCD applications. The dynamic behavior of the TN cells in a narrow‐gap range was studied and new tolane LC substances were introduced. The newly developed LC mixtures, having a narrow‐gap cell, enable a TN‐LCD to switch fast enough to be applied to field‐sequential‐color displays not only at a room temperature but also at low temperatures. It was also confirmed that the voltage‐holding ratio (VHR) is sufficiently high in field‐sequential addressing conditions and, therefore, the LC mixtures can be used in active‐matrix LCDs. For practical use, a storage test of the TN cells under light irradiation was performed to evaluate their voltage‐holding property. It was also confirmed that their high VHR can be maintained for over 10,000 hours under practical conditions.

Ion Generation in Liquid Crystals under Electric Field

Permittivity dispersion and transient current profiles of two classes of liquid crystal substances were analyzed, providing an idea of probable mechanisms of ion generation. Due to the strong local field caused by the electric double layers by ions, Schottky injection at electrodes can play an important role even under a small DC voltage application. In a higher voltage region, the dissociation of ion pairs and the recombination of free ions can be the dominant mechanisms, resulting in the Ohmic behavior of the voltage dependence of a steady-state current. Understandings of those mechanisms will be of a great help for development of advanced liquid crystal materials for uses in sophisticated displays.

Technical Note A study of microencapsulated chiral nematics in the presence of electric fields

Thermochromic liquid crystals have been used for the visualisation and measurement of heat transfer and temperature distributions since the late 1960s. The dawn of this rapidly exploited technology followed shortly after a review of liquid crystal materials [1, 2]where it was noted that a number of possible applications could arise from the ability of liquid crystal substances to register minute fluctuations in the local temperature, mechanical stress and electromagnetic radiation. Much work followed this period in the examination of the effect of electric fields on unencapsulated liquid crystals. Among which, Gerritsma and Van Zanten [3]showed that there was an electric field induced transition from the cholesteric mesophase to the nematic mesophase. During these tests it was noted that if the helical axes of the liquid crystals were parallel to the applied electric field, a periodic distortion in the planar texture occurred. At higher fields the helical axes were seen to rotate through 90° before the cholesteric to nematic transition occurred. Such transitions are easily detectable with the naked eye. The selectively reflected colours, characteristic of the cholesteric mesophase, would diminish in terms of saturation level as the focal conic structure was adopted. Untwisting of the helices would result in varying colour play properties being observed, until either the pitch of the helices was unsuitable for reflecting wavelengths of visible light [4]or, the molecules of the liquid crystal had adopted the structure of the nematic mesophase. Since this period, liquid crystals have been used in a variety of heat transfer studies : typical of which are Hippensteele et al. [5]and Ireland and Jones [6]. However, it should be noted that less emphasis has been placed on examining the properties of liquid crystals in recent studies, and as such heat transfer studies have been bound by examinations of unencapsulated rather than microencapsulated materials which are commonly in use today. Instead, techniques have been developed to help process the observed isochromes into desired temperature, and heat transfer coefficient, distributions [7, 8]. It is the aim of this work to examine the effect of an electric field on microencapsulated chiral nematics. From this it should be possible to identify whether electric fields have a significant effect on these liquid crystals and, if this is so, to what extent. In the present study a circular cylinder, coated with microencapsulated chiral nematic liquid crystals, was placed in electric fields up to 15 kV per meter parallel to the coated surface and up to 30 kV per meter perpendicular to this surface coating. Transitions to the nematic phase and variations in the colour play of the liquid crystal coating were investigated under these conditions.

Collective and non-collective excitations in antiferroelectric and ferrielectric liquid crystals studied by dielectric relaxation spectroscopy and electro-optic measurements

The dynamics of different molecular modes in four antiferroelectric liquid crystal substances have been studied by a combination of spectroscopic methods.The fastest motion is the reorientation around the molecular long axis, here found in the low GHz range by time domain spectroscopy. The reorientation around the short axis has a characteristic frequency of about 10kHz and is detected by frequency domain spectroscopy in the homeotropic configuration. As for the collective excitations, the Goldstone and soft modes, characteristic of the ferroelectric phase, have counterparts in the antiferroelectric phase which appear very different. There are two characteristic peaks in the spectrum, one at high frequency, about 100kHz, the other at low frequency, about 10 kHz. The latter has often been mistaken for short axis reorientation and both have been attributed to soft modes. By combining different experimental techniques and different geometries it can be shown that neither is a soft mode, but both are collective modes of different character: the high frequency mode corresponds to fluctuations where molecules in neighbouring layers are moving in opposite phase, the low frequency mode to phase fluctuations in the helicoidal superstructure. In materials exhibiting a C* phase in addition to the C*a or C* gamma phases, an additional strong peak appears in at least one lower-lying phase adjacent to the C* phase. We show that this peak, which we call a hereditary peak, has nothing to do with the antiferroelectric or ferrielectric order, but is just the Goldstone peak from a coexisting C* phase. In the same way, a Goldstone mode peak from the C* gamma phase may appear in the underlying C* a phase. In a general way, narrow phases like C* gamma, being bounded by first order transitions on both sides (C* a -C* gamma -C*) are likely to show non-characteristic (hereditary) peaks from both adjacent phases.

Liquid-crystals for multimedia display use: materials and their physical properties

AbstractDisplay devices are one of the key elements of the coming multimedia infrastructure, and Thin-Film-Transistor addressed Liquid Crystal displays are most promising for this application. Physical properties such as dielectric permitivities and electric resistivity are essentially important for TFT-addressing use LC materials. Both theoretical and experimental studies on these properties are reviewed and discussions are made on these in relation to the chemical structure of the liquid crystal substances, providing semiquantitative explanations on the superiority of fluorinated LC materials in respect to the coexistence of the high polarity and the high resistivity. Further studies to improve LC material properties are finally suggested.