Chiral Nematic Structure Research Articles

Chiral Nematic Coatings Based on Cellulose Nanocrystals as a Multiplexing Platform for Humidity Sensing and Dual Anticounterfeiting.

The need for a precise regulation of the properties of chiral nematic structures in response to external stimuli is addressed. Self-assembled iridescent coatings are produced under the effect of electrostatic interactions between cellulose nanocrystals and poly(acrylic acid), endowing a high anisotropic dissymmetry (>0.3) and sensitivity to environmental humidity (13.1 nm/1% at 68-75% relative humidity, RH). The phenomena associated with shifts in selective light reflection (green to orange) and polarization, facilitate tunable transmitted colors (blue to orange) at given rotation angles (RA). Such properties are conveniently integrated into a "RH-RA-color" ternary code that is introduced as an anticounterfeiting technology, taking advantage of multicolor patterns that conveniently track with changes in RH and RA. The proposed charge-driven assembly opens new opportunities for chiral nematic materials that enable precise optical sensing and information encryption.

Chiral Cellulose Nanocrystal Humidity-Responsive Iridescent Films with Glucan for Tuned Iridescence and Reinforced Mechanics.

The fabrication of biomimetic photonic materials with environmental stimuli-responsive functions from entirely biobased materials is becoming increasingly challenging with the growing demand for biodegradable materials. Herein, the effect of glucan with different molecular weights on the mechanical performance and tunable structural color of iridescent CNC composite films was investigated. The existence of glucan did not influence the self-assembly performance of CNCs, but rather led to an improvement in the mechanical performance, enabling cholesteric CNC composite films with an adjustable structural color. Simultaneously, the iridescent films showed a conspicuous redshift and enlarged initial pitch without obstruction of the chiral structure. In response to environmental humidity, the structural colors of the iridescent composite films can be changed by regulating their chiral nematic structure. In particular, the films demonstrate a reversible structural color change between blue and red at RH between 50 and 98%. The resulting biobased iridescent composite films have potential applications in decorative coating, optical and humidity sensing, and anticounterfeiting.

Multilayer photonic films based on interlocked chiral-nematic cellulose nanocrystals in starch/chitosan

In this study, a new approach to employ and control cellulose nanocrystal (CNC) chiral nematic structure as a biodegradable, intelligent material was investigated. Tuned CNC self-assembled films were interlocked between two layers of citric acid, cross-linked starch/chitosan (1:1) films through the solvent casting process. This method increased the mechanical properties of produced films and created a selective reflection band from UV to near-IR depending on the helical pitch of the chiral nematic CNC layer. The features of these intelligent films have potential for different applications, from UV protective packaging to biomedical uses. The water vapor permeability (WVP) of the produced films decreased considerably by adding a CNC layer into the cross-linked starch/chitosan structure. Also, the WVP was different for the different helical pitches of the CNC layer. The starch/chitosan (outer layer) also showed a remarkable antibacterial property against E. coli, P. fluorescens, S. Enteritidis, and S. aureus which could be useful for biomedical applications or antibacterial packaging.

Thermal annealing of iridescent cellulose nanocrystal films

The properties of chiral nematic and iridescent cellulose nanocrystal films with different monovalent cations (CNC-X) obtained through evaporation-induced self-assembly (EISA) can be modified by a variety of external stimuli. Here, we study the transformations of their optical and structural properties when the films are thermally annealed at 200 °C and 240 °C for up to 2 days. The chiral nematic structure of the most thermally stable films is not destroyed even after extensive heating due to the thermochemical stability of the cellulose backbone and the presence of surface alkali counterions, which suppress catalysis of early stage degradation. Despite the resilience of the cholesteric structure and the overall integrity of heated CNC-X films, thermal annealing is often accompanied by reduction of iridescence, birefringence, and transparency, as well as formation of degradation products. The versatility, sustainability, and stability of CNC-X films highlight their potential as temperature indicators and photonic devices.

Thixotropy of cellulose nanocrystal suspensions

The thixotropy of cellulose nanocrystal (CNC) water suspensions is intrinsically dependent on the hierarchical structure of the suspension. The diverse hierarchies that comprise individual CNC nanoparticles and mesophase liquid crystalline domains, chiral nematic and nematic structures, contribute selectively to the rheological material response. Here, we combine rheology with polarized light imaging (PLI) to elucidate the thixotropic behavior of CNCs suspended in water. The simultaneous monitoring of PLI and rheological tests enables the observation of mesogens and their orientation dynamics. Creep, dynamic time sweep, ramped hysteresis loop, and thixotropic recovery tests combined with PLI aim to differentiate the contribution of the different hierarchical levels of CNC suspensions to their thixotropy. The range of concentrations investigated comprised biphasic (4 and 5 wt. %) and liquid crystalline phase suspensions (6, 7, and 8 wt. %). The CNC suspensions exhibited complex thixotropy behavior, such as viscosity bifurcations in creep tests and overshoot in ramped hysteresis loop tests. The restructuring and destructuring appeared to correspond to different levels of their hierarchical structure, depending mainly on the phase, in agreement with previous studies. Restructuring was attributed to re-organizations of an individual CNC, e.g., in the isotropic fraction of biphasic suspensions and at the mesogen interfaces in liquid crystalline phase suspensions. However, by increasing liquid crystalline fraction in the biphasic concentrations, restructuring could also involve mesogens, as indicated in the creep tests. For flow conditions above the yield stress, as evidenced by the ramped hysteresis and thixotropy recovery tests, destructuring was dominated by orientation in the flow direction, a process that is readily observable in the form of PLI “Maltese-cross” patterns. Finally, we show that a simple thixotropy model, while unable to capture the finer details of the suspension’s thixotropic behavior, could be employed to predict general features thereof.

Handedness Inversion in Chiral Nematic (Ethyl)cellulose Solutions: Effects of Substituents and Temperature

Various acylates and phenylcarbamates of (ethyl)cellulose (EC) were synthesized by acylation and carbanilation, respectively, of the residual hydroxyls of an EC (ethyl DS = 2.50). The acyl substituents adopted were propionyl, butyryl, cyclohexanoyl, and adamantoyl groups, and the phenylcarbamoyl substituents included 3-chlorophenylcarbamoyl, 4-chlorophenylcarbamoyl, 3-methylphenylcarbamoyl, and bare phenylcarbamoyl groups. Chiral nematic mesophases of the EC derivatives, formed in chloroform, acetic acid (AA), and dichloroacetic acid (DCA), were examined by various optical techniques for evaluation of the helical pitch (P) in a set of absolute value and twist sense. The lyotropic samples prepared with DCA (20 °C) always assumed a right-handed chiral nematic structure, regardless of the degree of acyl (or phenylcarbamoyl) substitution (DSAcyl (or DSPC), ≤0.50). The chiral nematic series with AA (20 °C) and chloroform (5 °C) showed an inversion of the twist sense from being left-handed to right-handed, when DSAcyl (or DSPC) of each derivative was increased. The critical DS value for the inversion varied depending on the chain length, bulkiness, and polar nature of the employed substituent. For all of the lyotropic series explored, it was generalized that a temperature elevation strengthens a left-handed twisting power in each chiral nematic mesophase. Reversal of the twist handedness was also observed in a cycle of heating and cooling of some chiral nematics in AA. This was interpreted as being due to compensation in the strength of the two chiral interactions, i.e., steric repulsion and dispersion interaction between mesogenic molecules, contributing in mutually opposite signs to the twisting power (2πP–1) of the mesophase.

Chiral Photonic Liquid Crystal Films Derived from Cellulose Nanocrystals.

As a nanoscale renewable resource derived from lignocellulosic materials, cellulose nanocrystals (CNCs) have the features of high purity, high crystallinity, high aspect ratio, high Young's modulus, and large specific surface area. The most interesting trait is that they can form the entire films with bright structural colors through the evaporation-induced self-assembly (EISA) process under certain conditions. Structural color originates from micro-nano structure of CNCs matrixes via the interaction of nanoparticles with light, rather than the absorption and reflection of light from the pigment. CNCs are the new generation of photonic liquid crystal materials of choice due to their simple and convenient preparation processes, environmentally friendly fabrication approaches, and intrinsic chiral nematic structure. Therefore, understanding the forming mechanism of CNCs in nanoarchitectonics is crucial to multiple fields of physics, chemistry, materials science, and engineering application. Herein, a timely summary of the chiral photonic liquid crystal films derived from CNCs is systematically presented. The relationship of CNC, structural color, chiral nematic structure, film performance, and applications of chiral photonic liquid crystal films is discussed. The review article also summarizes the most recent achievements in the field of CNCs-based photonic functional materials along with the faced challenges.

Circularly Polarized Laser with Chiral Nematic Cellulose Nanocrystal Cavity.

Circularly polarized (CP) lasers derived from low-cost and renewable raw sources are attracting increasing attention in photonics and material science. Here, we present a facile and effective approach to fabricate CP lasers by the evaporation-induced assembly of cellulose nanocrystals (CNCs) and a laser dye. The obtained laser exhibits a controlled chiral nematic structure, which acts as a chiral optical cavity, and varied chiral coupling interactions. It is shown that the CNC-based laser can modify the polarization state of the laser into left-handed polarization, leading to strong CP laser emission (CPLE) with a dissymmetry factor up to 0.35. The chiral nematic CNC structure proves to be a versatile yet straightforward strategy to generate strong and tailored CPLE.

Infiltration of Proteins in Cholesteric Cellulose Structures.

Cellulose nanocrystals (CNCs) can spontaneously self-assemble into chiral nematic (cn) structures, similar to natural cholesteric organizations. The latter display highly dissipative fracture propagation mechanisms given their “brick” (particles) and “mortar” (soft matrix) architecture. Unfortunately, CNCs in liquid media have strong supramolecular interactions with most macromolecules, leading to aggregated suspensions. Herein, we describe a method to prepare nanocomposite materials from chiral nematic CNCs (cn-CNCs) with strongly interacting secondary components. Films of cn-CNCs were infiltrated at various loadings with strongly interacting silk proteins and bovine serum albumin. For comparison and to determine the molecular weight range of macromolecules that can infiltrate cn-CNC films, they were also infiltrated with a range of poly(ethylene glycol) polymers that do not interact strongly with CNCs. The extent and impact of infiltration were evaluated by studying the optical reflection properties of the resulting hybrid materials (UV–vis spectroscopy), while fracture dissipation mechanisms were observed via electron microscopy. We propose that infiltration of cn-CNCs enables the introduction of virtually any secondary phase for nanocomposite formation that is otherwise not possible using simple mixing or other conventional approaches.

Natural cuticle-inspired chitin/silk fibroin/cellulose nanocrystal biocomposite films: fabrication and characterization

This study aims at developing a green sustainable bio-inspired composite film by combining the flexible chitin/silk fibroin matrix (CT/SF) and the rigid cellulose nanocrystals (CNC) with the multi-layer chiral nematic structure. The effects of CNC concentration on the chemical bonding, optical- and micro-morphology, thermal stability and mechanical properies of chitin/silk fibroin/cellulose nanocrystal (CT/SF/CNC) were comprehensively investigated. The results indicate that a critical concentration for CNC at 2 wt% can display the obvious iridescent color domains of biocomposite films with potential optical functional property. There is a color shift from blue to yellow-red corresponding to the higher concentrations at 3.5 and 5.0 wt%, respectively. The concentration of CNC also affects the transition of conformation of SF from the random to the β-turn as well as the bonding ability between SF and CT. The CT/SF/CNC-3.5 wt% with the excellent optical and mechanical properties as well as the improved thermal stability breaks the conventional conflict between increased strength and compromised ductility. Its tensile strength and elongation at break effectively elevate by 282% and 42%, respectively.

Bio-based visual optical pressure-responsive sensor

A bio-based pressure-responsive sensor with adjustable structural color is prepared by combining aerogel skeleton of cellulose nanocrystals (CNCs)/poly(ethylene glycol) (PEG) obtained via the ice-templating method with flexible polyacrylamide (PAAM) elastomer. The white aerogel is composed of consecutive ribbons, demonstrating chiral nematic structure. These ribbons are rearranged to be vertical to the force direction, leading to immediate appearance of the structural color when the 3D aerogel transforms to a 2D plane. Helical pitches are regulated by the PEG content that the wavelength of structural color covers up to 178 nm. There is an excellent linear correlation between pressure and transmittance of reflectance peak, and the sensitivity to pressure can be regulated by changing solid content of PAAM. Furthermore, the pressure-responsive color is still vivid after 16 cycles of compression. This flexible material with pressure-responsive structural color is promising in sensing, intelligent display, information transmission, and etc.

Pearl-inspired polymer films with chiral nematic and multi-lamellar structures

ABSTRACT Structural coloured materials have been widely found in nature. More and more artificial materials were created based on structural colour. However, fabrication of polymer films with pearly lustre remains a challenging task. Herein, a strategy is reported to prepare broadband reflection polymer films based on the fluorophilic association. A monomer with a perfluorooctyl chain is synthesised. After photo-crosslinking a mixture of a chiral nematic liquid crystal and the fluorinated monomer, broadband reflection film with pearly lustre is obtained. The broadband reflection originates from the broad distribution of the helical pitch. The pearly lustre is caused by the periodically ordered layers formed by the microphase separation between hydrocarbon and fluorocarbon chains.A strategy for the preparation of color liquid crystal polymer film with pearly luster is developed. The Bragg-reflection color originates from the supramolecular helical structure of the cholesteric liquid crystal and the pearly luster originates from the multilayer structure formed by the fluorocarbon/hydrocarbon microphase separation

Photoluminescence-enhanced cholesteric films: Coassembling copper nanoclusters with cellulose nanocrystals

Iridescent and luminescent composite films were fabricated through a coassembly strategy, in which glutathione-stabilized copper nanoclusters (GSH-CuNCs) were incorporated into chiral nematic structures of a cellulose nanocrystal (CNC) film. Through variations in the helical pitch, these composite films exhibited broadband reflection. The fluorescence emission spectrum of the composite film exhibited peaks at 439 and 600 nm, corresponding to crystallization-induced emission from CNCs and assembly-induced emission from CuNCs. The enhanced luminescence and prolonged lifetime of the composite film were attributed to the confinement effect of solid layers and attendant intermolecular interactions. By tuning the reaction time, temperature, and pH of the solution, the emission color and intensity of the CuNCs could be changed. At appropriate GSH and Cu2+ concentrations, the chiral organization of GSH-CuNCs enabled the composite CNC film to exhibit right-handed chiral fluorescence with an asymmetry factor of −0.16. Luminescent composite films were employed to fabricate LEDs with custom colors and patterns.

Concentric chiral nematic polymeric fibers from cellulose nanocrystals.

Hierarchical biological materials, such as osteons and plant cell walls, are complex structures that are difficult to mimic. Here, we combine liquid crystal systems and polymerization techniques within confined systems to develop complex structures. A single-domain concentric chiral nematic polymeric fiber was obtained by confining cellulose nanocrystals (CNCs) and hydroxyethyl acrylate inside a capillary tube followed by UV-initiated polymerization. The concentric chiral nematic structure continues uniformly throughout the length of the fiber. The pitch of the chiral nematic structure could be controlled by changing the CNC concentration. We tracked the formation of the concentric structure over time and under different conditions with variation of the tube orientation, CNC concentration, CNC type, and capillary tube size. We show that the inner radius of the capillary tube is important and a single-domain structure was only obtained inside small-diameter tubes. At low CNC concentration, the concentric chiral nematic structure did not completely cover the cross-section of the fiber. The highly ordered structure was studied using imaging techniques and X-ray diffraction, and the mechanical properties and structure of the chiral nematic fiber were compared to a pseudo-nematic fiber. CNC polymeric fibers could become a platform for many applications from photonics to complex hierarchical materials.

Uniformly Assembly of Filamentous Phage/SiO <sub>2</sub> Composite Films with Tunable Chiral Nematic Structure in Capillary Confinement

Uniformly Assembly of Filamentous Phage/SiO <sub>2</sub> Composite Films with Tunable Chiral Nematic Structure in Capillary Confinement

Synthesis of optically active chiral mesoporous molybdenum carbide film

A chiral nematic mesoporous molybdenum carbide film (CN-Mo2C) was fabricated through the self-assembly of cellulose nanocrystal with peroxomolybdate. CN-Mo2C exhibits a tunable chiral nematic structure by varying the ratio of constituents, enabling a surface area of 210m2g−1. Importantly, it shows a typical chiroptical feature through polarized optical microscopy and circular dichroism spectral analysis. As it has an excellent conductivity of 2.4×10−2 S cm−1, we try to use CN-Mo2C in the electrocatalytic HER production, presenting an efficient H2 production capacity. It is a versatile synthetic strategy that can conveniently enable other large-area macroscopic chiral metal-based materials.

Patterned surface alignment to create complex three-dimensional nematic and chiral nematic liquid crystal structures

ABSTRACT Combining liquid crystals (LCs) with well-designed anchoring patterns at the substrates offers tremendous potential for the development of functional electro-optic devices or stimuli-responsive actuators. Photo-alignment techniques nowadays allow an almost arbitrary control over the surface anchoring and this flexibility is used to design highly efficient flat optical LC components with different functionalities. Part of this research, dealing with nematic and chiral nematic LC between substrates with patterned azimuthal anchoring, is reviewed here. The focus is on understanding the self-assembly of complex structures, steered by an interplay between surface anchoring and LC elasticity. Additional insight into the LC bulk behaviour is obtained by comparing experimental results with numerical simulations of the director configuration. Periodic anchoring patterns with azimuthal rotation at the top and bottom substrate are studied, as well as ring-shaped alignment patterns with a 180° or 360° azimuthal rotation in a confined region in space. Different combinations of anchoring patterns at the top and bottom substrates are investigated and in addition to nematic liquid crystal (NLC), also short and long-pitch chiral nematic liquid crystal (CLC) is considered.

Chameleon-Inspired Variable Coloration Enabled by a Highly Flexible Photonic Cellulose Film.

Due to spontaneous organization of cellulose nanocrystals (CNCs) into the chiral nematic structure that can selectively reflect circularly polarized light within a visible-light region, fabricating stretching deformation-responsive CNC materials is of great interest but is still a big challenge, despite such a function widely observed from existing creatures, like a chameleon, because of the inherent brittleness. Here, a flexible network structure is introduced in CNCs, exerting a bridge effect for the rigid nanomaterials. The as-prepared films display high flexibility with a fracture strain of up to 39%. Notably, stretching-induced structural color changes visible to the naked eye are realized, for the first time, for CNC materials. In addition, the soft materials show humidity- and compression-responsive properties in terms of changing apparent structural colors. Colored marks left by ink-free writing can be shown or hidden by controlling the environmental humidities. This biobased photonic film, acting as a new "smart skin", is potentially used with multifunctions of chromogenic sensing, encryption, and anti-counterfeit.

Applications of Cellulose Nanomaterials in Stimuli-Responsive Optics.

As one of the most abundant biopolymers, cellulose has been a basic but essential building block of human society, with its use dating back thousands of years. With recent developments in nanotechnology and increasing environmental concerns, cellulose-based nanomaterials are now gaining attention as promising green material candidates for many high-value applications as a result of their biocompatibility and advantageous physical and chemical properties. In particular, cellulose nanocrystals are notable for their optical properties that can respond to various environmental stimuli as a result of the unique chiral nematic structure of the material. Compositing cellulosic materials with functional polymers, small molecules, and other nanomaterials can further stabilize and amplify these responsive optical signals and introduce multiple new functionalities. On the basis of these capabilities, many advanced applications of cellulose nanomaterials have been proposed, including chemical sensors, photonic papers, decorative coatings, data security, and smart textiles. In this review, we discuss and summarize recent advances in this emerging field of stimuli-responsive optics based on cellulose nanomaterials.

Adsorption of phosphorus on lanthanum doped carbon films guided by self-assembly of cellulose nanocrystalline

Phosphorus has been an important main reason for water eutrophication. Reducing the concentration of phosphate to prevent eutrophication is one of the crucial goals of sewage engineering and surface water management. For the first time, cellulose nanocrystalline was used as biomass template and tetramethoxysilane as mesoporous template. A highly efficient and selective self-supporting mesoporous composite membrane adsorbent was prepared by one-step method. The film material has a highly ordered chiral nematic phase structure. It adsorbed phosphate well. In batch adsorption test, through lanthanum loading, the adsorbent material can selectively adsorb phosphate, and the adsorption kinetics followed a pseudo second-order model, especially in the case of phosphate. At the same time, cellulose nanocrystalline doped with trace lanthanum have been investigated to achieve high phosphate adsorption performance. The effect of Lanthanum on flocculation of cellulose nanocrystalline was discussed, and the adsorbent material with the best adsorption performance was obtained. The adsorption isotherm Langmuir model of the adsorbent was more representative than the Friedrich model. In addition, the adsorbent was achieved a high adsorption capacity in the range of pH values between 3.0 and 7.0. Phosphate selectivity was also soaring. The mesoporous films adsorbent has the advantages of high cycle times, repeatability and easy separation from solution.