Nematic Liquid Crystalline Phase Research Articles

Long-time relaxation dynamics in nematic and smectic liquid crystals of soft repulsive colloidal rods.

Understanding the relaxation dynamics of colloidal suspensions is crucial to identifying the elements that influence the mobility of their constituents, assessing their macroscopic response across the relevant time and length scales, and thus disclosing the fundamentals underpinning their exploitation in formulation engineering. In this work, we specifically assess the impact of long-ranged ordering on the relaxation dynamics of suspensions of soft repulsive rodlike particles, which are able to self-organize into nematic and smectic liquid-crystalline phases. Rods are modeled as soft repulsive spherocylinders with a length-to-diameter ratio L^{★}=5, interacting via the truncated and shifted Kihara potential. By performing dynamic Monte Carlo simulations, we analyze the effect of translational and orientational order on the diffusion of the rods along the relevant directions imposed by the morphology of the background phases. To provide a clear picture of the resulting dynamics, we assess its dependence on temperature, which can dramatically determine the response time of the system relaxation and the self-diffusion coefficients of the rods. The computation of the van Hove correlation functions allows us to identify the existence of rods that diffuse significantly faster than the average and whose concentration can be accurately adjusted by a suitable choice of temperature.

Substrate induced freezing, melting and depinning transitions in two-dimensional liquid crystalline systems.

We use molecular dynamics simulations to investigate the ordering phenomena in two-dimensional (2D) liquid crystals over the one-dimensional periodic substrate (1DPS). We have used Gay-Berne (GB) potential to model the interaction between a pair of liquid crystalline (LC) particles. The underlying substrate potential with which the GB particles interact varies sinusoidally in one direction only. At a given temperature and density of the GB system, we varied the substrate's periodicity (as) but fixed the substrate strength. We observed that with a small value of as, an underlying substrate helps to stabilize a disordered LC nematic phase to a 2D solid phase. However, for an intermediate range of as, the system melts and transitions to a modulate-smectic. Finally, with a further increase in as, the system undergoes a structural depinning transition and returns to an LC nematic phase like a free system with no substrate. We argue that a three-way interplay of the energies arising from orientation-dependent particle-particle and particle-substrate interaction makes it possible for the system to undergo substrate-periodicity-dependent multiple phase transitions in the GB LC system.

Synthesis of cholesterol containing unsymmetrical dimers: a new series of liquid crystals

ABSTRACT We report an efficient and practical procedure for the synthesis of cholesterol containing unsymmetrical liquid crystals dimers. Liquid crystalline and thermal properties were studied by POM, TGA, DSC, and SAXS analysis. All final products (3a-d) presented liquid crystal properties. The presence of smectic (compounds 3a, 3b, and 3d) and chiral nematic (3 c) mesophases evidenced by polarised optical microscopy (POM) and further confirmed by small-angle X-ray scattering (SAXS) analyses. The compound 3 c remains supercooled until room temperature and presented enantiotropic mesomorphism at a low temperature. This characteristic of organisation and mesomorphism at room temperature makes compound 3 c an ideal candidate for technological applications. The SAXS analyses further suggest possibly the presence of smectic clusters within the nematic liquid crystalline phase. All compounds presented high thermal stability at around 300°C when tested for their thermal properties. The highest occupied molecular orbital (HOMO) energies were estimated from cyclic voltammetry (CV) and used together with the optical absorption measurements to determine the band gap and lowest unoccupied molecular orbital (LUMO) energies. The values obtained were: HOMO (–5.81 to – 6.03 eV), LUMO (–1.53 to – 2.12 eV), and band gap (3.88 to 4.28 eV).

Shear-Coated Linear Birefringent and Chiral Cellulose Nanocrystal Films Prepared from Non-Sonicated Suspensions with Different Storage Time.

Nanocelluloses are very attractive materials for creating structured films with unique optical properties using different preparation techniques. Evaporation-induced self-assembly of cellulose nanocrystals (CNC) aqueous suspensions produces iridescent films with selective circular Bragg reflection. Blade coating of sonicated CNC suspensions leads to birefringent CNC films. In this work, fabrication of both birefringent and chiral films from non-sonicated CNC suspensions using a shear-coating method is studied. Polarization optical microscopy and steady-state viscosity profiles show that non-sonicated CNC suspensions (concentration of 6.5 wt%) evolve with storage time from a gel-like shear-thinning fluid to a mixture of isotropic and chiral nematic liquid crystalline phases. Shear-coated films prepared from non-sonicated fresh CNC suspensions are birefringent, whereas films prepared from suspensions stored several weeks show reflection of left-handed polarized light. Quantification of linear and circular birefringence as well circular dichroism in the films is achieved by using a Mueller matrix formalism.

Eco-friendly Biowaste-based natural surfactant for lyotropic assemblies and Bio-adsorbent for dye removal

Aggregation behavior of biowaste Litchi Chinensis (LC) seed in three different solvents of varying polarity [water (80), formamide (108), and glycerol (37)] has been studied. Nematic liquid crystalline phase has been obtained for studied systems as confirmed by a polarizing optical microscope (POM). Potential of the LC seeds has been checked to remove the Rhodamine B (RhB) dye from the wastewater. Effect of dye varying concentration (1 mg/L, 3 mg/L, 5 mg/L, 7 mg/L and 10 mg/L) varying pH (3,6 and 9), and contact time (0 to120 sec) on adsorption process have been evaluated. About 52% of dye removal efficiency is achieved with bare 50 mg adsorbent at pH≈ 6. Pseudo-first-order kinetics is found best fitted for the adsorption process (R2~0.98). Freundlich isotherm is found best fitted to the adsorption data which is also vindicated by a greater value of R2~0.95. Hydrogen bonding, electrostatic interactions, and pi-pi interaction between RhB and LC seeds (saponin) could be responsible for the adsorption process. These results indicate that LC seed can act as a perfect surfactant for lyotropic assemblies and also an effectual natural adsorbent for the dye adsorption and cleaning of aquatic systems.

Liquid Crystalline Properties of Symmetric and Asymmetric End-Grafted Cellulose Nanocrystals.

The hydrophilic polymer poly[2-(2-(2-methoxy ethoxy)ethoxy)ethylacrylate] (POEG3A) was grafted onto the reducing end-groups (REGs) of cellulose nanocrystal (CNC) allomorphs, and their liquid crystalline properties were investigated. The REGs on CNCs extracted from cellulose I (CNC-I) are exclusively located at one end of the crystallite, whereas CNCs extracted from cellulose II (CNC-II) feature REGs at both ends of the crystallite, so that grafting from the REGs affords asymmetrically and symmetrically decorated CNCs, respectively. To confirm the REG modification, several complementary analytical techniques were applied. The grafting of POEG3A onto the CNC REGs was evidenced by Fourier transform infrared spectroscopy, atomic force microscopy, and the coil-globule conformational transition of this polymer above 60 °C, i.e., its lower critical solution temperature. Furthermore, we investigated the self-assembly of end-tethered CNC-hybrids into chiral nematic liquid crystalline phases. Above a critical concentration, both end-grafted CNC allomorphs form chiral nematic tactoids. The introduction of POEG3A to CNC-I does not disturb the surface of the CNCs along the rods, allowing the modified CNCs to approach each other and form helicoidal textures. End-grafted CNC-II formed chiral nematic tactoids with a pitch observable by polarized optical microscopy. This is likely due to their increase in hydrodynamic radius or the introduced steric stabilization of the end-grafted polymer.

Large-Size Honeycomb-Shaped and Iris-Like Liquid Crystal Elastomer Actuators

Large-Size Honeycomb-Shaped and Iris-Like Liquid Crystal Elastomer Actuators

Enhancing the Catalytic Activity of Palladium Nanoparticles via Sandwich-Like Confinement by Thin Titanate Nanosheets.

As atomically thin oxide layers deposited on flat (noble) metal surfaces have been proven to have a significant influence on the electronic structure and thus the catalytic activity of the metal, we sought to mimic this architecture at the bulk scale. This could be achieved by intercalating small positively charged Pd nanoparticles of size 3.8 nm into a nematic liquid crystalline phase of lepidocrocite-type layered titanate. Upon intercalation the galleries collapsed and Pd nanoparticles were captured in a sandwichlike mesoporous architecture showing good accessibility to Pd nanoparticles. On the basis of X-ray photoelectron spectroscopy (XPS) and CO diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) Pd was found to be in a partially oxidized state, while a reduced Ti species indicated an electronic interaction between nanoparticles and nanosheets. The close contact of titanate sandwiching Pd nanoparticles, moreover, allows for the donation of a lattice oxygen to the noble metal (inverse spillover). Due to the metal–support interactions of this peculiar support, the catalyst exhibited the oxidation of CO with a turnover frequency as high as 0.17 s–1 at a temperature of 100 °C.

Synthesis and characterization of liquid crystalline epoxy resins to study the effect of mesogenic length on the physical properties

Two liquid crystalline epoxy (LCE) resins containing different mesogenic lengths were synthesized and cured with two different aromatic diamine curing agents, diaminodiphenylsulfone (DDS) and diaminodiphenylmethane (DDM) to prepare high performance LCE networks. The spectroscopic structures of LCE resins were identified by Fourier transform infrared (FT-IR) spectroscopy, Proton nuclear magnetic resonance (1H-NMR) spectroscopy, mass (MS) spectroscopy, and the formation of nematic and smectic liquid crystalline phase were observed by differential scanning calorimetry (DSC), and polarized optical microscopy (POM). The curing reactions and liquid crystalline textures of the LCE resins were also studied by DSC and POM on the process of curing. The thermal and mechanical properties were investigated for the cured LCE resins with different mesogenic lengths. These results show that the LCE cross-linked networks with long mesogenic group exhibited much higher storage modulus, glass transition temperature, thermal conductivity, thermal stability, and dimensional stability both in the regions of glassy and rubbery states. In addition, both LCE resins have superior thermal and mechanical properties compared to those of the commonly used epoxy resin.

Effect of a temperature threshold on the electrorheological performance of ionic liquid crystal polyanilines

A series of ionic liquid crystal polyanilines (ILCPs) electrorheological (ER) compounds were synthesized by use of polyaniline, naphthalene disulfonic acid and pyridinyl-containing mesogen. The chemical structure, thermal property, morphology, liquid-crystalline behavior, dielectric property and ER effect of the ILCPs-based ER fluids were characterized by use of various experimental techniques. The synthesized ILCPs displayed nematic liquid-crystalline phase, and the different mesogens played a determinant role in the liquid crystal behavior. The experimental results about dielectric property of the ILCPs suggested that both dielectric constant and dielectric loss increase with raising the temperature, showing a sudden change around glass-transition temperature. The ILCPs-based ER fluids contained an integrated character of liquid crystal, poly(ionic liquid) and polyaniline-base ER suspensions. These characteristics had strong dependence on operating temperature, and a temperature threshold was observed on the ER performance. The shear stress of the ER fluids increased abnormally near glass-transition temperature, which was due to the synergetic effect between the mesogenic orientation and the stretched polyaniline droplets induced by the electric field.

Chemical-Physical Characterization of a Binary Mixture of a Twist Bend Nematic Liquid Crystal with a Smectogen

Nematic twist-bend phases (NTB) are new types of nematic liquid crystalline phases with attractive properties for future electro-optic applications. However, most of these states are monotropic or are stable only in a narrow high temperature range. They are often destabilized under moderate cooling, and only a few single compounds have shown to give room temperature NTB phases. Mixtures of twist-bend nematic liquid crystals with simple nematogens have shown to strongly lower the nematic to NTB phase transition temperature. Here, we examined the behaviour of new types of mixtures with the dimeric liquid crystal [4′,4′-(heptane-1,7-diyl)bis(([1′,1″-biphenyl]4″-carbo-nitrile))] (CB7CB). This now well-known twist-bend nematic liquid crystal presents a nematic twist-bend phase below T ≈ 104 °C. Mixtures with other monomeric alkyl or alkoxy -biphenylcarbonitriles liquid crystals that display a smectic A (SmA) phase also strongly reduce this temperature. The most interesting smectogen is 4′-Octyl-4-biphenylcarbonitrile (8CB), for which a long-term metastable NTB phase is found at room and lower temperatures. This paper presents the complete phase diagram of the corresponding binary system and a detailed investigation of its thermal, optical, dielectric, and elastic properties.

Nanometer Confinement Induces Nematic Order in 1-Dodecanol.

The phase state and molecular dynamics of 1-dodecanol are studied in the bulk and under nanometer confinement within self-ordered nanoporous alumina templates. A rotator phase in the bulk is absent under confinement. A nematic liquid crystalline phase is formed instead in pores with diameters from 400 down to 25 nm. Results are based on the changes in temperature-dependence of dielectric permittivity and X-ray diffraction. The phase diagram under confinement is explored, and the limits of the nematic-to-isotropic and crystalline-to-nematic phase transitions are identified. The phase diagram allows for a direct transition from the liquid to the low-temperature crystalline phase in pores with a diameter below 20 nm. Furthermore, we report on the dielectric fingerprint of the rotator phase and the molecular dynamics in bulk 1-dodecanol.

Destabilization of the Nematic Phase of Clay Nanosheet Suspensions by Polymer Adsorption.

Complex aqueous mixtures comprised of swelling clays and hydrosoluble polymers naturally occur in soils and play a major role in pedogenesis. They are also very often used for formulating oil-well drilling fluids, paints, and personal-care products. The suspensions of some natural clays, thanks to their large nanoparticle aspect ratio, spontaneously form nematic liquid-crystalline phases where the particles align parallel to each other, which affects their flow properties. We observed that adding small amounts of hydrosoluble polymers to these clay suspensions destabilizes the nematic phase with respect to the isotropic (disordered) phase. The polymers that we used (poly(ethylene oxide) and dextran) were too small to adopt particle-bridging conformations and small-angle X-ray scattering experiments showed that the structure of the nematic phase is not altered by polymer doping. However, the adsorption isotherm shows that the macromolecules adsorb onto the clay nanosheets, effectively coating them with a polymer layer. Our extension of Onsager's theory for polymer-coated platelets properly captures the experimental phase diagram and shows how the nematic phase destabilization can be due to the polymer adsorbing more on the platelet faces than at the rim. Because the flow properties of the nematic phase are very different from those of the isotropic phase, the presence or absence of the former phase is an important factor to be determined and considered to explain the rheological behavior of these complex systems.

Structure and grating efficiency of thin cells filled by a twist-bend nematic liquid crystal.

A twist-bend nematic (N_{TB}) liquid crystalline phase spontaneously forms modulated structures on a microscale level when confined in thin planar cells. Preliminary studies showed that these cells can be used as polarization gratings. Here we present a theoretical description of the formation of a two-dimensionally modulated structure. By considering the N_{TB} phase as a pseudolayer medium, a threshold condition for the onset of a modulated structure is calculated for weak and strong boundary conditions in the case of initially bookshelf or pretilt alignment of pseudolayers. Based on the modeled structure we determine spatial variation of the optic axis and calculate properties of the transmitted diffracted light. Results of the beam propagation method (BPM) and transfer matrix method are compared and it is shown that a more complex BPM gives better agreement with experimental results, meaning that even in thin cells the diffraction of light inside the grating should not be neglected.

Reversible On–Off of Chirality and Anisotropy in Patterned Coexistence of Achiral‐Anisotropic and Chiral‐Isotropic Soft Materials

AbstractAchieving a coexistence of two or multiple phases of soft matters via a delicate trade‐off of free energy and long‐range order with one another has been inspiring abundant interest on fundamental sciences and engineering. In this work, a stable coexistence of the optically achiral‐anisotropic liquid crystalline nematic phase and the optically chiral‐isotropic liquid crystalline blue phase is proposed and demonstrated, with their distribution tailored in a micro‐pattern by the developed localized micro‐regional polymer templating technique. Such a stable patterned coexistence of the two different phases with a distinct molecular arrangement, optical chirality, and anisotropy is achieved depending on a delicate matching of the elastic energy on the interface between liquid crystals and polymer networks. In contrast to the majority of soft coexistence systems, a specific dynamic and reversible on–off response of chirality and anisotropy is observed in such a system driven by an electric field. The structure of patterned coexistence system in the presence and absence of external stimulation is explored by microscopy and optical technique, and furthermore, the prospective photonic applications are demonstrated. This investigation indicates another category of functional soft material with fantastic adaptive characteristics for application in optics, electronics, interface physics, and chemistry.

Linearly Polarized Emission from Shear-Induced Nematic Phase Upconversion Nanorods.

Lanthanide-doped particles exhibit unique polarization-dependent luminescence due to the anisotropic crystalline local symmetry surrounding the emitter. Precise control of the orientation of particles shows great significance for exploiting the luminescent polarization and their potential applications. Here, we demonstrated a facile polypropylene-aided shear-driven method to obtain large-scale orientationally ordered upconversion nanorods, showing a liquid-crystalline nematic phase. Upconversion nanorods with low aspect ratios were well-aligned with the crystalline c-axis along the shearing direction using monodispersed colloid nanorods as the nanoink. The order parameter of aligned upconversion nanorods can reach up to 0.95. The nematic upconversion nanorods demonstrated strong polarization-dependent luminescence with the high degrees of polarization of the 4F9/2 sublevels at 657 and 661 nm being 0.47 and 0.59, respectively. Taking advantage of these mesoscopic well-aligned upconversion nanorods, their peculiar polarized emissions are potentially useful for some interdisciplinary applications such as polarization-sensitive bioprobes and anticounterfeiting.

Retrieving the Coassembly Pathway of Composite Cellulose Nanocrystal Photonic Films from their Angular Optical Response.

Aqueous suspensions of cellulose nanocrystals (CNCs) are known to self-assemble into a chiral nematic liquid crystalline phase, leading to solid-state nanostructured colored films upon solvent evaporation, even in the presence of templating agents. The angular optical response of these structures, and therefore their visual appearance, are completely determined by the spatial arrangement of the CNCs when the drying suspension undergoes a transition from a flowing and liquid crystalline state to a kinetically arrested state. Here, it is demonstrated how the angular response of the final film allows for retrieval of key physical properties and the chemical composition of the suspension at the onset of the kinetic arrest, thus capturing a snapshot of the past. To illustrate this methodology, a dynamically evolving sol-gel coassembly process is investigated by adding various amounts of organosilica precursor, namely, 1,2-bis(trimethoxysilyl)ethane. The influence of organosilica condensation on the kinetic arrest can be tracked and thus explains the angular response of the resulting films. The a posteriori and in situ approach is general; it can be applied to a variety of additives in CNC-based films and it allows access to key rheological information of the suspension without using any dedicated rheological technique.

Vertical Lamellae Formed by Two-Step Annealing of a Rod-Coil Liquid Crystalline Block Copolymer Thin Film.

Silicon-containing block copolymer thin films with high interaction parameter and etch contrast are ideal candidates to generate robust nanotemplates for advanced nanofabrication, but they typically form in-plane oriented microdomains as a result of the dissimilar surface energies of the blocks. Here, we describe a two-step annealing method to produce vertically aligned lamellar structures in thin film of a silicon-containing rod–coil thermotropic liquid crystalline block copolymer. The rod–coil block copolymer with the volume fraction of the Si-containing block of 0.22 presents an asymmetrical lamellar structure in which the rod block forms a hexatic columnar nematic liquid crystalline phase. A solvent vapor annealing step first produces well-ordered in-plane cylinders of the Si-containing block, then a subsequent thermal annealing promotes the phase transition from in-plane cylinders to vertical lamellae. The pathways of the order–order transition were examined by microscopy and in situ using grazing incidence small-angle X-ray scattering and wide-angle X-ray scattering.

Self‐Assembly of Chiral Nematic Liquid Crystalline Phases of AgNR@SiO2@Cysteine@CsPbBr3 Hybrid Nanorods with Plasmon‐Dependent Photoluminescence

AbstractThe direct synthesis of a chiral nematic liquid crystalline phase of AgNR@SiO2@cysteine@CsPbBr3 hybrid nanorods (HNRs) is reported. The circular dichroism spectra can be divided into three components: (1) the interband absorption–enhanced optical activity of structural arrangement of cysteine (cys) molecules, 200–320 nm, (2) the chiral nematic liquid crystalline arrangement of the Ag nanorods (AgNRs), 350–450 nm, and (3) the exciton adsorption edge of the perovskite, 500–550 nm. The polarizing optical microscope images indicate that the chiroptical response of perovskite arises from chiral nematic crystalline arrangement rather than cys‐induced electronic coupling between a chiral ligand and otherwise achiral perovskite quantum dots (QDs). The luminescent intensity of CsPbBr3 QDs in AgNR@SiO2@cys@CsPbBr3 HNRs is boosted 87‐fold due to the local surface plasmon resonance field enhancement effect. Furthermore, the high‐performance green light emitting diode is constructed employing AgNR@SiO2@cys@CsPbBr3 complexes, which exhibit excellent luminescent properties. This work contributes insights into structure–property relationships and this strategy promisingly provides guidance for the other inorganic chiral semiconductor suprastructures.

Additive-Free MXene Liquid Crystals and Fibers.

The discovery of liquid crystalline (LC) phases in dispersions of two-dimensional (2D) materials has enabled the development of macroscopically aligned three-dimensional (3D) macrostructures. Here, we report the first experimental observation of self-assembled LC phases in aqueous Ti3C2Tx MXene inks without using LC additives, binders, or stabilizing agents. We show that the transition concentration from the isotropic to nematic phase is influenced by the aspect ratio of MXene flakes. The formation of the nematic LC phase makes it possible to produce fibers from MXenes using a wet-spinning method. By changing the Ti3C2Tx flake size in the ink formulation, coagulation bath, and spinning parameters, we control the morphology of the MXene fibers. The wet-spun Ti3C2Tx fibers show a high electrical conductivity of ∼7750 S cm–1, surpassing existing nanomaterial-based fibers. A high volumetric capacitance of ∼1265 F cm–3 makes Ti3C2Tx fibers promising for fiber-shaped supercapacitor devices. We also show that Ti3C2Tx fibers can be used as heaters. Notably, the nematic LC phase can be achieved in other MXenes (Mo2Ti2C3Tx and Ti2CTx) and in various organic solvents, suggesting the widespread LC behavior of MXene inks.