Orientational Order Of Liquid Crystals Research Articles

Theoretical and experimental study of elastocaloric responses in liquid crystalline elastomers

We carry out systematic experimental and theoretical study of elastocaloric (eC) response in nematic liquid crystal elastomers (LCEs), which possess the elastic properties of elastomers combined with the orientational order of liquid crystals. The chemical linking of mesogens to the polymer backbone enables thermomechanical response yielding a large spontaneous change in the LCE geometry on varying the temperature. In LCEs, a relatively large eC effect could be obtained upon adiabatic application or removal of a stress field in the isotropic state near phase transformation to the more ordered state. In these cases a change in the isothermal entropy or adiabatic temperature is compensated with the temperature change of liquid crystal elastomers. This effect could be exploited towards soft condensed state cooling technologies that gain increasing interest due to environmental issues, low-cost and low stimuli field. In the present study we measure directly the elastocaloric temperature change. Experimental measurements are interpreted using a modified Landau-de Gennes mesoscopic model taking into account both internal and external stress fields. Our systematic study identifies conditions yielding responsivity of a few K/MPa, which is two orders of magnitude larger than in the shape memory alloys, the most intensively studied materials for this effect.

A simple and robust aptasensor assembled on surfactant-mediated liquid crystal interface for ultrasensitive detection of mycotoxin

Here, a simple aptasensing approach is represented to sensitively detect ochratoxin A (OTA) as one of the most perilous mycotoxins with carcinogenic, nephrotoxic, teratogenic, and immunosuppressive sequels on human health. The aptasensor is based on the alteration in the orientational order of liquid crystal (LC) molecules at the surfactant-arranged interface. Homeotropic alignment of LCs is achieved by the interaction of the surfactant tail with LCs. By perturbing the alignment of LCs due to the electrostatic interaction of the aptamer strand with the surfactant head, a colorful polarized view of the aptasensor substrate is induced drastically. While OTA causes the re-orientation of LCs to a vertical state by forming an OTA-aptamer complex that induces darkness of the substrate. This study shows that the length of the aptamer strand impacts the efficiency of the aptasensor; longer strand results in the greater disruption of LCs, and therefore, increases the aptasensor sensitivity. Hence, the aptasensor can determine OTA in the linear concentration range of 0.1 fM-1 pM as low as 0.021 fM. The aptasensor is capable to monitor OTA in grape juice, coffee drink, corn, and human serum real samples. The proposed LC-based aptasensor provides a cost-effective, easy-to-carry, operator-independent, and user-friendly array with great potential to develop portable sensing gadgets for food quality control and health care monitoring.

A novel liquid crystal-based aptasensor for ultra-low detection of ochratoxin a using a π-shaped DNA structure: Promising for future on-site detection test strips

Ochratoxin A (OTA) as the most dangerous mycotoxin is produced by Aspergillus Ochraceus and Penicillium verrucosum. OTA can be found in beverages and foodstuffs that induces the teratogenic, nephrotoxic, carcinogenic, and immunosuppressive effects on humans. Hence, developing highly sensitive methods for its detection is of great importance. Herein, a novel aptasensor was designed for the label-free monitoring of the ultra-low OTA levels by a combination of the superiority of aptamers and long-range orientational order of liquid crystals (LCs). The aptasensing strategy was based on the conformational switch of the immobilized π-shaped DNA structure on the glass substrate in presence of the target. A shift in the orientation of LCs from random to homeotropic state led to the apparent alteration of the optical appearance of the aptasensor platform from bright to dark. The LC-based aptasensor especially detects OTA at the ultra-trace level as low as 0.63 aM with comparable selectivity. The aptasensor could detect OTA successfully in the grape juice, coffee, and human serum samples. The LC-based aptasensor paves a way for developing portable and real-time sensing probes with high performance for food safety control and clinical application.

Dynamical Landau-de Gennes theory for electrically-responsive liquid crystal networks.

Liquid crystal networks combine the orientational order of liquid crystals with the elastic properties of polymer networks, leading to a vast application potential in the field of responsive coatings, e.g., for haptic feedback, self-cleaning surfaces, and static and dynamic pattern formation. Recent experimental work has further paved the way toward such applications by realizing the fast and reversible surface modulation of a liquid crystal network coating upon in-plane actuation with an AC electric field [Liu, Tito, and Broer, Nat. Commun. 8, 1526 (2017)10.1038/s41467-017-01448-w]. Here, we construct a Landau-type theory for electrically-responsive liquid crystal networks and perform molecular dynamics simulations to explain the findings of these experiments and inform on rational design strategies. Qualitatively, the theory agrees with our simulations and reproduces the salient experimental features. We also provide a set of testable predictions: the aspect ratio of the nematogens, their initial orientational order when cross-linked into the polymer network, and the cross-linking fraction of the network all increase the plasticization time required for the film to macroscopically deform. We demonstrate that the dynamic response to oscillating electric fields is characterized by two resonances, which can likewise be influenced by varying these parameters, providing an experimental handle to fine-tune device design.

Differentiating Conformationally Distinct Alzheimer's Amyloid-β Oligomers Using Liquid Crystals.

Soluble oligomers of amyloidogenic proteins like an amyloid-β (Aβ) peptide are believed to exhibit toxic effects in neurodegenerative diseases. The structural classification of oligomers indicates two fundamentally distinct oligomers, namely, fibrillar and prefibrillar oligomers that are recognized by OC and A11 conformation-specific antibodies, respectively. Previous studies have indicated that the interaction of Aβ oligomers with the lipid membrane is one of the mechanisms by which these oligomers exert their toxic effects in Alzheimer's disease. Here, we report that the orientational ordering of liquid crystals (LC) can be used to study the membrane-induced aggregation of Aβ oligomers at nanomolar concentrations. Our results demonstrate a faster fibrillation kinetics of OC-positive fibrillar Aβ oligomers with the lipid monolayer in comparison to that of the A11-positive prefibrillar Aβ oligomers. Our findings suggest a general strategy for distinguishing conformationally distinct soluble oligomers that are formed by a number of amyloidogenic proteins on lipid-decorated aqueous-LC interfaces.

The integrals determining orientational order in liquid crystals by x-ray diffraction revisited

ABSTRACTThe orientational distribution function f(β) relative to the average orientation direction or director of liquid crystals and related materials can be obtained from the angular variation of the diffracted x-ray intensity distribution I(θ) of the wide-angle or equatorial arcs. The two quantities f(β) and I(θ) are related by an integral equation. Two different integral equations, one by Kratky and the other by Leadbetter and Norris describing the same phenomena, are the basis of the analyses. There has been discussion of which of the equations is correct; however, the form first derived by Kratky is correct. In this paper, solutions to the Kratky form of the equation are presented. Analytical, closed-form expressions for for n = 0, 1, 2 and 3 are presented. These allow calculation of the first three non-zero orientational order parameters. Experimental data are analysed using solutions to the Kratky kernel and the often used Leadbetter–Norris kernel as a series and in integral form, and the method of Lovell and Mitchell, based on symmetry considerations. The results show that the five approaches obtain indistinguishable values for the second- and fourth-order parameters in the range commonly encountered in liquid crystal studies.

Theory of liquid crystal elastomers and polymer networks : Connection between neoclassical theory and differential geometry.

In liquid crystal elastomers and polymer networks, the orientational order of liquid crystals is coupled with elastic distortions of crosslinked polymers. Previous theoretical research has described these materials through two different approaches: a neoclassical theory based on the liquid crystal director and the deformation gradient tensor, and a geometric elasticity theory based on the difference between the actual metric tensor and a reference metric. Here, we connect those two approaches using a formalism based on differential geometry. Through this connection, we determine how both the director and the geometry respond to a change of temperature.

Liquid crystal elastomer actuators: Synthesis, alignment, and applications

ABSTRACTLiquid crystal elastomers (LCEs) are a unique class of materials which combine rubber elasticity with the orientational order of liquid crystals. This combination can lead to materials with unique properties such as thermal actuation, anisotropic swelling, and soft elasticity. As such, LCEs are a promising class of materials for applications requiring stimulus response. These unique features and the recent developments of the LCE chemistry and processing will be discussed in this review. First, we emphasize several different synthetic pathways in conjunction with the alignment techniques utilized to obtain monodomain LCEs. We then identify the synthesis and alignment techniques used to synthesis LCE‐based composites. Finally, we discuss how these materials are used as actuators and sensors. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2017,55, 395–411

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.

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.

P‐92: Orientational Ordering of Nematic Liquid Crystal Aligned with a Directly Spinnable Carbon Nanotube Web

We studied about the orientational ordering of liquid crystal (LC) aligned with a carbon‐nanotube (CNT) web and its temperature dependence. The CNT web works as an alignment layer as well as an electrode. The LC molecules were aligned along the drawing direction of the CNT web but the order parameter S of the unrubbed CNT web sample was lower than the polyimide (PI) sample. To improve S of the CNT web sample, we directly rubbed the CNT web. S of the rubbed CNT web sample was nearly same with the PI sample and also showed similar temperature dependence. We also measured voltage‐transmittance curve and the residual dc of the CNT web sample. The CNT web sample showed a good electrooptical modulation of the retardation and smaller residual dc than the PI sample.

Xenon NMR of liquid crystals confined to cylindrical nanocavities: a simulation study.

Applications of liquid crystals (LCs), such as smart windows and the ubiquitous display devices, are based on controlling the orientational and translational order in a small volume of LC medium. Hence, understanding the effects of confinement to the liquid crystal phase behaviour is essential. The NMR shielding of (129)Xe atoms dissolved in LCs constitutes a very sensitive probe to the details of LC environment. Linking the experimental results to microscopic phenomena calls for molecular simulations. In this work, the NMR shielding of atomic (129)Xe dissolved in a uniaxial thermotropic LC confined to nanosized cylindrical cavities is computed from coarse-grained (CG) isobaric Monte Carlo (MC) simulations with a quantum-chemically (QC) pre-parameterised pairwise-additive model for the Xe nuclear shielding tensor. We report the results for the (129)Xe nuclear shielding and its connection to the structure and order of the LC appropriate to two different cavity sizes, as well as a comparison to the results of bulk (non-confined) simulations. We find that the confinement changes the LC phase structure dramatically and gives rise to the coexistence of varying degrees of LC order, which is reflected in the Xe shielding. Furthermore, we qualitatively reproduce the behaviour of the mean (129)Xe chemical shift with respect to temperature for atomic Xe dissolved in LC confined to controlled-pore glass materials. In the small-radius cavity the nematic - paranematic phase transition is revealed only by the anisotropic component of the (129)Xe nuclear shielding. In the larger cavity, the nematic - paranematic - isotropic transition is clearly seen in the Xe shielding. The simulated (129)Xe NMR shielding is insensitive to the smectic-A - nematic transition, since in the smectic-A phase, the Xe atoms largely occupy the imperfect layer structure near the cavity walls. The direct contribution of the cavity wall to (129)Xe nuclear shielding is dependent on the cavity size but independent of temperature. Our results show that the combination of CG simulations and a QC pre-parameterised (129)Xe NMR shielding allows efficient studies of the phase behaviour and structure of complex systems containing thousands of molecules, and brings us closer to the simulation of NMR experiments.

Electrically tunable birefringence of a polymer composite with long-range orientational ordering of liquid crystals.

We report an optical film with electrically tunable birefringence in which the liquid crystals (LCs), mixed with the host polymer, form long-range ordering. The film was prepared through polymerization without phase separation between the LCs and polymers. Driving voltage below 30 V for full switching of birefringence is achieved in a 6 μm-thick film. Electro-optical investigations for the film suggest that the long-range ordering of the LCs mixed in the film caused by polymerization lead to rotations of the LCs as well as optical anisotropy in the film. These films with electrically tunable birefringence could have applications as flexible light modulators and phase retardation films for 2D-3D image switching.

Conductivity Studies on the Effect of a Nematic Liquid Crystal on Polyvinyl Alcohol-Based Electrolytes

Recently, a few studies have shown that the introduction of liquid crystals (LCs) in polymer electrolytes would lead to an increase in the chain mobility and the ionic conductivity. It is believed that this enhancement of the polymer electrolyte performance is greatly influenced by the order parameter of the liquid crystal in this system. In this study, a deuterated 4-pentyl-4-cyano-biphenyl (5CB-d2) nematic liquid crystal-doped polyvinyl alcohol (PVA) polymer electrolyte were prepared. The orientational order of the nematic liquid crystal is then investigated via the quadrupolar splittings of the deuterium Nuclear Magnetic Resonance (NMR) spectra. The quadrapolar splitting, which is directly related to the orientational order of the liquid crystal director, was measured and compared between the embedded 5CB-d2 in the PVA electrolyte to that of the pure 5CB-d2. The conductivity of a 5CB-d2 embedded in PVA reached up to 3.28x10-1 S/cm compare to that of without 5CB-d2 which is only 2.89x10-1S/cm. The presence of 5CB-d2 in the PVA polymer electrolyte improved the electrical conductivity of the mixture through an improved charge transfer mechanism, which improves its electrical properties, a criterion useful for a device that needs high conductivity.

Impact of graphene incorporation on the orientational order of graphene/liquid crystal composites

2H NMR spectroscopy was used to characterize changes in the liquid crystal (LC) phase behavior and orientational order resulting from the dispersion of graphene nanoplatelets within the nematic 4-cyano-4′-n-pentyl-biphenyl (5CB) liquid crystal. The graphene/5CB composites revealed a decreased isotropic to nematic phase transition temperature (TIN) and a reduction in LC orientational order, with increasing graphene volume fraction. In contrast, the graphene/benzene/5CB composites revealed both an increased TIN and an increase in the LC orientational order with graphene addition. These changes result from anisotropic surface interactions between the graphene nanoplatelets and LC, and are discussed in terms of the mean-field theory for nanoparticle doped LCs.

Generation of harmonics and supercontinuum in nematic liquid crystals

Nonlinear optical properties of nematic liquid crystals (NLC) have been investigated. A technique for efficient laser frequency conversion in a microscopic NLC volume deposited on an optical fibre end face is experimentally demonstrated. An efficient design of a compact NLC-based IR frequency converter with a fibre input and achromatic collimator is proposed and implemented. Simultaneous generation of the second and third harmonics is obtained for the first time under pumping NLC by a 1.56-mm femtosecond fibre laser. The second-harmonic generation efficiency is measured to be about 1 %, while the efficiency of third-harmonic generation is several tenths of percent. A strong polarisation dependence of the third-harmonic generation efficiency is revealed. When pumping NLC by a cw laser, generation of spectral supercontinua (covering the visible and near-IR spectral ranges) is observed. The nonlinear effects revealed can be due to the light-induced change in the orientational order in liquid crystals, which breaks the initial symmetry and leads to formation of disclination structures. The NLC optical nonlinearity is believed to be of mixed orientationalelectronicnature as a whole.

Memory effect in composites of liquid crystal and silica aerosil

Aerosil silica nanoparticles dispersed in a liquid crystal (LC) possess the interesting property of keeping memory of an electric- or magnetic-field-induced orientation. Two types of memory have been identified: thermally erasable memory arising from the pinning of defect lines versus a "permanent" memory where the orientation persists even after thermal cycling the samples up to the isotropic phase. To address the source of the latter type of memory, solid-state nuclear magnetic resonance spectroscopy and conventional x-ray diffraction (XRD) were first combined to characterize the LC orientational order as a function of multiple in-field temperature cycles. Microbeam XRD was then performed on aligned gels of different concentrations to gain knowledge of the structural properties at the origin of the memory effect. No detectable anisotropy of the gel or significant breaking of silica strands with heating ruled out the formation of an anisotropic silica network as the source of the permanent memory as previously proposed. Instead, support for a role of the surface memory effect, well known for planar substrates, in stabilizing the permanent memory was deduced from "training" of the composites, that is, optimizing the orientational order through the thermal in-field cycling. The ability to train the composites is inversely proportional to the strength of the random-field disorder. The portion of thermally erasable memory also decreases as the silica density increases. We propose that the permanent memory originates from the surface memory effect operating at points of intersection in the silica network. These areas, where the LC is strongly confined with conflicted surface interactions, are trained to achieve an optimized orientation and subsequently act as sites from which the LC orientational order regrows after zero-field thermal cycling up to the isotropic phase.

Infrared Linear-Dichroism (IR-LD) Study of the Nematic Liquid Crystals Doped with Polymer

Recently liquid crystal doped with polymer is used to enhance physical and photo physical properties of these materials. The primary subject in the doped liquid crystal is the qualitative determination of the order parameter. Infrared spectroscopy has become the most informative tool in the study of conformation and orientational order of liquid crystalline materials. In the present paper, we used polarized Fourier transform infrared (FT-IR) spectroscopy to analyze the orientational ordering of the liquid crystal doped with polymer. We obtained information about the head orientational order of liquid crystal in these systems by observing the band corresponding to the vibrations of the relevant functional groups.

Orientational order of liquid crystals by 11B NMR spectroscopy

The use of 11B NMR spectroscopy to obtain information on the orientational order of a borinated calamitic liquid crystal (4DBF2) was here shown for the first time. The combination of spectra recorded on a liquid crystalline sample uniformly aligned in the magnetic field, giving access to quadrupolar splittings, with solid state stationary and MAS spectra recorded on a powdered sample, providing the quadrupolar parameters useful for the interpretation of the quadrupolar splittings, can be seen as a self-consisting method of general applicability to liquid crystals containing boron atoms on the molecular core.

Orientational order in liquid crystals exhibited by some binary mixtures of rod-like and bent-core molecules

We report measurements of the temperature variations of the optical birefringence in the nematic (N) and partial bilayer SmA (SmA(d)) phases in 4-n-octyloxy 4(')-cyanobiphenyl made of rod-like (R) molecules and five mixtures of this compound with 1,3-phenylene bis[4-(3-methylbenzoyloxy)] 4(')-n-dodecylbiphenyl 4(')-carboxylate, made of bent-core (BC) molecules. The birefringence decreases with the concentration x of the BC molecules but the macroscopic order parameter initially decreases upto 11 mol% of BC molecules and subsequently increases with x. This is attributed to the possible formation of polar clusters of BC molecules. Orientation of BC molecules changes between the N and SmA(d) phases and the birefringence data in the two phases imply that the kink angle of the BC molecules is approximately 90 degrees rather than approximately 110 degrees as obtained from calculations which minimize the energy of the molecule. IR spectroscopic measurements on the mixture with 11 mol% of BC molecules have been used to estimate the molecular order parameter S of the R molecules, and to provide additional support for a relatively small kink angle of BC molecules.