Nematic Phase Research Articles

Magnetic ordering in strong anisotropic ferromagnetic bilayers with dipolar and antiferromagnetic interlayer exchange interactions: Monte Carlo approach.

In this work, multilayer films consisting of two strong-anisotropic ferromagnetic layers antiferromagnetically coupled by a nonmagnetic spacer are studied by Monte Carlo simulations. The system is modeled by an Ising-based Hamiltonian that depends on both the intralayer exchange and dipolar constants and on the interlayer exchange constant (IEC). The ground state of the monolayers (null IEC) corresponds to alternate stripe domains with width h defined by the ratio between the exchange and dipolar constants (δ). The results show that IEC alters the energy balance that controls the stripe domain formation, leading to a ground state characterized by in-plane stripes out-plane antiferromagnetically coupled. When temperature increases two regimes are identified: an IEC-dominated regime where the orientational and positional orders are simultaneously lost in both layers, driving the system to the tetragonal liquid (TL) phase, and a dipolar-dominated one where signs of layers decoupling and the onset of positional disorder are observed. The last could be related with an intermediate nematic phase (NM). From the study of the nonequilibrium dynamics, the phase transitions to TL phase are characterized as continuous and those to the NM one as Kosterlitz-Thouless type. Also, for both layers the critical temperatures are the same and increase with IEC magnitude. Furthermore, the obtained critical exponents depend on the IEC values, which is indicative of a weak universality. For the dipolar-dominated regime, the decoupling between layers is also evidenced by the difference between their critical exponents.

Thermomechanical response of liquid crystal elastomers: role of crosslinker density

In this work thermomechanical properties of main-chain liquid crystal elastomers (MC-LCEs) with different degrees of crosslinking were investigated, and gradual loss of thermomechanical response was observed on repetitive measurements. Specifically, six samples of MC-LCEs were prepared, with crosslinker-to-mesogen relative concentration ranging from 5% to 10% in steps of 1%. The obtained results were then compared to thermomechanical response of side-chain liquid crystal elastomers (SC-LCEs). Additionally, thermomechanical response of polymer dispersed main-chain liquid crystal elastomers (MC-PDLCEs) was investigated. Results indicate that in MC-LCEs the concentration of crosslinker defines thermomechanical response and affects stability of the system. The loss of thermomechanical response is negligible in the case of crosslinker to mesogen ratio being the smallest, namely in 5% sample, and it is unaffected by glasslike to nematic phase transition. SC-LCEs do not show any sign of such behaviour and remain stable after several cycles of thermomechanical measurements.

Michael Addition Inducing Self-Assembly to Construct Mechanochromic BP Film.

Liquid crystalline blue phase (BP) with 3D cubic nanostructure has attracted much interest in the fields of photonic crystals due to their unique optical properties and the ability to control the flow of light. However, there remains a challenge for simultaneously achieving self-assembly and mechanochromic response of soft 3D cubic nanostructures. Herein, a scalable strategy for the preparation of soft 3D cubic nanostructured films using oligomerization of the Michael addition reaction, which can induce the assembly of double-twisted cylinders for collective replication, remodeling, recombination, and growth, with a phase transition from BPII to BPI, and to chiral nematic phase, is presented. The prepared BP patterns can be obtained by Michael addition oligomerization reaction and composite mask photopolymerization, which present distinct mechanochromic sensitive due to patterns derived from different BP state, and the pattern can be reversibly erased and recurred by mechanical force and temperature. The average domain size of BPII prepared using this strategy can achieve 96µm, which is 2.5 times larger than that obtained using the conventional cooling approach. This work provides new insights into the self-assembly and selective chemochromism of functional materials and devices.

Circularly Polarized Luminescence Switching of Chiral Perylene Diimide‐Doped Nematic Liquid Crystal Using DC Electric Field

AbstractTo obtain chiral luminescent nematic liquid crystals (N*‐LCs), two sets of chiral perylene luminescent materials (R,R)/(S,S)‐N,N′‐bis(1‐cyclohexylethyl)perylene‐3,4,9,10‐tetracarboxylic diimide [(R,R)/(S,S)‐CPDI] and (R,R)/(S,S)‐N,N′‐bis(1‐phenylethyl)perylene‐3,4,9,10‐tetracarboxylic diimide [(R,R)/(S,S)‐BPP] were prepared and doped into a nematic liquid crystal (N‐LC), 5CB. The obtained liquid crystals N*‐LC‐CPDI and N*‐LC‐BPP emitted stronger circularly polarized luminescence (CPL) signals than the CPDI‐ and BPP‐containing chiral poly(methyl methacrylate) (PMMA) luminescent films owing to higher helical twist power. When a direct current (DC) electric field was applied to N*‐LC, a reversible CPL response was obtained due to the field‐induced phase transition from the chiral nematic phase to the nematic phase. This demonstrated the successful construction of an ′′on‐off‐on′′ CPL system based on a DC electric field and was attributed to the change in the liquid crystalline organization from a uniformly lying helical structure to a planar one. This work can provide an effective strategy for the development of functional CPL devices in which the CPL can be regulated by applying a DC electric field to the N*‐LC.

Selenium-linked cyanobiphenyl-based liquid crystal dimers: the effects of chalcogen linkage and spacer length on the twist-bend nematic phase

ABSTRACT We synthesised selenium-linked liquid crystal dimers and examined their phase behaviour to determine the effects of chalcogen linkage and spacer length on the formation of a twist-bend nematic (NTB) phase. Two new homologous series of nonsymmetric selenoether- and ether- or thioether-linked cyanobiphenyl-based dimers (CBSenOCB and CBSenSCB, respectively) comprising an alkyl spacer chain with a carbon number (n) of 3, 5, 6, 7, 9, and 11 were developed. We also synthesised n = 6 and 11 of previously reported symmetrically selenoether-linked CBSenSeCB homologues. Conventional nematic (N) and NTB phases were observed for nearly all odd-n CBSenSeCB, CBSenOCB, and CBSenSCB dimers. The phase transition behaviours of the three homologous series were compared with those of the known symmetric ether-linked CBOnOCB, thioether-linked CBSnSCB, and asymmetric thioether- and ether-linked CBSnOCB dimers. Replacing ether with thioether or selenoether resulted in lower LC phase-transition temperatures, particularly for selenoether. Single-crystal structural analyses revealed that CBS3SCB and CBSe3SeCB exhibited folded U-shaped molecular shapes that were bent more than the V-shapes of CBOnOCB (n = 3 and 5) and CBS5SCB. In addition, thioether and selenoether linkages rendered the molecular forms of odd-spacer dimers in the crystals more bent.

The influence of molecular shape and electronic properties on the formation of the ferroelectric nematic phase

ABSTRACT The synthesis and characterisation of two series of ferroelectric nematogens based on RM734 having an additional methoxy group on the central phenyl ring are reported, the 3-methoxy-4-((4-nitrophenoxy)carbonyl)phenyl 2-alkoxy-4-alkoxybenzoates (7-m-n) and the 3-methoxy-4-((3-fluoro-4-nitrophenoxy)carbonyl)phenyl 2-alkoxy-4-alkoxybenzoates (8-m-n). In order to compare the behaviour of these series to those of the corresponding materials that do not contain the methoxy group on the central phenyl ring, we also report the synthesis and characterisation of 4-[(4-nitrophenoxy)carbonyl]phenyl 4-methoxybenzoate (11-0-1), 4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 4-methoxybenzoate (12-0-1) and 4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 2,4-diethoxybenzoate (12-2-2). Two compounds in which a lateral ethoxy chain is attached to the central ring, 3-ethoxy-4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (18-2-1) and 3-ethoxy-4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (19-2-1), are also described. The behaviour of these materials shows that the relative stabilities of the ferroelectric nematic, NF, and conventional nematic, N, phases are governed by a subtle interplay of steric and electronic factors. Furthermore, the electronic factors are better understood in terms of isolated regions of electron density rather than by a single large longitudinal dipole moment. In terms of molecular shape, to observe the NF phase it appears that the molecular structure must include one or more lateral substituents that enhance molecular biaxiality and destabilise the N phase.

Transformation of polar nematic phases in the presence of an electric field.

Only a few years have passed since the discovery of polar nematics, and now they are becoming the most actively studied liquid-crystal materials. Despite numerous breakthrough findings made recently, a theoretical systematization is still lacking. In the present paper, we take a step toward systematization. The powerful technique of molecular-statistical physics has been applied to an assembly of polar molecules influenced by electric field. Three polar nematic phases were found to be stable at various conditions: the double-splay ferroelectric nematic N_{F}^{2D} (observed in the lower-temperature range in the absence of or at low electric field), the double-splay antiferroelectric nematic N_{AF} (observed at intermediate temperature in the absence of or at low electric field), and the single-splay ferroelectric nematic N_{F}^{1D} (observed at moderate electric field at any temperature below transition into paraelectric nematic N and in the higher-temperature range (also below N) at low electric field or without it. A paradoxical transition from N_{F}^{1D} to N induced by application of higher electric field has been found and explained. A transformation of the structure of polar nematic phases at the application of electric field has also been investigated by Monte Carlo simulations and experimentally by observation of polarizing optical microscope images. In particular, it has been realized that, at planar anchoring, N_{AF} in the presence of a moderate out-of-plane electric field exhibits twofold splay modulation: antiferroelectric in the plane of the substrate and ferroelectric in the plane normal to the substrate. Several additional subtransitions related to fitting the confined geometry of the cell by the structure of polar phases were detected.

The influence of cholesteric helical ordering on the dynamics of liquid crystal microdroplet coalescence

ABSTRACT The dynamics of liquid crystal droplet coalescence in an isotropic melt was studied using polarised optical microscopy. Droplets of both nematic and cholesteric phases, formed from a mixture of nematic liquid crystal and chiral-photochromic dopant, were investigated. It was found that the characteristic coalescence times were significantly increased in the cholesteric phase compared to the nematic phase due to the presence of cholesteric helical ordering. In addition, these characteristic times increased as the cholesteric pitch decreased.

Cinnamic acid and biphenyl core calamitic asymmetric molecules: preparation and characterization for mesomorphic properties

ABSTRACT Some cinnamic acid and biphenyl core-derived calamitic molecules were synthesized and characterized for mesomorphic properties. Newly synthesized compounds showed well-defined schlieren textures and mosaic textures for the nematic phase and a relatively well-organized smectic A phase with focal conic textures. The chemical structures were confirmed by FT-IR, 1H-NMR, 13C-NMR and elemental analysis. The thermotropic behavior, optical textures and thermal stability have been investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), X-ray diffraction (SAXS) and thermal gravimetric analysis (TGA) respectively.

Topological interfaces crossed by defects and textures of continuous and discrete point group symmetries in spin-2 Bose-Einstein condensates

We systematically and analytically construct a set of spinor wave functions representing defects and textures that continuously penetrate interfaces between coexisting, topologically distinct magnetic phases in a spin-2 Bose-Einstein condensate. These include singular and nonsingular vortices carrying mass or spin circulation that connect across interfaces between biaxial- and uniaxial nematic, cyclic and ferromagnetic phases, as well as vortices terminating as monopoles on the interface (“boojums”). The biaxial-nematic and cyclic phases exhibit discrete polytope symmetries featuring non-Abelian vortices and we investigate a pair of noncommuting line defects within the context of a topological interface. By numerical simulations, we characterize the emergence of nontrivial defect core structures, including the formation of composite defects. Our results demonstrate the potential of spin-2 Bose-Einstein condensates as experimentally accessible platforms for exploring interface physics, offering a wealth of combinations of continuous and discrete symmetries. Published by the American Physical Society 2024

Stable graphene oxide-based lyotropic liquid crystals for interfacial lubrication

Lyotropic liquid crystals have lubricating properties due to their ordered assembly and fluidity, whose mesogens are often characterized by amphiphilic properties. Despite the attention that graphene oxide (GO) has been studied as a novel amphiphilic lyotropic mesogen this decade, and GO applied as a lubrication additive has been demonstrated in both oil and water-based systems, little research reveals the interfacial lubrication of GO liquid crystals yet. This work reports that GO aqueous dispersion can form lyotropic liquid crystals above a specific critical concentration of 5.00 mg/mL, providing a form of stable water-based lubricant, which can keep stable for several months and can reduce friction by 37.3% and wear by 25.24%. The liquid crystal phase was verified by polarizing microscope and synchrotron radiation small-angle X-ray scattering, and its rheological properties and viscoelasticity were studied by interfacial rheometer. The formation of lyotropic liquid crystals can enhance the stability of GO aqueous dispersions at high density, simultaneously ensuring friction decrease and anti-wear effect. It is attributed to the stable nematic network by the ordered GO sheets. The ordered assembly structure bears vertical shear force, therefore, reducing the wear. It is also assumed that the wide lateral size of graphene oxide promotes the nematic phase thus smoothes the graphene oxide film composed spontaneously under the coincidence of lamellar liquid crystal and 2D layered material. Through this work, the interlayer lubrication of GO was optimized, and the problem of GO dispersion sedimentation was solved by self-assembly. The range of interfacial lubrication of GO aqueous dispersion has been expanded and the synergistic effect is conducive to the environmentally friendly lubricants.

Sustainable Cellulose-Derived Organic Photonic Gels with Tunable and Dynamic Structural Color.

Structural color is a fascinating optical phenomenon arising from intricate light-matter interactions. Biological structural colors from natural polymers are invaluable in biomimetic design and sustainable construction. Here, we report a renewable, abundant, and biodegradable cellulose-derived organic gel that generates stable cholesteric liquid crystal structures with vivid structural colors. We construct the chromatic gel using a 68 wt % hydroxypropyl cellulose (HPC) matrix, incorporating distinct polyethylene glycol (PEG) guest molecules. The PEGs contain peculiar end groups with tailored polarity, allowing for precise positioning on the HPC helical backbone through electrostatic repulsion between the PEG and HPC chains. This preserves the HPC's chiral nematic phase without being disrupted. We demonstrate that the PEGs' polarity tunes the HPC gel's reflective color. Additionally, gels with variable polarities are highly sensitive to temperature, pressure, and stretching, resulting in rapid, continuous, and reversible color changes. These exceptional dynamic traits establish the chiral nematic gel as an outstanding candidate for next-generation applications across displays, wearables, flexible electronics, health monitoring, and multifunctional sensors.

Preparing a liquid crystalline dispersion of carbon nanotubes with high aspect ratio.

We successfully prepared a surfactant-assisted carbon nanotube (CNT) liquid crystal (LC) dispersion with double-walled CNTs (DWCNTs) having a high aspect ratio (≈1378). Compared to dispersions of single-walled CNTs (SWCNTs) with lower aspect ratio, the transition concentrations from isotropic phase to biphasic state, and from biphasic state to nematic phase are lowered, which is consistent with the predictions of the Onsager theory. An aligned DWCNT film was prepared from the DWCNT dispersion by a simple bar-coating method. Regardless of the higher aspect ratio, the order parameter obtained from the film is comparable to that from SWCNTs with lower aspect ratios. This finding implies that precise control of the film formation process, including a proper selection of substrate and deposition/drying steps, is crucial to maximize the CNT-LC utilization.

A cinnamate liquid crystal for rapid optical recording

ABSTRACT Photorecording materials have been applied for information recording. Herein, a cinnamate liquid crystal with an enantiotropic nematic phase was synthesised, which can be isomerised and polymerised under the irradiation of the 365-nm UV light. Cholesteric liquid crystal polymer network (CLCN) films were prepared using the mixtures of it, LC242, a chiral dopant and a photoinitiator. The CLCN films possess a slight gradient helical pitch which increases from the bottom to the top of the films. The formation of this structure should be driven by the photoisomerisation of the cinnamate. Under the irradiation of 365-nm UV light with a low intensity, the CLC mixtures show a photochromic behaviour which is proposed to be driven by the formation of oligomers. Based on this, the CLC mixtures can be applied for optical recording.

A dilute nematic gel produced by intramicellar segregation of two polyoxyethylene alkyl ether carboxylic acids

MotivationSurfactants like C8E8CH2COOH have such bulky headgroups that they cannot show the common sphere-to-cylinder transition, while surfactants like C18:1E2CH2COOH are mimicking lipids and form only bilayers. Mixing these two types of surfactants allows one to investigate the competition between intramicellar segregation leading to disc-like bicelles and the temperature dependent curvature constraints imposed by the mismatch between heads and tails. ExperimentsWe establish phase diagrams as a function of temperature, surfactant mole ratio, and active matter content. We locate the isotropic liquid-isotropic liquid phase separation common to all nonionic surfactant systems, as well as nematic and lamellar phases. The stability and rheology of the nematic phase is investigated. Texture determination by polarizing microscopy allows us to distinguish between the different phases. Finally, SANS and SAXS give intermicellar distances as well as micellar sizes and shapes present for different compositions in the phase diagrams. FindingsIn a defined mole ratio between the two components, intramicellar segregation wins and a viscoelastic discotic nematic phase is present at low temperature. Partial intramicellar mixing upon heating leads to disc growth and eventually to a pseudo-lamellar phase. Further heating leads to complete random mixing and an isotropic phase, showing the common liquid–liquid miscibility gap. This uncommon phase sequence, bicelles, lamellar phase, micelles, and water-poor packed micelles, is due to temperature induced mixing combined with dehydration of the headgroups. This general molecular mechanism explains also why a metastable water-poor lamellar phase quenched by cooling can be easily and reproducibly transformed into a nematic phase by gentle hand shaking at room temperature, as well as the entrapment of air bubbles of any size without encapsulation by bilayers or polymers.

Probing holographic flat bands at finite density

Flat band electronic systems exhibit a rich landscape of correlation-driven phases, both at the charge neutrality and finite electronic density, featuring exotic electromagnetic and thermodynamic responses. Motivated by these developments, in this paper, we explicitly include the effects of the chemical potential in a holographic model featuring approximately flat bands. In particular, we explore the phase diagram of this holographic flat band system as a function of the chemical potential. We find that at low temperatures and densities, the system features a nematic phase, transitioning into the Lifshitz phase as the chemical potential or temperature increases. To further characterize the ensuing phases, we investigate the optical conductivity and find that this observable shows strong anisotropies in the nematic phase.

An Electrically Tunable Dual‐Mode Laser Based on Self‐Assembled Soft Photonic Liquid Crystals

AbstractSoft photonic liquid crystals with electrically responsive properties are among the most promising intelligent materials for optoelectronic applications. This work demonstrates an ultra‐fast electrically tunable dual‐mode laser by introducing dye‐doped chiral liquid crystals. Due to the strong chirality of the chiral liquid crystal, two self‐assembled structures form depending on the temperatures. One is the chiral nematic phase (N*) with a 1D helical structure; the other is the blue phases (BPs) with 3D cubic structures. By tuning the strength of an electric field, the dye‐doped chiral liquid crystals are operated in two different modes: a band‐edge laser and a random laser. In the band‐edge mode, due to the electrostriction effect of the BPs, the electric‐tuning range of the laser emission is from 537 to 645 nm, and the switching times are less than 20 ms. When the electric field goes beyond a threshold value to force the occurrence of the BPs‐chiral‐nematic transition, the band‐edge laser with elliptical polarization emission is switched to a random laser with an immediate unpolarization emission (0.9 ms). Furthermore, the reversibility of the effects mentioned above points out the potential applications of self‐assembled soft photonic materials, spanning from solid‐state lighting to bio‐imaging.

Luminous and fluorescent cyanobiphenyls exhibiting super-cooled nematic phases

ABSTRACT A new series of liquid crystal dimers, 4-cyano-1,3-phenylene bis(4-((4’-cyano-[1,1’−biphenyl]-n-yl)oxy)alkanoate), has been synthesised and confirmed for their molecular structure using standard spectroscopic techniques. Thermal behaviour is assessed using optical polarising microscope (OPM), differential scanning calorimetry (DSC), and X-ray diffraction. Interestingly, all the dimers exhibit a super-cooled nematic phase without crystallisation during cooling cycles. These molecules self-assemble with head-on aggregation assessed by UV–Visible absorption spectra, and they also have a distinct fluorescence characteristic. A super-cooled nematogens self-assemble into head-on aggregation with a fluorescence property. Moreover, the materials were studied for their fluorescence properties and proved to be fluorescence active, making them excellent candidates to be used in designing photo-responsive optical sensor devices.

A new approach to the textural interpretation of magnetically aligned nematic and cholesteric lyotropic mesophases by using plate-λ accessory

ABSTRACT Lyotropic liquid crystals are substances formed by anisotropic micelles. Their unit structure change with the concentration and temperature and, as a consequence, the optical anisotropy changes. A measurable optical parameter, which reflects these anisotropies, is the refractive indices. Using these indices, it is possible to represent the anisotropic medium by indicatrix figures. In this sense, the indicatrix orientations will be interpreted in this work along the discotic nematic ( N D ) – biaxial nematic ( N B ) – calamitic nematic ( N C ) phases using a plate- λ accessory, from the polarized light microscopy, to modify the interference colors along the Michel-Lévy chart of the textures exhibited by the sample. Furthermore, the discotic cholesteric ( C h D ) – biaxial cholesteric ( C h B ) – N C ∗ [ unwounded calamitic cholesteric ( C h C ) ] phases are also investigated by the same methodology. The optic sign is also discussed here. In addition, we will also present a procedure that may help us to recognize the N B − N C phase transition from the microscopic textures.

Chiral induction of helical mesophase by amphiphilic chlorin e6 derivatives and their metal complexes: Effect of chiral and achiral aliphatic chains

An experimental and theoretical study of structure and properties of chiral amphiphilic chlorin e6 13(N)-methylamide-15-methyl,17-alkyl esters (17-pentadecyl 1 and 17-phytyl 2) and their Ni complexes (Ni-1 and Ni-2) and their helical twisting efficiency in liquid crystal were carried out. Quantum chemical simulation of their structure was performed, the adequacy of which was established by comparing the calculated and experimental UV/Vis, CD, 1H and 13C NMR spectra as well as optical rotation angles. Low values of g-factors of dissymmetry for macroheterocycle (MHC) with the phytol substituent were experimentally revealed. The reason for this effect, associated with the transfer of the chirality from the stereogenic centers of the substituent to macrocyclic chromophore in the associate, was established by quantum-chemical calculation. Using polarization microscopy, the formation of chiral nematic phases was detected by doping a mixture of cyanobiphenyls with macroheterocycles 1, 2, Ni-1 and Ni-2. The values of the clearance temperature and helical pitch were measured, the slopes of phase diagrams β and helical twisting power (HTP) were calculated. The orientation order parameters of dopants in liquid crystal (LC) were determined by polarization spectroscopy. The dielectric properties of LC–MHC systems were studied within the chiral nematics and isotropic phases, and the values of the Kirkwood’s correlation factors were calculated. Using quantum chemical simulation of MHC solvates, the features of the influence of aliphatic chains structure and complex formation on the physical properties of induced mesophases were established.