Nematic Liquid Crystal Phase Research Articles

Microfluidic Exploitation of Liquid Crystal Properties of Boron Nitride Nanotubes and Enhancing Neutron Shielding with Aligned Structure

Boron nitride nanotubes (BNNTs) are cylindrical nanostructures characterized by alternating boron and nitrogen atoms. Unlike carbon nanotubes (CNT), BNNTs are electrically insulating and exhibit high thermal stability and neutron shielding capabilities. Their unique tubular structure enables the formation of 1D arrays, which can achieve a nematic liquid crystal phase, ideal for fabricating high‐density structures. However, the widespread application of BNNTs has been hindered by challenges in purifying and dispersing high‐purity BNNTs. This study aims to exploit the liquid crystal properties of BNNTs by generating high‐purity BNNT dispersions through a series of dispersion and purification processes. Furthermore, this study utilizes microfluidics technology to encapsulate the BNNT dispersions. Using this approach, the transition of liquid crystal phases in relation to BNNT concentration is efficiently examined. This process results in the production of high‐density BNNT films characterized by pronounced anisotropy, which in turn provides significant thermal neutron shielding effects.

Photochemistry with Plane-Polarized Light: Controlling Selectivity of a Photochemical Rearrangement.

Over the past 50 years, a principal approach to controlling conventional photochemical reactions has relied on imposing geometric constraints on reactant or transition state via conducting photochemistry in the organized or constraining media. Herein, we describe a fundamentally different approach to affect the course of photochemical reactions (photochemical rearrangements) by utilizing spatially selective excitation of specific electronic transitions with plane-polarized light in the reactant molecules uniformly aligned in the nematic liquid crystal phase. In particular, we focused on the Type B enone rearrangement of 4,4-diarylcyclohexenones -one of the most common photochemical rearrangements. We demonstrated that the aryl migratory aptitude in this reaction was attenuated in response to changing an angle between the polarization plane of the incident light and the alignment direction of nematic liquid crystal, with the enhanced aryl migration achieved when the polarization plane coincided with the transition dipole moment leading to the excited state responsible for this migration. The spatially-selective initial excitation therefore was overruling the electronic factors responsible for the relative ratio of the two alternative photoproducts. The experimental findings were further supported by the results of a computational study. This work showcases a new fundamental paradigm in controlling photochemical reactivity and selectivity of photoreactions.

Microsecond electro-optic switching in the nematic phase of a ferroelectric nematic liquid crystal

Microsecond electro-optic switching in the nematic phase of a ferroelectric nematic liquid crystal

Additive-Free Gelation of Graphene Oxide Dispersions via Mild Thermal Annealing: Implications for 3D Printing and Supercapacitor Applications.

Herein, a mild thermal annealing (MTA) process is presented for additive-free gelation of graphene oxide (GO) dispersions. This process transitions the GO from a nematic liquid crystal phase to a random network structure, significantly enhancing its rheological properties by order of magnitude. This transition is facilitated by the diffusion of functional groups on the GO surface, which induces hydrophobic attractions, leading to a stable network structure. Employing rheo-SAXS experiments, detailed insights are provided into the microstructural changes of GO gel under shear stress, establishing a direct correlation between its rheological behavior and microstructure. The distinctive properties of MTA-processed inks are illustrated, seamlessly integrating with 3D printing, to yield a highly porous lattice structure that demonstrates promising potential as a supercapacitor electrode. The MTA process, an additive-free approach to gelation, maintains the inherent dispersion properties of GO while offering scalability. Thus, this method brings significant economic and environmental advantages compared to conventional gelation techniques. The findings not only advance the fundamental understanding of 2D colloidal network gels but also increase the potential of GO for a wide range of applications, from gas and liquid absorbers to electrodes for energy storage and conversion, and biomedical fields.

Prolonged memory effect in smectic A phase of a thermotropic liquid crystal material

The long-duration memory effect in thermotropic liquid crystal (LC) materials has long been a topic of substantial interest and numerous pioneering works. Although the idea of exploring the memory effect had been started from the nematic (N) phase of LC itself, but due to its very short duration, it was soon shifted to the other LC phases such as ferroelectric LC (FLC) or their derivatives and composites. Therefore, we demonstrate here the prolonged memory effect in a smectic A (SmA) phase of a pristine thermotropic LC material namely 4-octyl-4′-cyanobiphenyl (8CB). The memory effect has also been investigated in the N phase to estimate its duration as compared to the SmA phase. To examine the memory effect, firstly, we measured the dielectric permittivity using the dielectric spectroscopic technique with and without 40 V DC biasing. Furthermore, these results have been confirmed by means of optical textures recorded through a polarizing optical microscope. Moreover, we have also observed the removal and retrieval of two stable states by sequent application of heat and DC biasing to examine the cyclability of the LC compound under testing. We anticipate that our results will definitely bridge the gap that occurred in the advancement of emerging futuristic LC-based memory devices.

Synergistically controlled in situ growth of silver nanoparticles leading to organization of anisotropic liquid crystalline medium

ABSTRACT An in-depth macroscopic and microscopic investigation is carried out regarding understanding phase ordering and organisation of nematic phase of a room temperature nematic liquid crystal (NLC) MBBA (N-(4-methoxybenzylidene)-4-butylaniline) in the presence of silver nanoparticles (AgNPs) which are synthesised in situ in a single step using MBBA as a template as well as a reducing and stabilising unit. A synergistic effect between anisotropic LC structure and morphologies of AgNPs is observed. The LC-AgNP composite exhibits better thermal stability and phase ordering compared to its neat LC counterpart as indicated by combined study of optical polarization microscopy (OPM) and differential scanning calorimetry (DSC). While OPM study exhibits emergence of smectic mesophase in LC-AgNP composites, DSC manifests significant enhancement in the isotropic temperature and associated enthalpy as well as a change in the kinetic behaviour in LC-AgNP composites compared to neat LC. Also, an alternation in the electronic properties of LC-AgNP composites compared to neat LC is recognised. Such ordering at the macroscopic scale is well supported by our microscopic observations through spectroscopic studies such as Fourier transform infrared (FTIR) and X-ray diffraction (XRD) which provides valuable insights regarding understanding the organisation process more closely.

Multifaceted regulation of chiroptical properties and self‐assembly behaviors of chiral fluorescent polymers

AbstractThe multifaceted regulation of the chiroptical properties and self‐assembly behaviors of chiral fluorescent polymers is of great significance yet remains challenging to achieve. Herein, a series of novel salen‐based chiral fluorescent polymers with aggregation‐induced emission and varied substitution manners were facilely and efficiently synthesized. Multiple factors were systematically investigated on the chiroptical properties and self‐assembly performance of these polymers, which include molecular structures, solvent environments, metal coordination, and liquid crystal (LC) assemblies. Sutle change in the solvent composition can lead to diverse assembly morphologies of all these chiral polymers, from single‐handed helical fibers, helical toroids or loops, to spherical structures, consequently leading to an aggregation‐reduced circular dichroism (CD) phenomenon. The polymers bearing salen units show highly selective and reversible coordination with Zn2+ and can also induce multiple responses in the absorption, luminescence, CD, and circularly polarized luminescence (CPL) of these chiral fluorescent polymers via a coordination‐ and dissociation‐initiated self‐assembly tuning. Furthermore, a small amount of the chiral fluorescent polymer in can induce achiral nematic 4‐cyano‐4′‐n‐pentylbiphenyl (5CB) to form ordered chiral nematic LC phase with significant improvement in their CD and CPL signal. The absolute absorption and luminescent dissymmetry factors of the resulting supramolecular assemblies can reach the order of 10−1.

Emergence of cellular nematic order is a conserved feature of gastrulation in animal embryos.

Cells undergo dramatic changes in morphology during embryogenesis, yet how these changes affect the formation of ordered tissues remains elusive. Here we find that the emergence of a nematic liquid crystal phase occurs in cells during gastrulation in the development of embryos of fish, frogs, and fruit flies. Moreover, the spatial correlations in all three organisms are long-ranged and follow a similar power-law decay with less than unity for the nematic order parameter, suggesting a common underlying physical mechanism unifies events in these distantly related species. All three species exhibit similar propagation of the nematic phase, reminiscent of nucleation and growth phenomena. Finally, we use a theoretical model along with disruptions of cell adhesion and cell specification to characterize the minimal features required for formation of the nematic phase. Our results provide a framework for understanding a potentially universal features of metazoan embryogenesis and shed light on the advent of ordered structures during animal development.

Double-Helical Tiled Chain Structure of the Twist-Bend Liquid Crystal Phase in CB7CB

The twist-bend nematic liquid crystal phase is a three-dimensional fluid in which achiral bent molecules spontaneously form an orientationally ordered, macroscopically chiral, heliconical winding of a ten nanometer-scale pitch in the absence of positional ordering. Here, the structure of the twist-bend phase of the bent dimer CB7CB and its mixtures with 5CB is characterized, revealing a hidden invariance of the self-assembly of the twist-bend structure of CB7CB, such that over a wide range of concentrations and temperatures, the helix pitch and cone angle change as if the ground state for a pitch of the TB helix is an inextensible heliconical ribbon along the contour formed by following the local molecular long axis (the director). Remarkably, the distance along the length for a single turn of this helix is given by 2πRmol, where Rmol is the radius of bend curvature of a single all-trans CB7CB molecule. This relationship emerges from frustrated steric packing due to the bent molecular shape: space in the fluid that is hard to fill attracts the most flexible molecular subcomponents, a theme of nanosegregation that generates self-assembled, oligomer-like correlations of interlocking bent molecules in the form of a brickwork-like tiling of pairs of molecular strands into duplex double-helical chains. At higher temperatures in the twist-bend phase, the cone angle is small, the director contour is nearly along the helix axis z, and the duplex chains are sequences of biaxial elements formed by overlapping half-molecule pairs, with an approximately 45° rotation of the biaxis between each such element along the chain.

Simulating the Lyotropic Phase Behavior of a Partially Self-Complementary DNA Tetramer.

DNA oligomers in solution have been found to develop liquid crystal phases via a hierarchical process that involves Watson-Crick base pairing, supramolecular assembly into columns of duplexes, and long-range ordering. The multiscale nature of this phenomenon makes it difficult to quantitatively describe and assess the importance of the various contributions, particularly for very short strands. We performed molecular dynamics simulations based on the coarse-grained oxDNA model, aiming to depict all of the assembly processes involved and the phase behavior of solutions of the DNA GCCG tetramers. We find good quantitative matching to experimental data at both levels of molecular association (thermal melting) and collective ordering (phase diagram). We characterize the isotropic state and the low-density nematic and high-density columnar liquid crystal phases in terms of molecular order, size of aggregates, and structure, together with their effects on diffusivity processes. We observe a cooperative aggregation mechanism in which the formation of dimers is less thermodynamically favored than the formation of longer aggregates.

Room temperature stable twist-bend nematic materials without crystallization over 1 year

The twist-bend nematic (NTB) phase of achiral liquid crystals (LCs) manifests a unique self-assembled heliconical structure with nanometer-scale pitch length, mirroring the chiral symmetry-breaking phenomena in nature, thus sparking widespread research interest. However, the ingenious NTB phase is only stable at high temperatures within a very limited temperature interval, often undergoing inevitable crystallization at low temperatures. Herein, room temperature supercooled NTB material systems composed of meticulously designed LC dimer mixtures with varying molecular curvatures and central flexibility were developed, resulting in complete resistance to crystallization even after 1 year of storage. Furthermore, the proposed NTB material systems demonstrated exceptional compatibility with common nematic LCs, facilitating the tailoring of overall physical parameters, particularly to achieve a sufficiently low bend elastic constant with excellent stability. This work represents a paradigmatic advancement forward in realizing stable NTB phase materials with a broad temperature range and resistance to crystallization, thereby tackling the enduring and seemingly insurmountable challenge while providing impetus for further exploration of their applications in soft matter, crystallography, and advanced photonics.

Structural color control by remote electrical manipulation of chiral dopant molecules in a chiral nematic phase

Abstract 1,2-Bis((cholesteryloxy)octyloxy)-4-nitrobenzene was synthesized as a chiral dopant that exhibited electroresponsiveness in nematic liquid crystal (LC) phases. Even in a nonelectroresponsive host nematic LC, the helical pitch of the chiral nematic phase, generated by doping the chiral dopant molecules, could be altered in response to an external electric field, thus controlling the selective reflection color.

Microfluidics-Based Drying-Wetting Cycles to Investigate Phase Transitions of Small Molecules Solutions.

Drying-wetting cycles play a crucial role in the investigation of the origin of life as processes that both concentrate and induce the supramolecular assembly and polymerization of biomolecular building blocks, such as nucleotides and amino acids. Here, we test different microfluidic devices to study the dehydration-hydration cycles of the aqueous solutions of small molecules, and to observe, by optical microscopy, the insurgence of phase transitions driven by self-assembly, exploiting water pervaporation through polydimethylsiloxane (PDMS). As a testbed, we investigate solutions of the chromonic dye Sunset Yellow (SSY), which self-assembles into face-to-face columnar aggregates and produces nematic and columnar liquid crystal (LC) phases as a function of concentration. We show that the LC temperature-concentration phase diagram of SSY can be obtained with a fair agreement with previous reports, that droplet hydration-dehydration can be reversibly controlled and automated, and that the simultaneous incubation of samples with different final water contents, corresponding to different phases, can be implemented. These methods can be further extended to study the assembly of diverse prebiotically relevant small molecules and to characterize their phase transitions.

Effect of magnitude of voltage and magnetic field on memory effect in nematic phase of liquid crystal and their composites with aerosil and goethite nanoparticles

In liquid crystals doped with aerosil nanoparticles, the state induced by applying voltage or a magnetic field stays remembered after removing the power. The ability to remember the induced state after removing the power is known as non-volatile memory effect. The present paper describes how the magnitude of voltage and magnetic field affects the memory effect in the nematic phase of liquid crystal 4-cyano-4′-pentylbiphenyl doped with aerosil nanoparticles and with a combination of aerosil and magnetic goethite nanoparticles. Capacitance measurements revealed increasing in memory when the magnitude of voltage and magnetic field is increased. Applying fields with various magnitudes provides the possibility of fabrication a multilevel memory device based on the response to electric or magnetic field. The memory can be erased, and the initial state can be restored by heating the composites to the same temperature.

Chiral nematic cellulose nanocrystal composites: An organized review

AbstractCellulose nanocrystals (CNCs) are commercially available materials derived from cellulose, the most abundant biopolymer on our planet. Due largely to their high strength, high surface area‐to‐volume ratio, tailorable surface chemistry, and the abundance of biomass feedstocks with which to produce them, CNCs have attracted significant interest in applications spanning the paints and coatings, composites, packaging, and biomedical sectors. However, and perhaps most interestingly, CNCs will self‐assemble (or, as I've teased in the title, organize) to form highly ordered chiral nematic liquid crystal phases when concentrated in suspension. Upon complete solvent evaporation, this chiral nematic order is ‘locked’, yielding films with structural colour—colour arising not due to chemical pigments, but rather due to the physical structure of a material itself. In the pursuit of novel multi‐functional materials, research interest has shifted recently towards the incorporation of functional additives to form composite chiral nematic films. Along with introducing the basics of liquid crystals and self‐assembly, this review discusses the main approaches used in order to form CNC‐based composite films: co‐assembly, templating, and post‐processing, and highlights exceptional examples in each case. Finally, I give my uniquely Canadian perspective on the current status, future prospects, and major challenges associated with the development of CNC‐based chiral nematic composite materials.

Dielectric spectroscopy of ferroelectric nematic liquid crystals: Measuring the capacitance of insulating interfacial layers

Numerous measurements of the dielectric constant ɛ of the recently discovered ferroelectric nematic (NF) liquid crystal (LC) phase report extraordinarily large values of ɛ′ (up to ∼30 000) in the NF phase. We show that what is in fact being measured in such experiments is the high capacitance of the nonferroelectric, interfacial, insulating layers of nanoscale thickness that bound the NF material in typical cells. Polarization reorientation as a linear response to AC electric field drive renders the NF layer effectively electrically conductive, exhibiting low resistance that enables the charging of the interfacial capacitors. We analyze the commonly employed parallel-plate cell filled with NF material of high polarization , oriented parallel to the plates at zero applied voltage. Minimization of the dominant electrostatic energy renders spatially uniform and orients the polarization to make the electric field in the NF as small as possible, a condition under which the voltage applied to the cell appears almost entirely across the high-capacity interfacial layers. This coupling of orientation and charge creates a combined polarization-external capacitance (PCG) Goldstone reorientation mode requiring applied voltages orders of magnitude smaller than that of the NF layer alone to effectively transport charge across the NF layer. The NF layer acts as a low-value resistor, and the interfacial capacitors act as reversible energy storage reservoirs, lowering the restoring force (mass) of the PCG mode and producing strong reactive dielectric behavior. Analysis of data from a variety of experiments on ferroelectric LCs (chiral smectics C, bent-core smectics, and the NF phase) supports the PCG model, showing that deriving dielectric constants from electrical impedance measurements of high-polarization ferroelectric LCs, without properly accounting for the self-screening effects of polarization charge and the capacitive contributions of interfacial layers, can result in overestimation of the ɛ′ values of the LC by many orders of magnitude. Published by the American Physical Society 2024

Enhanced liquid crystal performance through alkoxybenzoic acid doping

ABSTRACT This study investigates the impact of doping agents, 4-octyloxybenzoic acid (8OBA) and 4-octyloxy-3-fluoro benzoic acid (8OBAF), on the thermodynamic and electro-optical properties of the nematic liquid crystal 5CB. The differential scanning calorimetry, dielectric, and polarising optical microscopic measurements revealed intriguing properties. The doped liquid crystal samples exhibit higher phase transition temperatures, increasing birefringence, and order parameters. Dielectric measurements disclose reduced free ion concentration with 8OBA and increased concentration with 8OBAF in the liquid crystal matrix. The threshold voltage, switching time, and rotational viscosity were also diminished in the 5CB + 8OBA mixture, whereas these parameters were increased in the 5CB + 8OBAF mixture due to the strong intermolecular interaction with liquid crystals. The results presented here demonstrate that 8OBA dopant can be used to achieve high thermal stability of the nematic liquid crystal phase with fast response times and low threshold.

Ultrahigh Polarization of Emitted Light by Quenched Emissive Material

AbstractThe emission of highly polarized light by organic light‐emitting diodes (OLEDs) is crucial for various applications; however, achieving such emission requires the use of polarizers, which reduce the device efficiency and durability. In this study, ultrahigh‐polarized light with a polarization ratio (PR) of 407:1 is achieved via the rapid thermal quenching (RTQ) of OLEDs. Poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) films, molybdenum disulfide (MoS2) nanosheets, and tungsten disulfide (WS2) nanosheets are applied as hole transport layers and surface‐alignment layers through surface treatment, including rubbing and/or ion‐beam exposure to align the molecules of the emissive materials, namely, {poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT). Quenching is performed at 240 °C, at which F8BT has a nematic liquid crystal (NLC) phase with high molecular ordering. Upon quenching, the high molecular ordering of the NLC‐phased emissive layer instantaneously froze via RTQ, leading to the emission of highly polarized light. Consequently, the PEDOT:PSS‐, MoS2‐, and WS2‐based OLEDs exhibit ultrahigh PRs of 407:1, 349:1, and 328:1, respectively, for photoluminescence at a 540 nm peak wavelength. In contrast, for electroluminescence, the PEDOT:PSS‐based OLED exhibits a high PR of 395:1. This result is the best reported to date and is comparable to the PRs generated by polarizers, indicating that quenching can be utilized for the development of ultrahigh‐polarized light‐emitting devices.

Shear-induced flow and dynamics of the nematic phase of Sunset Yellow lyotropic chromonic liquid crystal

ABSTRACT Fundamental understanding of rheological properties of lyotropic chromonic liquid crystals is inadequate compared to their thermotropic counterparts in spite of the growing interest in these materials. Here, we investigate the rheological properties of a lyotropic chromonic nematic liquid crystal (LCLC), prepared by dissolving 32 wt% of Sunset Yellow (SSY) in water. The flow curves (stress-strain rate, σ − γ ˙ ) at different temperatures exhibit three flow regimes R1, R2 and R3, respectively. At room temperature, in R1 ( γ ˙ < γ ˙ cl ) and R3 ( γ ˙ > γ ˙ cu ) the sample exhibits Newtonian flow behaviour ( σ ∝ γ ˙ ), whereas, in R2 ( γ ˙ cl < γ ˙ < γ ˙ cu ), it shows a power-law dependence ( σ ∼ γ ˙ β ). With increasing shear rate the change of stress from R1 to R2 is continuous, whereas the change is discontinuous from R2 to R3. We observe stress fluctuations under steady shear in regime R2 similar to those reported in worm-like micellar systems and in thermotropic twist-bend nematic liquid crystals. The non-monotonic flow behaviour and the stress dynamics are corroborated using in situ rheo-microscopy.

Effect of the Structure of Epoxy Monomers and Curing Agents: Toward Making Intrinsically Highly Thermally Conductive and Low-Dielectric Epoxy Resins.

The low intrinsic thermal conduction and high dielectric properties of epoxy resins have significantly limited their applications in electrical and electronic devices with high integration, high frequency, high power, and miniaturization. Herein, a liquid crystalline epoxy (LCE) monomer with a biphenyl mesogenic unit was first synthesized through an efficient one-step reaction. Subsequently, bisphenol AF (BPAF) containing low-polarizable -CF3 groups and 4,4'-diaminodiphenylmethane (DDM) were applied to cure the LCE and commercial diglycidyl ether of bisphenol A-type epoxy (E-51), respectively, to afford four kinds of epoxy resins with various intrinsic thermal conductivity and dielectricity values. Owing to the dual effect of microscopically stacking of mesogens and the contribution of fluorine to the formation of liquid crystallinity, ordered microstructures of the nematic liquid crystal phase were formed within the cross-linking network of LCE as confirmed by polarized optical microscopy and X-ray diffraction. Consequently, phonon scattering was suppressed, and the intrinsic thermal conductivity was improved considerably to 0.38 W/(m·K), nearly twice as high as that of E-51 cured with DDM (0.20 W/(m·K)). Additionally, the ordered microstructure and ultralow polar -CF3 groups within LCE cured with BPAF enabled the epoxy resin to exhibit a remarkably lower and stable dielectric constant (ε) and dielectric loss tangent (tan δ) over both low and high frequencies compared to E-51 cured with DDM. The ε decreased from 3.40 to 2.72 while the tan δ decreased from 0.044 to 0.038 at 10 GHz. This work presents a scalable and facile strategy for breaking the bottleneck of making epoxy resins simultaneously with high inherent thermal conduction and low dielectric performance.