Liquid Crystal Order Research Articles

Theory of nematic ordering in supramolecular systems: Self-assembly of mesogenic groups and self-organization of the system

The behaviors of two systems are considered: a low-molecular system (a binary mixture of low-molecular components) and a polymer system (mixture of a macromolecules and a low-molecular dopant) whose components reversibly bind to form dimers capable of liquid-crystal ordering. The general theory of binary-mixture structuring, where two processes occur—self-assembly of mesogenic dimers of the mixture components and their self-organization—is presented. The theory demonstrates general laws of the behavior of low-molecular and polymer systems as well as differences related to the features of polymer thermodynamics.

Electric field effects on phase transitions in the 8CB liquid crystal doped with ferroelectric nanoparticles.

The influence of a low ac electric field on phase transitions is discussed in the case of a nematic liquid crystal 4-n-octyl-4^{'}-cyanobiphenyl (8CB) doped with Sn_{2}P_{2}S_{6} ferroelectric nanoparticles. The phase-transition temperatures obtained from temperature-dependent dielectric measurements were higher than those determined by the calorimetric method. This difference is explained by the presence of the measuring electric field which induces two effects. The first one is the amplification of the interactions between the nanoparticle polarization and the liquid-crystal order parameter. The second one is the field-induced disaggregation or aggregation process at high nanoparticle concentrations.

Search for microscopic and macroscopic biaxiality in the cybotactic nematic phase of new oxadiazole bent-core mesogens

The possibility of biaxial orientational order in nematic liquid crystals is a subject of intense current interest. We explore the tendencies toward local and global biaxial ordering in the recently synthesized trimethylated oxadiazole-based bent-core mesogens with a pronounced asymmetric (bow-type) shape of molecules. The combination of x-ray diffraction and optical studies suggests that the biaxial order is expressed differently at the short- and long-range scales. Locally, at the scale of a few molecules, x-ray-diffraction data demonstrate biaxial packing. However, above the mesoscopic scale, the global orientational order in all three compounds is uniaxial, as evidenced by uniform homeotropic alignment of the nematic phase which is optically tested over the entire temperature range and by the observations of topological defects induced by individual and aggregated colloidal spheres in the nematic bulk.

Nematic liquid crystal order reconstruction in ultraconfinement, from density-functional theory

ABSTRACTI use the Onsager approximation of density-functional theory with a simple Parsons–Lee re-scaling to study a hybrid-aligned liquid crystal under very strong confinement. The system is modelled as a film of hard Gaussian overlap particles in a slit geometry, where anchoring is planar at one wall and homeotropic at the other. As the film density is increased from the isotropic phase, a planar-disordered layer grows from the planar-anchoring wall, and a homeotropic layer from the homeotropic-anchoring wall. For film thicknesses between 2 and 7 particle lengths, the degree of overall nematic order as measured by the spatial average of (where is the nematic order parameter tensor) is a continuously increasing function of the density, with a pronounced rise around the bulk isotropic-nematic transition. This rise is steeper the thicker the film, and is preceded by a large amount of biaxiality growing from the planar-anchoring wall. Deeper into the nematic phase, most of the film is either planar or homeotropic, but some biaxiality persists in a region of otherwise reduced overall nematic order whose position varies with film thickness. This is similar to the Schopohl–Sluckin scenario of a biaxial defect core.

Orientational Order of a Lyotropic Chromonic Liquid Crystal Measured by Polarized Raman Spectroscopy.

Lyotropic chromonic liquid crystals are distinct from thermotropic nematics from a fundamental standpoint as the structure of the aggregating columns is a function of both the temperature and concentration. We report on the thermal evolution of orientational order parameters, both the second (=scalar) (⟨P200⟩ (=S)) and fourth (⟨P400⟩) order, of sunset yellow FCF aqueous solutions, measured using polarized Raman spectroscopy for different concentrations. The order parameter increases with the concentration, and their values are high in comparison with those of thermotropic liquid crystals. On the basis of Raman spectroscopy, we provide the strongest evidence yet that the hydrozone tautomer of SSY is the predominant form in aqueous solutions in the isotropic, nematic, and columnar phases, as well as what we believe to be the first measurements of (⟨P400⟩) for this system.

Phenomenological description of semi-soft nematic elastomers

Nematic elastomers are new materials that have many remarkable properties. In this article, we study how nonlinear elasticity of semi-soft nematic elastomers can be described phenomenologically. We start with a theory based on strain tensor only, and then continue to develop a phenomenological description with the liquid crystal order tensor included explicitly. Such a description has the virtue of being able to treat the strain tensor and the liquid crystal order tensor equally and thus making the complicated symmetries of nematic elastomers easier to understand.

Photoinduced deformation of crosslinked liquid crystalline polymers and soft actuators

Photoresponsive polymeric materials are a type of functional materials that can absorb photo energy and undergo intra- or inter-molecular physical or chemical reactions. As a result, the materials exhibit macroscopic property changes when exposed to light, such as shape, color or refractive index. They attract more and more attention for the reason that light is an environmental-friendly and remotely controllable stimulus source. Liquid crystals exhibit cooperative motion of molecules in changing the molecular alignment by external stimuli. That means that only a small amount of energy is needed to bring about the alignment change in the whole system. Crosslinked liquid crystalline polymers (CLCPs) can be prepared by crossliking conventional liquid crystal polymers into a network, which possess both the order of liquid crystals and the elasticity of elastomers. The cooperative effect of liquid crystals coupled with polymer network structure gives rise to the characteristic properties of CLCPs. The initial ordering of mesogens can be fixed by the crosslinkers in CLCPs, which might cause a quick change in shape due to a fast order-disorder transition. CLCPs have been a hot topic because large deformation can be induced by changing the alignment of mesogens in CLCPs by external stimuli such as electric fields, changes in temperature, humidity and light. Incorporation of photochromic moieties into CLCPs provides the materials with photo controllable properties. Photoresponsive CLCPs can generate sophisticated movements including contraction/expansion, bending and twisting which convert photo energy into mechanical energy directly. Therefore, the CLCPs can be fabricated into various soft smart actuators by rational design, which brings about broad applications in artificial muscles, microrobots, microvalves, sensors and so on. In this review, the photoresponsive mechanism is firstly elucidated involving photochemical phase transition and photo realignment. Then, we detail the historical development of photoresponsive CLCPs, with emphasis on light- driven mechanical responses concerning two dimensional and three dimensional movements and surface property changes caused by deformation. We also summarize the recent progress of light-driven soft actuators made of photoresponsive CLCPs. We end with an outlook of existing challenges and opportunities.

Electrochemical Preparation of Electro-active Polymers having Nitrogen Atom

New functions of polymer film can be developed by using template. We synthesized polypyrrole (PPy) as a conducting polymer in liquid crystal electrolyte solution. The liquid crystal electrolyte solution for the electrochemical polymerization contains monomer, electrolyte, and matrix liquid crystal. The electrochemical polymerization in liquid crystal was simply performed by using sandwich cell electrochemical polymerization method, which we have originally developed. Application of constant voltage between the two electrodes produces a solid polymer film at anode side through electrochemical oxidation polymerization process. Polarizing optical microscopy observation revealed that the polymer film thus obtained show beautiful optical texture like liquid crystal. The surface texture was derived from molecular structure transcription from the liquid crystal electrolyte solution. The transcription is occurred from molecular level to macroscopic level. Infrared absorption spectroscopy measurements, UV-Vis absorption measurements confirm that the polymer film is exactly pi-conjugated film, although the optical structure is very similar to that of liquid crystal. Furthermore, the polymer film can be tuned its color via electrochemical redox reaction (doping-dedoping). Therefore, the polymer has both liquid crystal order and electro-active property. Lyotropic liquid crystals have good affinity with water, and form crystal like aggregation structure via a self-assembly. Lyotropic liquid crystal shows variety in liquid crystal type, such as columnar phase, smectic phase, nematic phase, and cholesteric phase. In this research, we employed lyotropic liquid crystals for electrolyte solution. Columnar liquid crystals have paid attention as an electrical conduction materials having good carrier mobility. Therefore, the polymer thus prepared can be expected to show structure based unique electrical conduction function through columnar form produced by the transcription from the liquid crystal electrolyte solution in the electrochemical polymerization. Cholesteric liquid crystals show chiral aggregation form. The polymer prepared in cholesteric liquid crystal shows fingerprint texture under the observation of polarizing optical microscope, and optical rotation and circular dichroism. The optical rotation angle can be controlled with electrochemical redox reaction. In this presentation, we report, 1) Preparation of liquid crystal electrolyte solution and characterization 2) Electrochemical polymerization 3) Spectroelectrochemistry. This research can be involved molecularly imprinting (MIP) [3,4]. We discuss the method of electrochemical polymerization, functions, and future prospective of the polymer films prepared in liquid crystals based on electrochemistry.

Stabilizing the uniform lying helix alignment in chiral nematic liquid crystals using direct laser writing

abstractWe investigate a novel approach for controlling the alignment of a fast-switching chiral nematic liquid crystal state using a high resolution two-photon absorption laser scanning lithography technique with aberration correction that permits the engineering of photonic structures in-situ. Walls of polymer network are engineered parallel and perpendicular to the helical axis of a uniform lying helix in chiral nematic liquid crystals in order to stabilize the alignment in the absence of an electric field and enhance the flexo-electro-optic response.

Reliability of Orientational Order Parameters Determined from Two-dimensional X-ray Diffraction Patterns: A Simulation Study.

The orientational order parameter S2 is one of the most important quantities to describe the degree of long-range orientational ordering of liquid crystals. There are several approaches to experimentally measure this order parameter of liquid crystalline phases but every method includes substantial simplifications and assumptions. We present a simulation-based approach to elucidate the reliability of the method of Davidson, Petermann and Levelut to measure S2 via 2D X-ray experiments. We have found that this method slightly underestimates S2 by an absolute value of only 0.05 and thus provides reliable measures of S2 by X-ray diffraction.

Liquid Crystal Biosensors: New Approaches

This review describes recent advances in the area of research involving the interfacial design of liquid crystal (LC) based biosensors. The first advance revolves around the design and modulation of LC based interfaces for developing LC based stimuli responsive materials. For example, designing nanostructured thin films of LC-based colloidal gels exhibit the sensitivity and specificity with the added benefits of mechanical robustness and processability and can be used to report adsorption of biological and synthetic amphiphiles at aqueous-LC interfaces. In addition, a new pathway for the easy formation of spontaneous uniform LC droplets has been reported that provides a high spatial resolution of micrometers with a very high sensitivity. A second development has focused on the investigations of the ordering of LCs at aqueous interfaces for qualitative and quantitative understanding of important biomolecular interactions for bedside diagnostics and laboratory applications. These important biomolecular interactions include: (i) protein endotoxin interactions as these lead to divergent effects on lipopolysaccharide (LPS)-induced responses, (ii) pH induced conformation change of cardiolipin (CL) which is known to affect a range of cellular processes and (iii) endotoxin interactions with bacterial cell wall components.Thirdly this study involves design of LC based sensors that hold promise to act as a marker for cells and cell based interactions. Overall, this review illustrates new approaches for developing LC based stimuli responsive materials that are anticipated with fundamental understanding of biomolecular interactions and provide a gateway for further advances involving LCs.

Orientational order at finite temperature on curved surfaces

We study the effect of thermal fluctuations in the XY model on surfaces with unequal principal curvatures. Unlike Gaussian curvature that typically frustrates orientational order, the extrinsic curvature of the surface can act as a local field that promotes long-range order at low temperature. We find numerically that the transition from the high temperature isotropic phase to the true long-range ordered phase is characterized by critical exponents consistent with those of the flat space Ising model in two dimensions, up to finite size effects. Our results suggest a versatile strategy to achieve geometric control of liquid crystal order by suitable design of the underlying curvature of a substrate.

Dielectric and electro-optical properties of polymer-stabilized liquid crystal system

In this work, we report the results of dielectric and electro-optical properties as a function of temperature for both pure liquid crystal matrix and polymer-stabilized liquid crystal (PSLC). The threshold and saturation voltages have been determined from transmission–voltage curves. We have studied the polymer domains formation in PSLC with variation of concentration of polymer in liquid crystal matrix. It is observed that the dielectric anisotropy of PSLC is significantly influenced by the polar order present in the polymer domains environment. A delicate interplay between the orientational order of liquid crystal and polymeric domains determines the molecular orientations of PSLC with respect to the director of the LC system.

Investigation of orientational order in a nematic liquid crystal with semiconductor CdSe/Zns QDs by IR spectroscopy

The dichroic ratio and orientational-order parameter of fundamental bands have been studied in the IR spectrum for a nematic liquid crystal based on alkylcyanobiphenyls and its suspensions with semiconductor CdSe/ZnS QDs with a core size of 5 nm. Analyses have been performed by the method of multiple frustrated total internal reflection. It is shown that an increase in the nanoparticle concentration from 0.5 to 1.3 wt % improves the orientation uniformity of the liquid crystal near the interface with the orienting surface. The orientational order parameter in a suspension with 1.3 wt % QDs has increased in comparison with the initial liquid crystal by more than 10% for the fundamental bands in alkylcyanobiphenyl molecules, the stretching vibrations of which correspond to the direction of their long axes.

Biofilm assembly becomes crystal clear – filamentous bacteriophage organize the Pseudomonas aeruginosa biofilm matrix into a liquid crystal

Pseudomonas aeruginosa is an opportunistic bacterial pathogen associated with many types of chronic infection. At sites of chronic infection, such as the airways of people with cystic fibrosis (CF), P. aeruginosa forms biofilm-like aggregates. These are clusters of bacterial cells encased in a polymer-rich matrix that shields bacteria from environmental stresses and antibiotic treatment. When P. aeruginosa forms a biofilm, large amounts of filamentous Pf bacteriophage (phage) are produced. Unlike most phage that typically lyse and kill their bacterial hosts, filamentous phage of the genus Inovirus, which includes Pf phage, often do not, and instead are continuously extruded from the bacteria. Here, we discuss the implications of the accumulation of filamentous Pf phage in the biofilm matrix, where they interact with matrix polymers to organize the biofilm into a highly ordered liquid crystal. This structural configuration promotes bacterial adhesion, desiccation survival, and antibiotic tolerance – all features typically associated with biofilms. We propose that Pf phage make structural contributions to P. aeruginosa biofilms and that this constitutes a novel form of symbiosis between bacteria and bacteriophage.

Large Colloids in Cholesteric Liquid Crystals

We describe a coarse-grained Landau-de Gennes model of liquid crystals (LCs) including hydrodynamics based on the Beris-Edwards equations. The model is employed to study the impact of large colloids on the long range LC defect structure in the cholesteric LC blue phases. 'Large' here means that the particle size is comparable to the cholesteric pitch, the length scale on which the LC order undergoes a helical twist. We investigate the case of a single particle, with either normal or degenerate planar anchoring, placed initially in an equilibrium blue phase LC. It is found that in some cases, well defined steady disclination structure emerges at the particle surface, while in other cases no clear steady state is reached inthesimulations,anddisclinationreorganisationappearstoproliferatethroughthebulkLC. These systems are of potential interest in the context of using LCs to template self-assembly of colloid structure, e.g., for opto-electronic devices. Computationally, we demonstrate a parallel approach using mixed message-passing and threaded model on graphical processing units allows effective and efficient progress for this problem.

Synthetic Mimics of Bacterial Lipid A Trigger Optical Transitions in Liquid Crystal Microdroplets at Ultralow Picogram-per-Milliliter Concentrations.

We report synthetic six-tailed mimics of the bacterial glycolipid Lipid A that trigger changes in the internal ordering of water-dispersed liquid crystal (LC) microdroplets at ultralow (picogram-per-milliliter) concentrations. These molecules represent the first class of synthetic amphiphiles to mimic the ability of Lipid A and bacterial endotoxins to trigger optical responses in LC droplets at these ultralow concentrations. This behavior stands in contrast to all previously reported synthetic surfactants and lipids, which require near-complete monolayer coverage at the LC droplet surface to trigger ordering transitions. Surface-pressure measurements and SAXS experiments reveal these six-tailed synthetic amphiphiles to mimic key aspects of the self-assembly of Lipid A at aqueous interfaces and in solution. These and other results suggest that these amphiphiles trigger orientational transitions at ultralow concentrations through a unique mechanism that is similar to that of Lipid A and involves formation of inverted self-associated nanostructures at topological defects in the LC droplets.

Thermo-responsive chiral column by scissoring motion from rigid–flexible aromatic rod assembly

Here, we report thermo-responsive chiral columns from self-assembled coil-rod-coil molecules. The rigid rods self-assemble into multiplying dimers which stack on the top of one another to form non-chiral columnar crystal. Notably, the achiral crystal transforms into 2D hexagonally ordered liquid crystal with supramolecular chirality as heating. The reversible chiral switching behavior within columnar structures arises from scissoring motion from neighbored molecules driven by high thermal entropy of flexible chains relative to stiff-rod.

3-(4-Bromophenyl)-5-(4-hydroxyphenyl)isoxazole: a versatile, new molecular building block for side-chain liquid crystal oligomers and polymers

The synthesis and liquid crystalline behaviour of new comb-shaped methacrylate oligomers, derived from a new key intermediate in the synthesis of heterocyclic liquid crystals, i.e., 3-(4-bromophenyl)-5-(4-hydroxyphenyl)isoxazole, are reported. These new thermotropic liquid crystals exhibit a smectic A mesophase (SmA) above a soft crystal phase (CrE). The new methacrylate oligomers were synthesized by free radical polymerization of the corresponding monomers. Additionally, one main-chain liquid crystal polymer (MCLCPs) was prepared using a Mizoroki–Heck polymerization. 3-(4-Bromophenyl)-5-(4-hydroxyphenyl)isoxazole (2) is a new key intermediate in the synthesis of polymer and non polymer ordered liquid crystal phase.

New insights into photoactivated volume generation boost surface morphing in liquid crystal coatings.

Photoactivated generation of disorder in a liquid crystal network produces free volume that leads to the controlled formation of dynamic corrugations at its surface. The liquid crystal order amplifies the deformation of copolymerized azobenzene, which takes place on molecular length scales, to a micrometre-sized macroscopic phenomenon based on changes in density. We postulate a new mechanism in which continuous oscillating dynamics of the trans-to-cis isomerization of the azobenzene overrules the net conversion, which is currently considered as the origin. This is supported by a significant local density decrease when both the trans and cis isomers are triggered simultaneously, either by dual-wavelength excitation or by the addition of a fluorescent agent converting part of the light to the cis-actuating wavelengths. This new insight provides a general guideline to boost free volume generation leading not only to larger macroscopic deformations but also to controllable and faster non-equilibrium dynamics.