Nematic Liquid Crystal Phase Research Articles

Fast-response infrared phase modulator based on polymer network liquid crystal

We report a high birefringence terphenyl liquid crystal mixture, designated as M3, for infrared phase modulation with special emphasis on mid-wave infrared (MWIR). In addition to high birefringence, M3 exhibits excellent UV stability, modest dielectric anisotropy, and a very broad nematic range. The high birefringence enables a thin cell gap to be used for achieving a 2π phase change while maintaining a high transmittance (T>98%) in the MWIR region. To achieve fast response time, we employed a polymer network liquid crystal using M3 with 2π phase change at λ = 4μm and 3.6-ms response time. This response time is about 100X faster than that of a nematic LC phase modulator.

Effect of methoxy group instead of polar group in the nematic phase of four-ring bent-core liquid crystals

Two achiral four-ring bent-core liquid crystals are investigated by using the optical polarizing microscope, electro-optical studies, and dielectric spectroscopy.

Detecting, Visualizing, and Measuring Gold Nanoparticle Chirality Using Helical Pitch Measurements in Nematic Liquid Crystal Phases

Chirality at the nanoscale, or more precisely, the chirality or chiroptical effects of chiral ligand-capped metal nanoparticles (NPs) is an intriguing and rapidly evolving field in nanomaterial research with promising applications in catalysis, metamaterials, and chiral sensing. The aim of this work was to seek out a system that not only allows the detection and understanding of NP chirality but also permits visualization of the extent of chirality transfer to a surrounding medium. The nematic liquid crystal phase is an ideal candidate, displaying characteristic defect texture changes upon doping with chiral additives. To test this, we synthesized chiral cholesterol-capped gold NPs and prepared well-dispersed mixtures in two nematic liquid crystal hosts. Induced circular dichroism spectropolarimetry and polarized light optical microscopy revealed that all three gold NPs induce chiral nematic phases, and that those synthesized in the presence of a chiral bias (disulfide) are more powerful chiral inducers than those where the NP was formed in the absence of a chiral bias (prepared by conjugation of a chiral silane to preformed NPs). Helical pitch data here visually show a clear dependence on the NP size and the number of chiral ligands bound to the NP surface, thereby supporting earlier experimental and theoretical data that smaller metal NPs made in the presence of a chiral bias are stronger chiral inducers.

Flow properties of a twist-bend nematic liquid crystal

We present the first shear alignment studies and rheological measurements in the twist-bend nematic (Ntb) liquid crystal phase of odd numbered flexible dimer molecules. It is found that the Ntb phase is strongly shear-thinning. At shear stresses below 1 Pa the apparent viscosity of Ntb is 1000 times larger than in the nematic phase. At stress above 10 Pa the Ntb viscosity drops by two orders of magnitude and the material exhibits Newtonian fluid behavior. This is consistent with the heliconic axis becoming normal to the shear plane via shear-induced alignment. From measurements of the dynamic modulus we estimate the compression modulus of the pseudo-layers to be B ∼ 2 kPa; this value is discussed within the context of a simple theoretical model based upon a coarse-grained elastic free energy.

Tuning the iridescence of chiral nematic cellulose nanocrystal films with a vacuum-assisted self-assembly technique.

Iridescent films composed of the chiral nematic liquid crystal phase of cellulose nanocrystals (CNC) have attracted significant interest due to their fascinating optical properties. However, the current fabrication method, i.e., solution casting with a subsequent evaporation process, has significant limitations and therefore hinders the application of CNC iridescent films. In the present study, we demonstrate, for the first time, that vacuum-assisted self-assembly (VASA) can be used to fabricate highly oriented, large area, smooth, and structurally homogeneous CNC iridescent films. It was found that a long ultrasonic pretreatment is necessary for obtaining CNC iridescent films via VASA. Furthermore, it was also found that the iridescent color of the CNC films can be tuned by the sonication time, suspension volume, and degree of vacuum. By combining CD spectroscopy, SEM, and WAXD techniques, the internal structure of CNC iridescent films prepared by VASA has been investigated in detail. Moreover, the origin of the ultrasonic pretreatment effect on the self-assembly behavior of CNCs is also discussed.

Isotropic–nematic phase transition in aqueous sepiolite suspensions

Aqueous suspensions of sepiolite clay rods in water tend to form gels on increase of concentration. Here it is shown how addition of a small amount (0.1% of the clay mass) of a common stabiliser for clay suspensions, sodium polyacrylate, can allow the observation of an isotropic–nematic liquid crystal phase transition. This transition was found to move to higher clay concentrations upon adding NaCl, with samples containing 10-3M salt or above only displaying a gel phase. Even samples that initially formed liquid crystals had a tendency to form gels after several weeks, possibly due to Mg2+ ions leaching from the clay mineral.

Dynamic Switching of the Circularly Polarized Luminescence of Disubstituted Polyacetylene by Selective Transmission through a Thermotropic Chiral Nematic Liquid Crystal

AbstractThe circularly polarized luminescence (CPL) of chiral disubstituted liquid‐crystalline polyacetylene (di‐LCPA) can be dynamically switched and amplified from left‐ to right‐handed CPL and vice versa through the selective transmission of CPL across a thermotropic chiral nematic liquid crystal (N*‐LC) phase. By combining a chiral di‐LCPA CPL‐emitting film with an N*‐LC cell and tuning the selective reflection band of the N*‐LC phase to coincide with the CPL emission band, a CPL‐switchable cell was constructed. The phase change induced by the thermotropic N*‐LC cell by varying the temperature leads to a change in the selective transmission of CPL, which enables the dynamic switching and amplification of CPL. It is anticipated that CPL‐switchable devices might find applications in switchable low‐threshold lasers and optical memory devices.

Dynamic switching of the circularly polarized luminescence of disubstituted polyacetylene by selective transmission through a thermotropic chiral nematic liquid crystal.

The circularly polarized luminescence (CPL) of chiral disubstituted liquid-crystalline polyacetylene (di-LCPA) can be dynamically switched and amplified from left- to right-handed CPL and vice versa through the selective transmission of CPL across a thermotropic chiral nematic liquid crystal (N*-LC) phase. By combining a chiral di-LCPA CPL-emitting film with an N*-LC cell and tuning the selective reflection band of the N*-LC phase to coincide with the CPL emission band, a CPL-switchable cell was constructed. The phase change induced by the thermotropic N*-LC cell by varying the temperature leads to a change in the selective transmission of CPL, which enables the dynamic switching and amplification of CPL. It is anticipated that CPL-switchable devices might find applications in switchable low-threshold lasers and optical memory devices.

Liquid crystal-based biosensors using a strong polyelectrolyte-containing block copolymer, poly(4-cyanobiphenyl-4′-oxyundecylacrylate)-b-poly(sodium styrene sulfonate)

The interface between a nematic liquid crystal phase, 4-cyano-4′-pentylbiphenyl (5CB) and water was examined for protein detection by monitoring the formation of a complex between sodium polystyrene sulfonate (PSSNa) and a positively charged biological species on the 5CB in a transmission electron microscopy (TEM) grid cell coated with a strong anionic polyelectrolyte-containing block copolymer, LCP-b-PSSNa (LCP:poly(4-cyanobiphenyl-4′-oxyundecylacrylate)). This block copolymer was successfully synthesized by reversible addition-fragmentation chain transfer polymerization. A monolayer of LCP-b-PSSNa in a Langmuir Blodgett trough (in which PSSNa and LCP were located in and above water, respectively, in the TEM grid cell) was transferred to the 5CB/water interface in the 5CB-filled TEM grid that was already placed on octadecyltrichlorosilane-coated glass. Model proteins such as bovine serum albumin (BSA), hemoglobin (Hb), α chymotrypsinogen-A (ChTg), and lysozyme (LYZ) having different isoelectric points (pIs) were tested for non-specific protein detection. When the protein solutions were injected into the TEM grid cell, the initial homeotropic orientation of 5CB in the TEM grid cell changed to a planar one below the pIs of the proteins due to electrostatic interactions between PSSNa (- charge) and the proteins (+ charge); this did not occur above the pIs of the tested proteins. The minimum concentrations at which the homeotropic to planar configurational changes (H-P changes) occurred were 0.02, 0.04, 0.04, and 0.08 wt% for BSA, Hb, ChTg, and LYZ, respectively. Therefore, the positively charged biomaterials were visually detected at the PSSNa-coated LC/water interface during an H-P change by using polarized optical microscopy under crossed polarizers. This simple set-up for non-specific biomaterial detection paves a way for the development of efficient and excellent quality biosensors.

Fast-Response Liquid Crystal Microlens

Electrically tunable liquid crystal microlenses have attracted strong research attention due to their advantages of tunable focusing, voltage actuation, low power consumption, simple fabrication, compact structure, and good stability. They are expected to be essential optical devices with widespread applications. However, the slow response time of nematic liquid crystal (LC) microlenses has been a significant technical barrier to practical applications and commercialization. LC/polymer composites, consisting of LC and monomer, are an important extension of pure LC systems, which offer more flexibility and much richer functionality than LC alone. Due to the anchoring effect of a polymer network, microlenses, based on LC/polymer composites, have relatively fast response time in comparison with pure nematic LC microlenses. In addition, polymer-stabilized blue phase liquid crystal (PS-BPLC) based on Kerr effect is emerging as a promising candidate for new photonics application. The major attractions of PS-BPLC are submillisecond response time and no need for surface alignment layer. In this paper, we review two types of fast-response microlenses based on LC/polymer composites: polymer dispersed/stabilized nematic LC and polymer-stabilized blue phase LC. Their basic operating principles are introduced and recent progress is reviewed by examples from recent literature. Finally, the major challenges and future perspectives are discussed.

ЧАСТОТНЫЕ И ТЕМПЕРАТУРНЫЕ ЗАВИСИМОСТИ ДИЭЛЕКТРИЧЕСКОЙ ПРОНИЦАЕМОСТИ И ТАНГЕНСА УГЛА ПОТЕРЬ НЕКОТОРЫХ НЕМАТИЧЕСКИХ ЖИДКИХ КРИСТАЛЛОВ

The substances, capable to be in a liquid crystal state show the fluidity and anisotropy of properties and, as a result of it, easily change structure under the influence of external factors. By the preliminary researches made by a polarizing and optical method, it was revealed that connections form only a nematic liquid crystal phase. In work modern ideas of processes of formation of electric polarization, anisotropy in liquid crystals are studied. Depending on a temperature range in the studied samples dipole-group and dipole-segmental polarization may occur. Types (α and β) of the relaxation processes, lying on the basis of a method of dielectric spectroscopy are considered. The dielectric spectroscopy is one of the most effective methods of studying of molecular structure and intermolecular interaction in liquid crystal, polar substances. Modern methods of measurements and theories of permittivity and losses allow to connect in some cases their dependence on temperature and frequency of electric field with a structure, for example loudspeakers of molecules of liquid crystals. The technique of measurement of dielectric losses and permittivity of materials is described at various temperatures, frequencies and preparation of samples for measurements. Electric properties of liquid crystals at various temperatures are very important at creation of new technologies. Results of a experimental study of temperature dependence of dielectric parameters of three, earlier low-studied, connections in the temperature interval covering nematic and isotropic (near transition are presented to nematic) phases. The sign of dielectric anisotropy of a mesophase is defined by the magnitude and the direction of the molecular dipolar moment, anisotropy of polarizability, degree of orderliness of molecules and also a structure of supramolecular structures.

Effects of added silica nanoparticles on the nematic liquid crystal phase formation in beidellite suspensions.

In this article, we present a study of the liquid crystal phase behavior of mixed suspensions of the natural smectite clay mineral beidellite and nonadsorbing colloidal silica particles. While virtually all smectite clays dispersed in water form gels at very low concentrations, beidellite displays a first order isotropic-nematic phase transition before gel formation (J. Phys. Chem. B, 2009, 113, 15858-15869). The addition of silica nanospheres shifts the concentrations of the coexisting isotropic and nematic phases to slightly higher values while at the same time markedly accelerating the phase separation process. Furthermore, beidellite suspensions at volume fractions above the isotropic-nematic phase separation, trapped in a kinetically arrested gel state, liquefy on the addition of silica nanospheres and proceed to isotropic-nematic phase separation. Using small-angle X-ray scattering (SAXS), we probe the structural changes caused by the addition of the silica nanospheres, and we relate the modification of the phase transition kinetics to the change of the rheological properties.

Xenon NMR of phase biaxiality in liquid crystals

Biaxial thermotropic nematic liquid crystals would be of great importance in liquid crystal display technology. Less than a decade ago, such liquid crystals were suggested. The biaxiality of the phases was confirmed using (2)H NMR spectroscopy of deuterated probe molecules. The spectra were collected from a sample rotating around an axis perpendicular to the external magnetic field, resulting in a two-dimensional powder pattern. We have proposed an alternate technique that is based on the second order quadrupole shift detectable in (131)Xe NMR spectra of dissolved xenon. The method has many advantages, such as the NMR spectra are taken from a static sample and the (131)Xe quadrupole coupling tensor is extremely sensitive to the symmetry of the phase. In the present study, we report results obtained on a 600-MHz NMR spectrometer. Together with the data of our earlier study, they confirm that the asymmetry parameter of the (131)Xe quadrupole coupling tensor in the nematic phase of a ferroelectric liquid crystal is 0.85 and in the smectic A phase ca 0.62, indicating significant phase biaxiality.

Electro-Optical Properties of Electric Field-Induced-Polymer Stabilization at Blue Phase of Chiral Nematic Liquid Crystal

Polymer stabilization of the blue phase induced a wide temperature range of more than 60 K, but the high driving voltage and the large switching hysteresis compared to the low molecular weight blue phase of the chiral nematic liquid crystal. In this study, we newly proposed electric field-induced-polymer stabilization of blue phase (EFIPSBP) which the electric field was applied to the blue phase during polymer stabilization and investigated the driving voltage and the switching hysteresis using various voltage-applied EFIPSBP cells. We confirmed that the driving voltage and the switching hysteresis of the EFIPSBP cell were decreased compared to those of conventional polymer stabilized BP at 8OCB chiral nematic liquid crystal mixture. This could be suggested as a potential method for improving the driving voltage and the switching hysteresis of conventional polymer stabilized BP.

Separation of Isomers on Nematic Liquid Crystal Stationary Phases in Gas Chromatography: A Review

Understanding the composition of complex hydrocarbon mixtures is important for environmental studies in diverse fields, but many prevalent compounds cannot be confidently identified using traditional gas chromatography (GC) techniques. Increasing requirements on analyses of isomeric compounds and the problems encountered in their separation demand a study of more efficient systems which exhibit a high selectivity. Kelker and Fresenius first used nematic liquid crystals as stereospecific stationary phases in GC. Nematic liquid crystal has shown this particular selectivity and sensitivity as stationary phases for the separation of isomers having similar volatilities. Because of their unique selectivity towards rigid solute isomers, liquid crystal stationary phases were considered at one time to be a very promising class of materials that give gas chromatographic separations very different from those that can be obtained with any other stationary phase. Since then, a great deal of attention has been paid to the separation properties of this relatively wide group of substances. Liquid crystal can be used to separate a variety of compounds including isomer mixtures which cannot be separated on conventional stationary phases. This paper aims to review all specific experimental results and presents a comparative analytical study of monomeric nematic liquid crystal stationary phases used in GC. A further contribution of this review is in the field of isomeric compounds separation.

The Nematic Phases of Bent‐Core Liquid Crystals

Over the last ten years, the nematic phases of liquid crystals formed from bent-core structures have provoked considerable research because of their remarkable properties. This Minireview summarises some recent measurements of the physical properties of these systems, as well as describing some new data. We concentrate on oxadiazole-based materials as exemplars of this class of nematogens, but also describe some other bent-core systems. The influence of molecular structure on the stability of the nematic phase is described, together with progress in reducing the nematic transition temperatures by modifications to the molecular structure. The physical properties of bent-core nematic materials have proven difficult to study, but patterns are emerging regarding their optical and dielectric properties. Recent breakthroughs in understanding the elastic and flexoelectric behaviour are summarised. Finally, some exemplars of unusual electric field behaviour are described.

Low symmetry tetrahedral nematic liquid crystal phases: Ambidextrous chirality and ambidextrous helicity

We discuss the symmetry properties as well as the dynamic behavior of various non-polar nematic liquid crystal phases with tetrahedral order. We concentrate on systems that show biaxial nematic order coexisting with octupolar (tetrahedral) order. Non-polar examples are phases with D2 and S4 symmetries, which can be characterized as biaxial nematics lacking inversion symmetry. It is this combination that allows for new features in the statics and dynamics of these phases. The D2-symmetric phase is chiral, even for achiral molecules, and shows ambidextrous chirality in all three preferred directions. The achiral S4-symmetric phase allows for ambidextrous helicity, similar to the higher-symmetric D2d-symmetric phase. Such phases are candidates for nematic phases made from banana-shaped molecules.

Gelator-doped liquid-crystal phase grating with multistable and dynamic modes

We demonstrate a gelator-doped nematic liquid-crystal (LC) phase grating, which can be operated in both the multistable mode and the dynamic mode. Thermoreversible association and dissociation of the gelator molecules can vary and fix the multistable diffraction efficiencies of the gratings. A voltage (V) can also be applied to modulate dynamically the diffraction efficiencies of the grating, which behaves as a conventional LC grating. Experimental results show that the variations of the diffraction efficiencies in the multistable and dynamic modes are similar. The maximum diffraction efficiency is approximately 30% at V = 2 V.

Simple and highly efficient chiral dopant molecules possessing both rod- and arch-like units.

A simple chiral dopant molecule (R)-1 with both rod- and arch-like units was prepared, and extremely large helical twisting powers (+123 to +228 μm(-1)) in nematic liquid crystal phases were achieved. We have demonstrated that the introduction of an arch-like unit in addition to rod-like units is highly effective in controlling the helical molecular alignment. As an application of the dopant, induction of blue phases by addition of a small amount of it was achieved.

Ferroelectric-like switching in the nematic phase of four-ring bent-core liquid crystals

The recent discovery of a ferroelectric response to a switching electric field in nematic phases exhibited by bent-core polar molecules, particularly 1,2,4-oxdiazole derivatives, could lead to new avenues for the development of electro-optic devices. For the first time, we report ferroelectric-like switching under the influence of a triangular wave electric field in the nematic phase exhibited by unsymmetrical achiral four-ring bent-core compounds. These bent-core molecules, exhibiting a large nematic phase range (>70 °C), consist of two unequal lengths in two wings and possess a polar moiety at one end and an alkyloxy chain at the other end. An anomalous variation in spontaneous polarization as a function of temperature in the nematic phase is distinctly observed, which is similar to the results reported only in the low temperature region of the nematic phase. Electro-optical, current-response and dielectric studies of aligned samples corroborate earlier reports of the proposed polar structure of the cybotactic clusters and the ferroelectric-like polar switching of these nematic phases.