Cholesteric Liquid Crystal Films Research Articles

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.

Topological structures in chiral media: Effects of confined geometry.

We theoretically study orientational structures in chiral magnetics and cholesteric liquid crystal (CLC) nanosystems confined in the slab geometry. Our analysis is based on the model that, in addition to the exchange and the Dzyaloshinskii-Moriya interactions, takes into account the bulk and surface anisotropies. In CLC films, these anisotropies describe the energy of interaction with external magnetic/electric field and the anchoring energy assuming that magnetic/electric anisotropy is negative and the boundary conditions are homeotropic. We have computed the phase diagram and found that the ground state of the film is represented by various delocalized structures depending on the bulk and surface anisotropy parameters, κ^{b} and κ^{s}. These include the z helix and the z cone states, the oblique, and the x helicoids. The minimum energy paths connecting the ground state and metastable helicoids and the energy barriers separating these states are evaluated. We have shown that there is a variety of localized topological structures such as the skyrmion tube, the toron, and the bobber that can be embedded in different ground states including the z cone (conical phase) and tilted fingerprint states. We have also found the structure called the leech that can be viewed as an intermediate state between the toron and the skyrmion tube.

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.

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.

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.

Tuning the circularly polarized luminescence of polymer-stabilized cholesteric liquid crystal films using chiral dopants

PSCLC films embedded with a D-A type dye were obtained. Due to the synergetic effects of conformational/stacking chirality of the emissive dye and selective Bragg reflection, tuneable CPL performance was observed.

Electro-optical characteristics of cholesteric liquid crystal films based on CuS nanoparticles

The CuS nanoparticles (NPs) coated by poly(vinylpyrrolidone) (PVP) layer were doped into bistable liquid crystal (LC) film to provide an electric, thermal or optical phase-shift controllable films. The CuS/PVP NPs could form a compatible NIR absorbing film, which could generate heat to switch LC film from planar texture to focal conic texture. The appropriate content of CuS/PVP NPs was provided to achieve fast thermal response, satisfactory dispersion and clear display brightness. The novel LC film containing 0.8 wt.% CuS/PVP NPs could save almost 51% time at thermal elimination, comparing to LC mixture without NIR absorbent. The hybrid organic-inorganic functional bistable LC material reported here extend and open up new application of LC writing tablets.

Asymmetric chiral disazo dopants with high anisotropy for versatile modulation of liquid crystal optics

As key elements, anisotropic chiral dopants determine the helical twisting power (HTP) of doped cholesteric liquid crystal (CLC) system by the host–guest interaction. To reveal the intrinsic structure–function relationship, we developed 3 novel asymmetric and photo-responsive chiral derivatives bearing axial chiral binaphthol moiety and disazo chromophore. They exhibited reversible photoisomerization, and rapidly induced helical degeneration of the doped CLC mixture under UV irradiation. Furthermore, highly asymmetric (R)-2′-hydroxy-[1,1′-binaphthalen]-2-yl 3,5-bis((E)-(4-butoxyphenyl)diazenyl)benzoate (molar absorption coefficient: 28,090 L⋅mol−1⋅cm−1; half-life period: 10.28 h; HTP: −34 um−1 in E7) was adopted to construct the CLC film with excellent NIR-reflection adjusted by light, heat, and electric field. It is a promising photoswitch agent for application in advanced smart materials and optical devices.

Structure colorants based on cross‐linked cholesteric liquid crystalline polymeric slices

AbstractIn daily life, we usually make materials with certain colors via dyes or pigments. However, there is a methodology for making specific nanostructures that reflect light under certain wavelengths, and this is considered structural color. In this study, we demonstrate novel sliced polymeric films exhibiting reflected purple, blue, cyan, and red colors via Bragg reflection. The pitch of the cholesteric liquid crystal was changed by altering the ratio of chiral dopant inside the liquid crystal mixture. It was confirmed that the reflected colors remained stable and did not change as the size and thickness of the sliced films were decreased. Furthermore, colorful patterns were easily achieved by using CLC slices as colorants, and these were fixed with transparent glue for image production. Theoretically, nanosized structural colorants could be prepared via a femtosecond laser. It is anticipated that these sliced polymerized cholesteric liquid crystal films can be applied in the surface coating and painting industries.

Transparent UV-blocking photonic film based on reflection of cholesteric liquid crystals

An optically transparent bilayer cholesteric liquid crystal (CLC) solid film capable of almost completely rejecting ultraviolet (UV)-A light was fabricated with a bilayer CLC film comprising double handed CLC layers using UV-curing reactive mesogen mixtures (RMMs) doped with LC756 and S811. The concentrations of S811 and LC756 in RMMs and the film thickness were optimized to achieve more than 98% UV blocking (in the UV-A region) and more than 95% transparency (in the visible region). The proposed approach provides a simple and reliable fabrication process that can be employed in practical applications such as smart windows, eyeglasses, and lenses.

Self-diffusion method for broadband reflection in polymer-stabilized cholesteric liquid crystal films

ABSTRACT Chiral nematic liquid crystal (N*-LCs) films with wide pitch gradients covering the near infrared (NIR) region are facilely obtained by a simple controlling diffusion method. The polymer network can not only fix the orientation of N*-LCs, but also serves as a diffusion channel for substances, and has excellent compatibility. First, in the design of the pre-polymerisation (Incomplete polymerisation) layer, mainly by controlling the content of polymerisable monomer and UV-absorbing dyes in N*-LCs, a polymer network with density gradient distribution and suitable for material diffusion is formed, and the substrate of the oil repellent layer on the side of the low-density polymer network is easier to be peeled off. Then, nematic liquid crystals are injected between the pre-polymerisation layer and another oriented glass substrate. Due to the large concentration difference of chiral compounds and UV-327 between the two liquid crystals, the gradient distribution of spacing is formed by diffusion. Because of the gradient distribution of the UV-327, the gradient distribution of the pitch is further increased after polymerisation again. Finally, the effect of the UV-327 on the polymer network of the pre-polymerised layer is observed using SEM. Thus, a new method for reflective broadband in the near-infrared region is proposed.

From Single to Multi Mode Lasing: The role of materials revealed in optical simulations

ABSTRACT The comparative study of thin film Cholesteric Liquid Crystal (CLC) lasers made from different materials and optically pumped by an external solid state laser reveals a striking dependence of lasing behaviour (ranging from single mode at the edge of the selective reflection band to multimode lasing) on the morphology and microstructure of CLC films. The materials studied belong to two groups: low molar mass liquid crystals and polymers. It is shown that the orientation of individual chiral domains and fluctuations in helical pitch contribute significantly to the type of lasing displayed by the material. Different ways of preparing CLC cells that lead to predominantly one type of lasing are discussed. The importance of variations of the helical pitch and domain orientation in producing single and multimode lasing is justified by optical simulations (4x4 matrix method) of lasing in multilayered samples.

Chiroptical Smart Paints: Polymerization of Helical Structures in Cholesteric Liquid Crystal Films

Using polymerizable cholesteric liquid crystal (CLC) paints, we prepare circularly polarized freestanding films that enable students to acquire a basic understanding of circular polarization and molecular chirality. The polymerized helical structures of CLC films can exhibit brilliant structural colors by selective reflection of visible light corresponding to their helical pitch. For the fabrication of freestanding and circularly polarizing films of various colors and shapes, students control helical pitch with the amount of chiral dopants and stabilize them by photopolymerization. On the basis of an understanding of the circularly polarized reflection of CLC paints, students can encode secret letters that are visible only with a specific circular polarizer.

Preparation of cholesteric polymer networks with broadband reflection memory effect

ABSTRACT In this study, polymer network is prepared by broadband reflective cholesteric liquid crystal (CLC) film with washing out method. By the method of photoinduced molecular diffusion, the CLC film is prepared by adding different concentrations of ultraviolet (UV) absorption dyes into the LC system and irradiated with a series of UV light intensities for different times. The selective reflection property and broadband reflection memory effect of the polymer network are proved, and the response mechanism is given. By refilling the polymer network with different LCs, reflective bands with different central wavelengths and bandwidths can be obtained. It is confirmed that different fillers can recurrence the initial reflection bandwidth of CLCs in different degrees. These results may provide a new idea for total reflection CLC films.

Flexible, broadband, super-reflective infrared reflector based on cholesteric liquid crystal polymer

Cooling indoor spaces in hot weather conditions requires a significant amount of energy. To reduce energy consumption, light control of windows and facades in buildings is a great way. In this study, we propose a facile method to fabricate a flexible cholesteric liquid crystal (CLC) polymer reflector that can control the reflection of infrared (IR) light selectively without interfering with visible light. To this end, we fabricated a flexible super-wideband IR reflector using right- and left-handed chiral reactive mesogens. The fabricated IR reflector exhibited several desirable properties, including thinness, high mechanical stability and flexibility, strong durability, solvent- and acid resistance, high transparency, and low haze in visible light. The fabricated IR reflector was observed to reduce the indoor temperature by 6 °C compared to the outside temperature. This proves that, by insulating the incident heat, the proposed IR reflector can be utilized to maintain low indoor temperatures in working and living spaces. The flexible and super-reflective CLC film reported in this study is expected to be a promising candidate in several applications, including anti-IR devices, energy-saving windows for cars, and facades of modern buildings.

Broadband reflection prepared by loading chiral dopants in white carbon black

ABSTRACT Cholesteric liquid crystals with helicoidal molecular architecture are known for their ability to selectively reflect light with the wavelength that is determined by the periodicity of molecular orientations. In this paper, the broadband reflective polymer stabilised cholesteric liquid crystal (PSCLC) films in the near infrared region have been fabricated by introducing the chiral dopant-loaded white carbon black. The non-uniform pitch distribution can be anchored by adjusting the diffusion time of chiral dopants, UV intensity and polymerisation temperature, as well as the concentration of polymerisable monomers. The reflection bandwidth was broadened from 300 nm to 840 nm. The scanning electron microscope (SEM) proved the non-uniform pitch distribution in the composites. In this paper, the investigated results suggested the correlation between diffusion time, polymerisation conditions and reflection bandwidth.

Doubling the optical efficiency of color‐converted micro‐light‐emitting diode displays with a patterned cholesteric liquid crystal polymer film

AbstractWe propose a patterned cholesteric liquid crystal (CLC) polymer film as a self‐assembled Bragg reflector to enhance the performance of a color‐converted micro‐light‐emitting diode (LED) display system. In the display system, we firstly optimize the device structure of blue LED chip to maintain a high light extraction efficiency and low optical crosstalk. Next, we fabricate several patterned CLC films with feature sizes ranging from 10 to 80 μm. By integrating the patterned CLC film into a color‐conversion micro‐LED display, the optical efficiency is doubled while keeping a color gamut of about 90% Rec. 2020. Finally, we carry out a proof‐of‐concept experiment to validate the functionality of such a patterned CLC film.

Broadband reflection cholesteric liquid crystal film fabricated by near-infrared photothermal response technology

ABSTRACT A polyvinyl alcohol–modified copper sulphide (PVA-CuS) nanoparticlephotothermal film with modulated photothermal response properties is fabricated. Aliquid crystal cell with modulated photothermal conversion function is fabricated by using the glass substrate with this photothermal film as one side of this liquid crystal cell. In the formula of cholesteric liquid crystal (CLC), there is achiral compound with temperature-dependent helical twisting power and aultraviolet (UV)-polymerisable monomer. Therefore, under the irradiation of the near-infrared laser, the temperature of the sample gradually increases due to the photothermal conversion function of the liquid crystal cell, which causes the pitch of the CLC to gradually increase. Meanwhile, the liquid crystallinity polymerisable monomer polymerises under the irradiation of UV light to form apolymer network, and fix the pitch at each temperature section, then form anon-uniform distribution of the pitch to achieve broadband reflection. The influence of heating speed and UV light intensity is investigated under different modulation modes of near-infrared laser.

Label-free single-substrate quantitative protein assay based on optical characteristics of cholesteric liquid crystals

Cholesteric liquid crystal (CLC) is characterized by the selective reflection in the Bragg condition for constructive interference. Instead of assembling a LC cell with a pair of glass substrates, only a single glass substrate modified with vertical alignment reagent and layered with a CLC film of 3.4 ± 0.2 μm in thickness was utilized in this study for the detection of bovine serum albumin (BSA). By fine-tuning the ratio of the thickness d to the helical pitch P of the CLC film, both qualitative and quantitative biosensing capabilities were demonstrated with an E7/R811 CLC (d/P close to but smaller than unity) and an E7/R5011 CLC (d/P > > 1) film, respectively. The E7/R811 film was in the unwound homeotropic state in the absence of BSA, but transitioned to the fingerprint state at 10−9 to 10−6-g/ml BSA and to the planar state between 10−5 and 10−2-g/ml BSA. Quantitative protein assay based on E7/R5011 CLCs was established through transmission spectrometric analysis, and detection sensitivity was optimized by adjusting the Bragg reflection wavelength. A limit of detection of 5.9 × 10−9 g/ml was achieved for the CLC film consisting of 2.0-wt% R5011 in the nematic host E7 and exhibiting a Bragg reflection wavelength of 700 nm. As a majority of clinical assays are performed on a single solid substrate, the CLC-based, single-substrate biosensing platform allows for better integration with current technologies. Besides, the protein array employed in this work can be further miniaturized and converted to a protein microarray format to facilitate high-throughput analysis.

Thermally bandwidth-controllable reflective liquid crystal films prepared by doping nano-sized electrospun fibers

ABSTRACT In this study, a polymer-stabilised cholesteric liquid crystal film with broadband reflection was prepared by using a nanofibre film loaded with chiral dopants as the carrier. The nanofibre film prepared by the electrospinning technology ensures that chiral dopants can be uniformly distributed on the film and can be controlled release during the heating process. In order to obtain the maximum non-uniform distribution of the pitch, a series of investigations were conducted on the concentration of the polymerised monomer, polymerisation temperature, and intensity of the ultraviolet light. By screening different experimental conditions, the optimal conditions for broadening the reflection band were obtained, and the reflection bandwidth was broadened from 270 to 425 nm. The prepared cholesteric liquid crystal films with broadband reflection characteristics have potential application value in many fields, such as light-brightening films, smart windows, infrared shielding devices, laser protective coatings, etc.