Morphology Of Liquid Crystals Research Articles

Liquid crystal-based sensing platform for detection of Pb2+ assisted by DNAzyme and rolling circle amplification

Lead ions (Pb2+) are one of the most widespread heavy metal contaminants that pose detrimental impact on environment and human health. We demonstrate a highly sensitive and specific liquid crystal (LC)-based sensing platform for detecting Pb2+ assisted by DNAzyme and rolling circle amplification (RCA). Magnetic beads (MBs) are functionalized with DNA duplexes of the catalytic strands (DNAzymes) and the substrate strands. In the presence of Pb2+, the substrate strands are disassembled due to activation of the DNAzyme, which allows initiation of DNA RCA on MBs. The amplified DNA strands can disrupt arrangement of octadecy trimethyl ammonium bromide monolayers (OTAB), thereby inducing planar orientation of LC molecules at the interface of aqueous and LCs. Thus, LCs exhibit bright appearance. In contrast, RCA cannot be triggered in the absence of Pb2+. Therefore, LC molecules adopt perpendicular orientation at the interface, which induces the dark morphology of LCs. The limit of detection reaches as low as 16.7 pM. It is an improvement of more than two orders of magnitude compared to that of previously reported LC-based sensing approaches. This approach also shows excellent performance in monitoring Pb2+ in tap water and lake water.

Advection of nematic liquid crystals by chaotic flow

Consideration is given to the effects of inhomogeneous shear flow (both regular and chaotic) on nematic liquid crystals in a planar geometry. The Landau–de Gennes equation coupled to an externally prescribed flow field is the basis for the study: this is solved numerically in a periodic spatial domain. The focus is on a limiting case where the advection is passive, such that variations in the liquid-crystal properties do not feed back into the equation for the fluid velocity. The main tool for analyzing the results (both with and without flow) is the identification of the fixed points of the dynamical equations without flow, which are relevant (to varying degrees) when flow is introduced. The fixed points are classified as stable/unstable and further as either uniaxial or biaxial. Various models of passive shear flow are investigated. When tumbling is present, the flow is shown to have a strong effect on the liquid-crystal morphology; however, the main focus herein is on the case without tumbling. Accordingly, the main result of the work is that only the biaxial fixed point survives as a solution of the Q-tensor dynamics under the imposition of a general flow field. This is because the Q-tensor experiences not only transport due to advection but also co-rotation relative to the local vorticity field. A second result is that all families of fixed points survive for certain specific velocity fields, which we classify. We single out for close study those velocity fields for which the influence of co-rotation effectively vanishes along the Lagrangian trajectories of the imposed velocity field. In this scenario, the system exhibits coarsening arrest, whereby the liquid-crystal domains are “frozen in” to the flow structures, and the growth in their size is thus limited.

Composite liquid crystal-polymer electrolytes in dye-sensitised solar cells: effects of mesophase alkyl chain length

ABSTRACTThe doping of polymer electrolytes (PEs) with liquid crystal (LC) materials has been shown to improve the performance of dye-sensitised solar cells (DSSCs). This is achieved by promoting ionic conduction and increasing optical path length through multiple-light scattering within the photovoltaic devices. In LCs, it is well known that the length of the alkyl chain plays an important role since the LC morphology and mesophase stabilisation depend strongly on the alkyl group. In this work, liquid crystal-polymer composite electrolytes (LC-PEs) are prepared using nematic LCs with different alkyl chain lengths. The morphology of the LC-PEs is investigated and correlated with their electrical properties. Subsequently, DSSCs are prepared using the LC-PEs as a direct example of its application. It is shown that increasing the alkyl chain length of the LCs reduces the efficiency of the solar devices. The longer alkyl chains are speculated to intertwine, thus trapping the mobile ions and reducing the bulk ionic conductivity. For the same reason, longer alkyl chain LCs are thought to be unable to passivate the TiO2 surface through the adsorption of cyanobiphenyl groups and hence the higher probability of back recombination reaction between the electrons in TiO2 and PE.

Extended Chain Lamella Formation Characteristics of Main-Chain Smectic Liquid Crystalline Copolyesters Comprising Different Length Units

A series of liquid crystal (LC) PB-8/12 copolyesters have been synthesized from 4,4′-biphenol with sebacic acid and tetradecanedioic acid, and their LC structures and morphologies have been examined. The copolyesters formed smectic I (SmI) LCs similarly as to the corresponding PB-8 and PB-12 homopolyesters; however, dissimilarity of the comonomer lengths decreased the smectic layer order while sustaining the hexagonal order in the lateral packing of the chains. The SmI LCs consisted of 100-nm-thick lamellae stacked along the polymer chain direction. The lamella thicknesses are more than two times greater than the thicknesses of the homopolyesters and comparable to or greater than the chain contour lengths, indicating the formation of extended chain lamellae.

Directed self-assembly of nematic liquid crystals on chemically patterned surfaces: morphological states and transitions.

The morphology and through-film optical properties of nematic liquid crystals (LCs) confined between two surfaces may be engineered to create switches that respond to external electric fields, thereby enabling applications in optoelectronics that require fast responses and low power. Interfacial properties between the confining surfaces and the LC play a central role in device design and performance. Here we investigate the morphology of LCs confined in hybrid cells with a top surface that exhibits uniform homeotropic anchoring and a bottom surface that is chemically patterned with sub-micron and micron- wide planar anchoring stripes in a background of homeotropic anchoring. In a departure from past work, we first investigate isolated stripes, as opposed to dense periodic arrays of stripes, thereby allowing for an in-depth interpretation of the effects of patterning on LC morphology. We observe three LC morphologies and sharp transitions between them as a function of stripe width in the submicron and micron regimes. Numerical simulations and theory help explain the roles of anchoring energy, elastic deformation, entropy, pattern geometry, and coherence length of the LC in the experimentally observed behavior. The knowledge and models developed from an analysis of results generated on isolated features are then used to design dense patterned substrates for high-contrast and efficient orientational switching of LCs in response to applied fields.

Smectic assemblies in C₃-symmetric hexa-alkylated liquid crystals: transformation from smectogen to discogen via hydrogen bonding.

In this paper, we report a C3-symmetric liquid crystal (LC) with sixfold alkyl peripheries exhibiting an unusual smectic E-like organization in the LC state. Based on conformational considerations, the smectic assembly is attributed to the formation of an endo-type Y conformer of asymmetric triazolyl and benzylic groups that cannot be accessed in other C3-symmetric molecules exclusively showing columnar assemblies. The Y conformers form a two-dimensional oblique lattice in the aromatic layers of the ordered smectic phase. In addition, the Y-shaped molecule in the smectic phases can change into a circular shape by the 1 : 1 hydrogen-bonding interaction with a gallic acid derivative, which leads to a hexagonal columnar LC phase. The triazole-based LC design concept proves the smectic LC assembly in the C3-symmetric system, and provides the supramolecular manipulation of LC morphologies.

Liquid crystal micro-lenticular array assembled by a fringing field

Based on dielectrophoretic effect, the phase separation morphology of liquid crystal (LC) in a liquid monomer can be manipulated by a fringing field. Applying the fringing field generated from interdigitated electrode upon the LC/monomer mixture, the randomly dispersed LC droplets can be assembled to grating-like stripes. When the field is removed, the LC stripes break into tiny droplets again. This process is reversible and stable. Our results show that the surface profile of each LC stripe exhibits a lenticular shape. The response time of the LC morphology converting from droplets to stripes and from stripes to droplets is ~0.96 s. Potential applications of this controllable morphology can be found in optical communications, beam steering, imaging, and displays.

Electron-beam curing of a novel liquid crystal thermoset resin

In this paper we report on the processing and properties of a novel, “dual-curing” liquid crystal (LC) resin cured by electron-beam irradiation. The LC monomer contains both acrylate and acetylene reactive groups. Neat resin samples were e-beam cured in either the LC phase at 65 °C or the isotropic phase at 90 °C. The experimental variables included the total e-beam radiation dose (150 or 250 kiloGray {kGy or J/g}) and thermal post-cure cycle (none or 190 °C/1 h). Cured polymer specimens were characterized by dynamic mechanical analysis (DMA) and thermo-mechanical analysis (TMA). The results demonstrated that e-beam exposure alone at 150 or 250 kGy was not sufficient to fully cure the polymer, although it was sufficient to lock-in the isotropic or LC morphology. Differences in morphology had a significant impact on the DMA and TMA results for non-post-cured specimens, where LC samples demonstrated more robust mechanical properties at elevated temperatures compared to isotropic specimens. After a thermal post-cure cycle the DMA storage modulus differences between the LC and isotropic samples were not as great. However, large differences in thermal expansion coefficients remained. The data suggest the possibility that the acrylate bonds react during e-beam exposure, followed by conversion of the acetylene bonds during the thermal post-cure. Also, e-beam curing in the LC phase likely will allow for tighter molecular packing and more efficient polymerization resulting in higher crosslink densities.

Photopyroelectric structural and thermal characterization of first-order phase transition in liquid crystals

A photopyroelectric setup, which allows the study of the thermal behavior and the structural evolution of 8CB liquid crystal at the first-order nematic–isotropic phase transition, is presented. It is shown that the kinetics of the transition in liquid-crystal films are not the same on heating and cooling, and a quantitative determination of the width of the two-phase coexistence region is reported. A correlation between the photopyroelectric signal and the liquid-crystal morphology in the coexistence region has been found.