Liquid Crystal Films Research Articles

Broad-wave reflection of doping double-layered nanofibers in polymer-stabilized cholesterol-based liquid crystal films

In this study, polymer-stabilized cholesterol-based liquid crystal (PSCLC) films with broadband reflection were prepared using bilayer sodium alginate (SA)/polyvinyl alcohol (PVA) composite nanofiber films as a carrier for loaded photoinitiators as well as chiral compounds. The loaded photoinitiators and chiral compounds in the bilayer nanofibers could load a superposition of pitch inhomogeneous distribution to achieve broadband reflection. The effects of the concentration of chiral compounds in the films, the concentration of polymerization monomers, polymerization time, light intensity and temperature on the wave width were investigated. The morphology and diameter of the nanofibers were observed by SEM, and the weaving of the liquid crystals (LCs) was characterized by POM. The reflectance wavelengths were mainly measured by a UV–Vis spectrophotometer. This method broadens the wavelength of cholesteric phase liquid crystals from 256 nm to 436 nm, and lays the foundation for the study of the combination of wide wave reflection through nanofiber films.

Cholesteric liquid crystal films with adjustable wavelength band and reflectance by using wash-out/refill technique and light-responsive compounds

ABSTRACT Cholesteric liquid crystal (CLC) film with adjustable high reflection wavelength of light response is prepared by refilling the light-response CLC system into the polymer network with broadband reflection memory effect, which is the washing out/refilling method. The film is reusable with some treatment. The adjustable range of high reflection wavelengths in the near-infrared region and visible region were 1270–1750 nm and 640–1110 nm, respectively. In addition, a highly reflective film with broadband reflection has been successfully prepared in the wavelength range of 1238–1826 nm, and the waveband can reach 588 nm. This research has excellent application prospects in building energy conservation, radiation protection, aerospace and other fields.

Ultrasensitive fiber-optic temperature sensor based on cascaded Sagnac interferometers with a nematic liquid crystal film

We demonstrated an ultrasensitive fiber-optic temperature sensor based on cascaded Sagnac interferometers (CSIs). One of the Sagnac interferometers (SIs) consisted of a panda polarization-maintaining fiber (PMF) as the reference arm, while the other one contained two single-mode patch cables in a ferrule matching sleeve as the sensing arm, where a nematic liquid crystal (NLC) film was infiltrated into the micron-scale gap of the sleeve. In the sensing SI, the orientation of NLC film was induced by just rotating the two single-mode patch cables without any orientation agent, electric, or magnetic devices. Vernier effect was acquired by adjusting the thickness of NLC film. Experimental results revealed that the temperature sensitivity of the CSIs was up to 43.13 nm/°C, which was enhanced about 5 times compared to the single NLC SI. The designed optical fiber temperature sensor possessed the merits of compact structure, high stability, ultrahigh sensitivity, low hysteresis effect and high resolution, showing a promising application prospect in some scientific filed requiring accurate temperature measurement, such as biomolecules, medicine and other filed.

Electrically induced coloration of polymer-stabilized cholesteric liquid crystal films with broadband reflection capability for smart windows

In this work, a novel approach to realizing the electrically induced coloration of polymer-stabilized cholesteric liquid crystal (LC) films with broadband reflection capability for smart windows was reported. Only starting common materials and an established preparation method were used. The polymer-stabilized cholesteric LC films with broadband reflection capability were prepared via the method of polymerization-induced pitch gradient. After doping with a commercial ionic liquid which served as the supporting electrolyte in the system, the initial broadband reflection capability of the polymer-stabilized cholesteric LCs film was not damaged, and the film can change into a colored state from a colorless state under a low direct current (DC) voltage. The color of the polymer-stabilized cholesteric LCs also can gradually disappear after the removal of the field despite the slow response rate. The as-prepared polymer-stabilized cholesteric LCs in this work not only can be applied in the energy-saving smart windows and thermal management for buildings, but also presented the performance of electrically induced coloration under a low voltage. This work is expected to provide a creative approach for the preparation of electrochromic energy-saving smart windows.

A Light Field Display Realization with a Nematic Liquid Crystal Microlens Array and a Polymer Dispersed Liquid Crystal Film

This study demonstrates a light field display system using a nematic liquid crystal (LC) microlens array (MLA) and a polymer dispersed liquid crystal (PDLC) film. LC-MLA without polarization effects presented high-resolution intermediate 3D images by adopting a depolarization algorithm. The adopted PDLC film modulated the reconstructed 3D images to deliver full-parallax images efficiently with a wide FOV. The experimental result shows that the peak signal to noise ratio (PSNR) value of photograph accurate display results improves compared to the pure LC-MLA method. The proposed method is an essential step toward high-quality light field display.

Broadband Reflective Liquid Crystal Films Prepared by Rapid Inkjet Printing and Superposition Polymerization

Inkjet printing is a non-contact, material saving and on-demand material manufacturing technology, which is able to be applied to the fabrication of functional materials with high efficiency. A new method for preparing broadband reflective cholesteric films based on inkjet printing and non-stick technology was proposed in this paper. The feasibility of automatic mixing of liquid crystal and doped materials in inkjet printing was studied. The spectral data of samples prepared by manual mixing and automatic mixing by inkjet printing were compared. It was found that the spectral error of the printed film was only less than 0.17 wt%, which reached or even exceeded the effect of manual mixing. The feasibility of preparing liquid crystal films with broadband reflection characteristics by stacking polymerization based on in situ UV polymerization and non-stick technology was verified. By changing the printing amount of chiral doped ink, the bandwidth of PSCLC film can be accurately controlled. This technology is expected to play an important role in scientific research and practical application.

Mechanically strong and highly ion conductive graphene oxide liquid crystal film containing the poly(amic acid) salt

Mechanically strong and highly ion conductive graphene oxide liquid crystal film containing the poly(amic acid) salt

Principles of preparing broad-wave reflective films supported by nanofiber networks

ABSTRACT In this study, polymer-stabilised cholesterol-based liquid crystal (PSCLC) films with broadband reflection were prepared using nanofiber networks loaded with nanoparticles as the network supports throughout the liquid crystal cassette. The nanofiber prepared by electrostatic spinning technique ensures that the nanoparticles can be relatively uniformly distributed in the films, in addition to the formation of a fibre network that can replace the role of monomers and form network support. A series of studies on the concentration of nanoparticles and polymerisation conditions were carried out to obtain the maximum inhomogeneous distribution of spacing. The distribution of nanoparticles in the film was characterised by SEM and EDS; the stability of liquid crystals (LCs) and fibres with temperature was demonstrated by POM. The prepared broad-wave reflective films are of great value in intelligent windows and infrared shielding devices.

Programming Solitons in Liquid Crystals Using Surface Chemistry.

AC electric fields cause three-dimensional orientational fluctuations (solitons) to form and rapidly propagate in confined films of liquid crystals (LCs), offering the basis of a new class of active soft matter (e.g., for accelerating mixing and transport processes in microscale chemical systems). How surface chemistry impacts the formation and trajectories of solitons, however, is not understood. Here, we show that self-assembled monolayers (SAMs) formed from alkanethiols on gold, which permit precise control over surface chemistry, are electrochemically stable over voltage and frequency windows (<100 V; 1 kHz) that lead to soliton formation in achiral nematic films of 4'-butyl-4-heptyl-bicyclohexyl-4-carbonitrile (CCN-47). By comparing soliton formation in LC films confined by SAMs formed from hexadecanethiol (C16SH) or pentadecanethiol (C15SH), we reveal that the electric field required for soliton formation increases with the LC anchoring energy: surfaces patterned with regions of C16SH and C15SH SAMs thus permit spatially controlled creation and annihilation of solitons necessary to generate a net flux of solitons. We also show that solitons propagate in orthogonal directions when confined by obliquely deposited gold films decorated with SAMs formed from C16SH or C15SH and that the azimuthal direction of propagation of solitons within achiral LC films possessing surface-induced twists is not unique but reflects variation in the spatial location of the solitons across the thickness of the twisted LC film. Finally, discontinuous changes in LC orientation induced by patterned surface anchoring lead to a range of new soliton behaviors including refraction, reflection, and splitting of solitons at the domain boundaries. Overall, our results provide new approaches for the controlled generation and programming of solitons with complex and precise trajectories, principles that inform new designs of chemical soft matter.

Single‐, Dual‐, Triple, and Quadruple‐Wavelength Surface‐Emitting Lasing in Blue‐Phase Liquid Crystal (Adv. Mater. 9/2022)

Organic Lasers Single-, dual-, triple- to quad-wavelength lasers can be flexibly manipulated based on polymer-stabilized blue-phase liquid-crystal film, by tuning the bandgap, resonance cavity, order parameter of the laser dye, and even the pump energy. The single-to-quad-wavelength surface-emitting lasers proposed by Feng Jin, Jingxia Wang, and co-workers in article number 2108330, can be employed as coherent light sources for next-generation optical devices.

PDLC with controllable microstructure using wavelength-selective two-stage polymerization

A wavelength-selective, two-stage polymerization of acrylate-thiol for the formation of polymer dispersed liquid crystal (PDLC) films is developed. Via this method, the PDLC with microsphere polymer morphology was successfully prepared, which greatly improved the contrast ratio (CR) and adhesion without causing other performance degradation. The change of morphology from the porous to the microsphere structure could be controlled by adjusting the reaction time of the Michael addition reaction. Furthermore, the size and uniformity of the microsphere structure could be controlled by tuning the content of the free radical initiator. This work is expected to provide an effective method to control the microstructure of acrylate-thiol polymers in solution polymerization, and can possibly allow the fabrication of PDLCs with high CR and adhesion, which is of great significance in large, flexible displays, switchable windows, and light valves.

Hierarchically designed nanocomposites for triboelectric nanogenerator toward biomechanical energy harvester and smart home system

Triboelectric nanogenerator (TENG) has realized broad application prospects in the internet of things, human-machine interfaces, artificial intelligence. Among the abundant material options, polymer-based nanocomposites are highly potential for triboelectric devices, owing to their flexibility, easy processing and especially tunable properties (dielectric constant, polarity, etc.) by various additives. At present, the widely used approaches for output performance enhancement of nanocomposite-based TENG are to boost the dielectric constant for improved charge storage ability and/or to enlarge the polarity difference of these triboelectric materials. Herein, we discovered that large amounts of dielectric additives would boost the dielectric constant while impair the triboelectric polarity of nanocomposites with degraded output performance. A hierarchical design strategy by additionally constructing a triboelectric coating on the high dielectric nanocomposites is proposed to balance the dielectric constant and triboelectric polarity of nanocomposites for enhanced triboelectric performance. The hierarchical nanocomposites possess a high dielectric constant (εr ~ 82.35 at 103 Hz) and optimized triboelectric polarity. Owing to the synergistic influence of dielectric incorporation and hierarchical design by triboelectric coating, an enhancement of short-circuit current density and maximum instantaneous power density by 4.1 and 23.1 times, respectively, are achieved. Theoretical mode of hierarchical nanocomposites is established to guide the design and optimization. A polymer dispersed liquid crystal film driven by TENG and its application in smart home system are demonstrated successfully. This work facilitates the exploration of multi-layered nanocomposite materials for high-performance TENG in biomechanical energy harvester and smart home system applications.

Effect of Smectic Layers on Thermal Diffusivity of Side-Chain Polymer Liquid Crystals

The thermal conductivity (TC) of polymers is known to be effectively enhanced by liquid crystallinity. Liquid crystals (LCs) exhibit anisotropic TC with a greater magnitude in the orientation direction. However, the effects of other LC structures than the orientational order, such as smectic layer structures, on TC have not been well studied. This study investigated the correlation between LC structures and thermal diffusivity using a series of LC PMnMA polymethacrylates having azobenzene mesogens in the side chains. PMnMA formed nematic (N), smectic A (SmA), and smectic C (SmC) phases. These LCs were aligned by a magnetic field or large-amplitude oscillatory shear, yielding glassy LC films with mesogen long axes in the in-plane or out-of-plane direction. These glassy LCs increased thermal diffusivities in the LC orientation direction with the degree of LC orientation, whereas N and SmC LCs had smaller values than SmA LCs. These results demonstrated that smectic layers can enhance thermal diffusivity in their normal direction.

Electrotunable 180° achromatic linear polarization rotator based on a dual-frequency liquid crystal.

Linear polarization rotators have been widely used in optical systems. Commonly used polarization rotators are still beset by strong dispersion and thus restricted spectral bandwidth of operation. This leads to the development of achromatic or broadband alternatives, but most of them incorporate multiple waveplates for retardation compensation, which comes at the cost of increased complexity and reduced flexibility in operation and system design. Here, we demonstrate a single-element achromatic polarization rotator based on a thin film of dual-frequency chiral liquid crystal. The angle of polarization rotation is electrically tunable from 0° to 180° with low dispersion (±3°) in the entire visible spectrum, and a high degree of linear polarization (>95%) at the output.

Circularly polarized luminescence from cholesteric organic-inorganic hybrid silica films

Chiroptical functional materials with circularly polarized luminescence (CPL) properties have drawn increasing interest for their potential applications in optoelectronics and chiral sensing. However, the development of CPL materials with both large luminescence dissymmetry (glum) factors and high quantum yields (ΦF) in solid state is still a challenging task. In this work, organic–inorganic hybrid silica films have been prepared by the in-situ polycondensation of a cholesteric liquid-crystalline organosilane with the addition of a chiral dopant and an aggregation-induced emission (AIE)-active tetraphenylethene (TPE) derivative under an acidic condition. The resulting films emit bright CPL with the absolute glum values about 0.15, which were mainly caused by the long-range ordered supramolecular helical structure. It is found that the handedness of the CPL was dominated by the selective reflection of the cholesteric liquid crystal films.

Single-, Dual-, Triple, and Quadruple-Wavelength Surface-Emitting Lasing in Blue-Phase Liquid Crystal.

Soft organic lasers with multiwavelength output and high spectral purity are of crucial importance for versatile photonic devices, owing to their monochromaticity, coherence, and high intensity.However, there remain challenges for the achievement ofsurface-emitting multiwavelength lasing in soft photonic crystals, and the relative mechanisms need to be investigated. Herein, single-, dual-, triple-, and quadruple-wavelength lasers are successfully achieved in dye-doped blue-phase liquid crystal (BPLC) film. The number and wavelength of the lasing peaks can be manipulated by tuning the center of the bandgap, the order parameter of the laser dye, the quality of the resonance cavity, and even the pump energy. For single-wavelength lasing, a lasing peak withan ultranarrow linewidth of 0.04nm (Q-factor of 13454) is achieved. Multiwavelength lasing is attained based on the following aspects: i) the narrow bandgaps of the BPLCs withfull width at half maximum of 14-20nm; ii) a laser dye with high gain over a wide wavelength band, having a low-order parameter in the liquid crystal matrix; iii) appropriate relative positions between the reflection and fluorescence peaks; and iv) the highly ordered crystal lattice of BPLC film. The proposed single-to-quadruple-wavelength surface-emitting lasers can be employed as coherent light sources for next-generation optical devices.

Surfaces Decorated with Enantiomorphically Pure Polymer Nanohelices via Hierarchical Chirality Transfer across Multiple Length Scales.

Mesoscale chiral materials are prepared by lithographic methods, assembly of chiral building blocks, and through syntheses in the presence of polarized light. Typically, these processes result in micrometer-sized structures, require complex top-down manipulation, or rely on tedious asymmetric separation. Chemical vapor deposition (CVD) polymerization of chiral precursors into supported films of liquid crystals (LCs) are discovered to result in superhierarchical arrangements of enantiomorphically pure nanofibers. Depending on the molecular chirality of the 1-hydroxyethyl [2.2]paracyclophane precursor, extended arrays of enantiomorphic nanohelices are formed from achiral nematic templates. Arrays of chiral nanohelices extend over hundreds of micrometers and consistently display enantiomorphic micropatterns. The pitch of individual nanohelices depends on the enantiomeric excess and the purity of the chiral precursor, consistent with the theoretical model of a doubly twisted LC director configuration. During CVD of chiral precursors into cholesteric LC films, aspects of molecular and mesoscale asymmetry combine constructively to form regularly twisted nanohelices. Enantiomorphic surfaces permit the tailoring of a wide range of functional properties, such as the asymmetric induction of weak chiral systems.

Wavelength-Tunable and Water-Stable Cesium-Lead-Based All-Bromide Nanocrystal-Polymer Composite Films Using Ultraviolet-Curable Prepolymer as an Anti-Solvent.

All-inorganic metal halide perovskite nanocrystals (IPeNCs) have become one of the most promising luminescent materials for next-generation display and lighting technology owing to their excellent color expression ability. However, research on IPeNCs with stable blue emission is limited. In this paper, we report stable blue emissive all-bromide IPeNCs obtained through a modified ligand-assisted reprecipitation method using an ultraviolet (UV)-curable prepolymer as the anti-solvent at a low temperature. We found that the blue emission originates from quantum-confined CsPbBr3 nanoparticles formed together with the colorless wide-bandgap Cs4PbBr6 nanocrystals. When the temperature of the prepolymer was increased from 0 to 50 °C, CsPbBr3 nanoparticles became larger and more crystalline, thereby altering their emission color from blue to green. The synthesized all-bromide blue-emitting IPeNC solution remained stable for over 1 h. It also remained stable when it was mixed with the green-emitting IPeNC solution. By simply exposing the as-synthesized IPeNC–prepolymer solutions to UV light, we formed water-stable composite films that emitted red, green, blue, and white colors. We believe that this synthetic method can be used to develop color-emitting composite materials that are highly suitable for application as the color conversion films of full-color liquid crystal display backlight systems and lighting applications.

An Electrically and Thermally Erasable Liquid Crystal Film Containing NIR Absorbent Carbon Nanotube.

Carbon nanotubes (CNTs) coated by a poly(vinylpyrrolidone) (PVP) layer were doped in bistable cholesteric liquid crystal (ChLC) film to provide electric, thermal, or optical erasability controllable films. The CNT/PVP formed a compatible NIR-absorbing film that can generate heat to switch ChLC film from a planar texture to a focal conic texture. The appropriate content of CNT/PVP is provided to achieve a fast thermal response, satisfactory dispersion, and clear display brightness. The ChLC film containing CNT/PVP @ 0.8 (wt.%) saves 51% time at thermal erasing, compared to the ChLC mixture without NIR absorbent. The hybrid organic–inorganic bistable ChLC material reported here extends and offers new applications of ChLC writing tablets.

Liquid Crystalline Half‐Skyrmions and Their Optical Properties

AbstractLiquid crystals have intrigued physicists as a platform that facilitates direct visualization of topological concepts. Here, the theoretical and experimental studies demonstrating that a thin film of a chiral liquid crystal forms topological entities known as half‐Skyrmions, swirl‐like orientational order that spans the hemisphere of the order parameter space , are reviewed. They appear in the form of a hexagonal lattice or in an isolated manner, accompanied by topological line defects to satisfy topological constraints. Under an optical microscope, half‐Skyrmions appear as dark spots, and their hexagonal lattice yields a Kossel diagram comprising a hexagonal arrangement of circular arcs. These experimental observations are corroborated by numerical calculations of the orientational order based on the Landau‐de Gennes theory with an orientational order parameter of a second‐rank tensor, and construction of its optical images and Kossel diagrams by directly solving Maxwell equations for light wave. A thin film of a chiral liquid crystal thus provides an interesting system for the investigation of topological concepts, and spontaneous formation of a half‐Skyrmion lattice whose periodicity is of the order of the wavelength of visible light has a potential for photonics applications as well as poses a challenging problem on optics.