Pitch Gradient Research Articles

Anti-UV Passive Radiative Cooling Chiral Nematic Liquid Crystal Films for Thermal Management.

Passive radiative cooling (PRC) can spontaneously dissipate heat to outer space through atmospheric transparent windows, providing a promising path to meet sustainable development goals. However, achieving simultaneously high transparency, color-customizable, and thermal management of PRC anti ultraviolet (anti-UV) films remains a challenge. Herein, a simple strategy is proposed to utilize liquid crystalline polymer, with high mid-infrared emissive, forming customizable structural color film by molecular self-assembly and polymerization-induced pitch gradient, which guarantees the balance of transparency in visible spectrum and sunlight reflection, rendering anti-UV colored window for thermal management. By performing tests, temperature fall of 5.4 and 7.9°C are demonstrated at noon with solar intensity of 717Wm-2 and night, respectively. Vivid red-, green-, blue-structured colors, and colorless films are designed and implemented to suppress the solar input and control the effective visible light transmissivity considering the efficiency function of human vision. In addition, temperature rise of 11.1°C is achieved by applying an alternating current field on the PRC film. This study provides a new perspective on the thermal management and aesthetic functionalities of smart windows and wearables.

Quickly Formed Continuously Tuneable Laser in the Full Visible Spectrum Using Cholesteric Liquid Crystal Cells Based on the Electrothermal Effect

AbstractWavelength‐tuneable lasers can play a crucial role across many fields, and their applications continue to expand with advancements. Cholesteric liquid crystal (CLC) devices are developed, enabling good wavelength‐tuneable lasers. In CLC devices, pitch control is required to tune the desired wavelength. However, pitch control takes a long time and is difficult, which limits the speed and range of laser wavelength tuning. Here, by applying a voltage once and forming an electric field gradient along the wedge direction in the CLC wedge cell, we could realize CLC pitch gradient arrays that achieve continuous wavelength tuning over 500 nm in only 10 min through the electrothermal effect while continuously generating laser lines in the full visible spectrum. The laser wavelength could be rapidly tuned over the full visible spectrum by moving within the array in the wedge cell. The study of CLC cells for electric field application in real time is able to observe the dynamic process of pitch change directly. This new laser device and strategy has potential in many fields that require active laser tuning and is expected to lead to innovative changes in the development of future practical laser devices in the optical industry.

Chiral nematic nanocomposites with pitch gradient elaborated by filtration and ultraviolet curing of cellulose nanocrystal suspensions

An innovative method combining frontal filtration with ultraviolet (UV) curing has been implemented to design cellulosic nanocomposite films with controlled anisotropic textures from nanometric to micrometric length scales. Namely, an aqueous suspension containing poly (ethylene glycol) diacrylate polymer (PEGDA) as a photocurable polymer and cellulose nanocrystals (CNCs) at a 70/30 mass ratio was processed by frontal filtration, followed by in-situ UV-curing in a dedicated cell. This procedure allowed designing nanocomposite films with highly oriented and densely-packed CNCs, homogeneously distributed in a PEGDA matrix over a thickness of ca. 500 μm. The nanocomposite films were investigated with small-angle X-ray scattering (SAXS), by raster-scanning along their height with a 25 μm vertically-collimated X-ray beam. The CNCs exhibited a high degree of orientation, with their director aligned parallel to the membrane surface, combined with an increase in the degree of alignment as concentration increased towards the membrane surface. Scanning electron microscopy images of fractured films showed the presence of regularly spaced bands lying perpendicular to the applied transmembrane pressure, highlighting the presence of a chiral nematic (cholesteric) organization of the CNCs with a pitch gradient that increased from the membrane surface to the bulk.

Enhanced broadband reflection properties of PSCLCs films supported by electrospun nanofiber networks loaded with Cs0.33WO3 nanoparticles

ABSTRACT The introduction of nanoparticles into cholesteric liquid crystals has long provided inspiration for innovative device materials. A novel material design strategy was demonstrated to achieve a broadband reflection of polymer-stabilised cholesteric liquid crystals (PSCLCs) based on Cs0.33WO3 nanoparticles and the polymer nanonetworks in the PSCLCs. Through the adjustment of nanoparticle and chiral dopant (C6M) concentrations and meticulous control of polymerisation conditions, an appropriate pitch gradient distribution was achieved in the polymer-stabilised cholesteric liquid crystals (PSCLCs). This was attained by striking a delicate balance between the polymerisation speed and chiral dopant diffusion rate, resulting in optimal PSCLCs optical properties. Modification of Cs0.33WO3 nanoparticles by oleic acid was successfully accomplished as confirmed by IR spectroscopy. SEM and EDS analyses further demonstrated the distribution of nanoparticles in the spun fibre film. Moreover, POM observations showed that liquid crystals with nanofiber networks maintained the planar texture and temperature stability. The prepared broadband reflective film opens an avenue towards developing the reflection bandwidth in the PSCLCs, essential for intelligent windows and infrared shielding devices.

Multiscale study of the chiral self-assembly of cellulose nanocrystals during the frontal ultrafiltration process.

The structural organization of cellulose nanocrystal (CNC) suspensions at the membrane surface during frontal ultrafiltration has been characterized, for the first time, at the nano- and microscale by in situ small-angle X-ray and light scattering (SAXS and SALS, respectively). During filtration, the particles assembled at the membrane surface and formed the so-called concentration polarization layer (CPL), which contains CNCs arranged in a chiral nematic (cholesteric) helicoidal structure, with the long axis of the CNCs oriented parallel to the membrane surface, and the helical axis of the cholesteric structure oriented perpendicular to the membrane surface. The self-organization of CNCs in the form of oriented cholesteric structures was further characterized by a pitch gradient in the CPL. The structure of the CPL was also investigated upon release of the transmembrane pressure. SAXS data revealed a relaxation process associated with a diffusion of the CNCs from the membrane surface towards the bulk, while SALS measurements revealed a re-organization of the cholesteric phase that was preserved all along the deposit. The preservation of the observed structure after 14 days of continuous filtration followed by air-drying was confirmed using scanning electron microscopy and wide-angle X-ray diffraction, demonstrating the feasibility of the process scale-up.

Near-Infrared Reflective Polymer Films Based on UV-327-Doped Zinc Oxide Nanoparticles.

We prepared cholesteric liquid crystal (CLC) films with broadband reflective properties by admixing organic dye UV-327 into inorganic zinc oxide nanoparticles (ZnO NPs), utilizing the principle of pitch distribution from a large to a small gradient along the film thickness direction, leading to broadband reflection. ZnO NPs are poorly dispersed and easy to gather, but they do not decompose easily. The addition of UV-327 makes up for the above shortcomings. UV-327 is an organic compound with good compatibility and dispersion with liquid crystal systems. Therefore, we used the method of mixing two UV-absorbing dyes (UV-327 and ZnO NPs) to obtain CLC films. UV-absorbing dyes (UV-327 and ZnO NPs) made the liquid crystal films form a UV intensity gradient in the direction of thickness, prompting the polymerizable monomers to polymerize faster on the stronger side of the light, leading to the relative diffusion of chiral molecules and polymerizable monomers, forming the concentration gradient of chiral molecules in the direction of thickness. The pitch has a gradient distribution as the chiral concentration varies. Then, anchored by the polymer network, the pitch gradient distribution no longer changes. Broadened reflective bandwidth can reach up to 881 nm. Furthermore, the film covers the near-infrared wavelength band well, which can be applied to future smart windows or laser shielding for medical and military applications. It is also believed that this achievement will optimize the preparation technology of broadband reflective CLC films in the future.

Polymer-stabilized cholesteric liquid crystal films with broadband reflection formed by photomask polymerization

In this paper, polymer-stabilized cholesteric liquid crystal (PSCLC) films with broadband reflection are fabricated by photomask polymerization. Due to the small hole diffraction effect of light, a gradient of UV light intensity is formed in the upper part of the liquid crystal (LC) cell perpendicular to the thickness direction of the glass substrate, which induces the formation of the pitch gradient. The experimental conditions for the optimal reflection bandwidth were obtained by studying the concentration of polymerizable monomers, temperature, time, and UV light intensity of the mask polymerization. The results showed that the reflectance bandwidth was broadened from 293 nm to 508 nm. The method has the advantages of simple preparation process, effective broadening of reflection bandwidth and reusability, which provides new practical guidance for the preparation of PSCLC films with broadband reflectance.

Preparation of polymer stabilised cholesteric broadband reflection films based on ZIF-8 assisted bidirectional diffusion method

ABSTRACT The polymer-stabilised cholesteric liquid crystal (PSCLC) broadband reflective films are prepared by zeolite imidazole framework-8 (ZIF-8)-assisted bidirectional diffusion combined with light curing, altering the single pitch of cholesteric liquid crystals (CLCs). By changing temperature, chiral compounds and ultraviolet (UV) absorbing dyes adsorbed on porous ZIF-8 are able to diffuse bidirectionally into the CLCs’ system and produce concentration gradients, thereby inducing a pitch gradient, and the structure of broad reflection bandwidth can be fixed through UV irradiation. In addition, the effects of various factors, such as UV irradiation intensity, UV irradiation time, mass fraction of polymerisable monomers and curing temperature on the pitch gradient of the films are systematically investigated, and the maximum reflection bandwidth of the prepared films is 1274 nm. There is no doubt that new and modified processes and techniques provide significant possibilities for preparing and applying PSCLC films.

Research Progress of Cholesteric Liquid Crystals with Broadband Reflection.

Cholesteric liquid crystal (ChLC) materials with broadband reflection are witnessing a significant surge in interest due to their unique ability to self-organize into a helical supra-molecular architecture and their excellent selective reflection of light based on the Bragg relationship. Nowadays, by the virtue of building self-organized nanostructures with pitch gradient or non-uniform pitch distribution, extensive work has already been performed to obtain ChLC films with a broad reflection band. This critical review systematically summarizes the optical background of the ChLCs with broadband reflection characteristics, methods to obtain broadband reflection of ChLCs, as well as the application in this area. Combined with the research status and the advantages in the field, the challenges and opportunities of applied scientific problems in the research direction are also introduced.

Optical properties of broadband reflection of cholesteric liquid crystal by thermal diffusion of benzotriazoles.

Polymer stabilized cholesteric liquid crystal broadband reflective films are prepared by the thermal diffusion of a benzotriazoles organic ultraviolet (UV) absorber (UV-327) combined with photopolymerization. A gradient of UV intensity is established in the direction of film thickness, inducing the formation of a pitch gradient distribution, thus broadening the reflected bandwidth. The effects of UV-327 concentration, UV irradiation intensity, irradiation time, polymerization temperature, and C6M concentration on the reflected bandwidth of the sample are examined in detail. The results indicate that the diffusion method of the organic UV absorber enables the reflected bandwidth to be broadened under appropriate conditions.

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.

The smectogenity as a crucial factor of broadening of the selective light reflection peak in cholesteric photopolymerizable mixtures

ABSTRACT Cholesteric liquid crystals reflecting circularly polarised light are promising materials for creating flexible and compact polarisers. One of the issues of these systems is the narrow width of the photonic stop band. The range of selective light reflection can be increased by creating a gradient of the cholesteric pitch in the process of photopolymerisation of reactive cholesteric mixtures. The possibility of broadening the peak of selective light reflection during UV polymerisation of nematogenic diacrylate mixed with chiral monoacrylates was studied. The monoacrylates used had a different number of aromatic rings. A broadening of the peak of selective light reflection was observed during the photopolymerisation of mixtures containing a chiral monomer with three benzene rings. While before irradiation the width of the selective reflection peak was about 100 nm, after irradiation it was broadened to about 300 nm. A mechanism is proposed for the formation of the helical pitch gradient in the cholesteric, in which the smectogenity of the system plays a crucial role. The obtained results open up new possibilities for creating flexible and thin circular polarisers operating in a wide spectral range.

Preparation of chiral polymer/cholesteric liquid crystals composite films with broadband reflective capability for smart windows and thermal management of buildings

In this study, a new method to prepare the chiral polymer/cholesteric liquid crystals composite films with broadband reflective capability was reported. A negative dielectric anisotropy liquid crystal material was utilized as the mesogenic solvents and a chiral mesogenic monomer was employed as the chiral dopant. The chiral mesogenic monomer not only induced the liquid crystal molecules to align to form the helical structure, but also as the polymerizable monomer to be crosslinked under UV light to stabilize the pitch gradient in the materials. The UV absorbing dye was used to generate the intensity gradient of UV light and promoted the diffusion of the chiral mesogenic monomer from the backlight side to the light side. The effect of the content of the chiral mesogenic monomer and the intensity of the UV light applied during the preparation process of the composite film on the broadband reflective capability was studied systematically. The chiral polymer/cholesteric liquid crystals composite film that can reflect the light covering the wavelength from 820 nm to 2100 nm was achieved successfully and was used as the functional film in the windows for shielding part of near-infrared light to achieve the thermal management effect of buildings. This work is expected to provide a new method for the preparation of smart windows and thermal management of buildings. • A composite film with broadband reflective capability was presented. • The composite film contributed to shielding part of near-infrared light. • The film can reflect the light covering the wavelength from 820 nm to 2100 nm. • The composite film can be applied in the thermal management effect of buildings. • The film was capable of reducing the thermal effect of sunlight in the house.

Impact wave dispersion characteristics in Bouligand-type structures

A Bouligand structure is a special class of natural phononic crystals, which enables changes in configuration to achieve effective control of wave propagation. Here, we report an analytical modeling approach to study the impact energy filtering and the regulatory configuration mechanism in such structures. First, we demonstrate that by adjusting the helicity of structures and material properties of the Bouigand-type periodic structure, the size of bandgaps can be controlled and even implemented dispersion-free states. We then establish impact wave propagation models based on the propagator matrix method, which can be analytically evaluated for gradient-layered structures. Finally, we show that Bouligand-type materials with naturally occurring pitch gradient can effectively filter the impact energy generated by stomatopods. This is of interest for future design of vibration isolation devices, stealth aircraft skin, and offshore platform support and can provide important insight into structure–property relationships of the Bouligand-type structures.

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.

Electro-optic effect of graded-pitch chiral photonic structures under oblique illumination

We studied theoretically optical spectra when circularly polarized light obliquely impinges on a slab of a chiral photonic structure with linearly and uniformly spatial varying pitch. The material that we considered locally has a $\overline{4}2m$ point-group symmetry and presents Pockels effect, hence, we controlled optical spectra by a low frequency (DC) electric field applied along the nonhomogeneity axis. The spectra display a Bragg-type broadband, where the edge wavelengths of the photonic band are expressed in terms of the medium parameters and the external electric-field magnitude. This allows us to select the region of the electromagnetic spectrum, where the band could become as broad as we choose. We studied three samples with iridescent, silver, and golden colors due to their reflection properties. We observed an enhancement as well as a broadening of the optical band by the application of the electric field. We found that, under certain conditions, when the optical band is practically absent, a very broad band could be created (comparable to the whole visible spectrum) when a DC electric field is applied. This is the result of electro-optic contributions in the permittivity tensor elements, which give rise to a tremendous increase in both the rotatory power and dichroism. Therefore, these media could be used as electrically controlled broadband frequency and polarization filters. Moreover, we observed the usual blueshift of the band as the incidence angle of light increases and an asymmetry in the reflection band amplitude as the incidence angle of the light increases, depending on whether the pitch increases or decreases and the pitch gradient; which endows the sample with different reflection colors for different gradients and incidence angles of the light. The asymmetry in the reflection band vanishes as the magnitude of the electric field increases.

Lasing peculiarities in cholesteric multilayer structure with dye-doped polymer film depending on the concentration of laser dye and pumping energy

We have experimentally investigated lasing peculiarities in a cholesteric liquid crystal – dye-doped polymer layer – cholesteric liquid crystal three-layer wedge-cell system with different laser dye concentration changing pumping energy. It was theoretically and experimentally shown that there are photonic defect modes in cholesteric photonic bandgap of the wedge-cell system. The problem was solved by Ambartsumian’s layer addition modified method. It was shown that in the mentioned above system it is possible to obtain lasing generation and it should be noted that in our case the observed lasing peculiarities are not because of the isotropic defect (planar polymer layer) but the observed features are due to the cholesterics, namely by changing the thickness of the layer of cholesterics, we are able to ensure a change in the laser wavelength, due to the pitch jumps and therefore helix pitch gradient. The advantage of our experiment is the continuous change in the thickness of the cholesteric liquid crystal layer. To prevent degradation of the molecules, decrease of the fluorescence output as well as to except convective flows for higher pumping energies the dye concentration in the polymer was chosen 10−4mol/l. We have experimentally shown the dependences of the intensity and displacement of the lasing peaks’ on the pumping power.

Broadband reflection in polymer-stabilized cholesteric liquid crystal film with zinc oxide nanoparticles film thermal diffusion method

ABSTRACT Broadband reflective films were prepared by thermal diffusion of zinc oxide nanoparticles (ZnO NPs) in chiral nematic liquid crystals (N*-LCs). Ultraviolet (UV) light was irradiated in the direction of the thickness of the liquid crystal film to form the intensity gradient of UV light, to induce the gradient distribution of the pitch. The effects of the content of ZnO NPs, the UV light intensity, the UV irradiation time, the monomer concentration, and the diffusion temperature on the reflective bandwidth of the sample were studied. The results show that the appropriate content of ZnO NPs and the effective UV radiation intensity is beneficial to the broadband reflection of the sample. To elucidate the mechanism of photoinduced pitch gradients, the fracture surface of the sample film was observed by scanning electron microscopy (SEM), and the influence of experimental conditions on the broadband reflection performance of N*-LCs films was analysed theoretically.

Enantiomorphic double-polymerized chiral polymer composite template for highly efficient energy-saving green window

An infrared (IR) reflector that has ultra-wide reflection band and high reflectance was successfully fabricated based on an enantiomorphic chiral polymer composite template-made method involving UV-curing, washing-out, stacking, and refilling. In this study, an IR reflector reflecting >80% of incoming solar IR light from 800 nm to 1500 nm and retaining acceptable transmittance in the visible region is demonstrated. This undertaking is achieved by stacking two enantiomorphic double-polymerized chiral polymer templates with large longitudinal pitch gradients as a composite template cell designed for the near-IR region. Heat insulation test reveals that the temperature can be effectively lowered by 2 °C–3 °C by using the IR reflector, thereby indicating that a considerable amount of incident heat could be isolated and a significant amount of energy could be saved by cooling. The sample has potential in green and military applications, such as energy-saving green windows and anti-IR devices.

Design of a Pseudoperiodic Slow Wave Structure for a 6-kW-Level Broadband Helix Traveling-Wave Tube Amplifier

In this article, a potentially pseudoperiodic slow wave structure (SWS) for a broadband, high-power, helix traveling-wave tube (TWT) is studied. The structure considered here suppresses the back-wave oscillations caused by a large working current. These oscillations do not have conventional uniform pitch distributions in the radial dimension; rather, the two configurations studied in this article are a uniform pitch gradient and a conical helix. Through a reasonable design and a large number of calculations, the entire tube consists of four tapered, pitch-varying SWSs. In addition, to achieve an 8–18-GHz broadband, high-power output, a double-ridged waveguide window output coupler is designed, fabricated, and tested. The 3-D electromagnetic simulation software CST STUDIO SUITE is used to verify the performance of the tube. The simulation results show that the potentially pseudoperiodic SWS can adequately suppress the backward-wave oscillations, and the tube is capable of achieving an output power of over 6 kW, even with a very wide bandwidth of 8–18 GHz. Additionally, the electronic efficiency varies from 21.5% to 25.8%. The maximum output power is 7.45 kW, corresponding to an electronic efficiency of 25.8% at 14 GHz.