Chiral Nematic Films Research Articles

Optically active polyaniline film based on cellulose nanocrystals

Chiral transfer from cellulose nanocrystals (CNCs) chiral nematic liquid crystal to polyaniline (PANI) is successfully achieved through co-assembly method for the first time, affording the PANI film based on CNCs optical activity. Meanwhile, the CNCs/PANI composite film displays notable Cotton effect ascribed to the polaron band transitions of PANI in circular dichroism (CD) spectrum. Hydrogen bond and electrostatic attraction favor the combination of two molecular chains in the mixed aqueous suspension and will lead to the chiral assembly of the PANI in the host chiral nematic film. However, the CD signal disappears when cholesteric phase collapse by acid protonation, and which demonstrates the chiral nematic ordering of the CNCs is essential to the chiral transfer. With the assistance of CNCs, PANI chains form a helically stacked structure. Thus, the optical activity of PANI originates from its long range organization.

Effect of Anisotropy of Cellulose Nanocrystal Suspensions on Stratification, Domain Structure Formation, and Structural Colors.

Outstanding optical and mechanical properties can be obtained from hierarchical assemblies of nanoparticles. Herein, the formation of helically ordered, chiral nematic films obtained from aqueous suspensions of cellulose nanocrystals (CNCs) were studied as a function of the initial suspension state. Specifically, nanoparticle organization and the structural colors displayed by the resultant dry films were investigated as a function of the anisotropic volume fraction (AVF), which depended on the initial CNC concentration and equilibration time. The development of structural color and the extent of macroscopic stratification were studied by optical and scanning electron microscopy as well as UV–vis spectroscopy. Overall, suspensions above the critical threshold required for formation of liquid crystals resulted in CNC films assembled with longer ranged order, more homogeneous pitches along the cross sections, and narrower specific absorption bands. This effect was more pronounced for the suspensions that were closer to equilibrium prior to drying. Thus, we show that high AVF and more extensive phase separation in CNC suspensions resulted in large, long-range ordered chiral nematic domains in dried films. Additionally, the average CNC aspect ratio and size distribution in the two separated phases were measured and correlated to the formation of structured domains in the dried assemblies.

Iridescent Chiral Nematic Mesoporous Organosilicas with Alkylene Spacers

AbstractMesoporous organosilica films with chiral nematic structures are prepared with a bridging urea group and with alkylene bridges, where the length of the alkylene bridge varies from C1–C6. To synthesize these materials, cellulose nanocrystals (CNCs) are used as liquid crystal templates, which coassemble with the organosilica precursor to give composite materials with a chiral nematic structure of CNCs embedded within. Removal of the CNCs by acid hydrolysis results in free‐standing, mesoporous organosilica films with long‐range chiral nematic ordering of the porous network. It is found that increasing the length of the organosilica bridging groups causes phase separation between the organosilica precursors and the aqueous CNC suspension. This problem is solved through two distinct methods: combining the long alkylene‐bridged precursors with shorter precursors or by using a mixed solvent system of water and dimethylformamide during self‐assembly. The new chiral nematic mesoporous organosilica films display photonic properties, good thermal stabilities, and interconnected pore structures.

Tactoid Annealing Improves Order in Self-Assembled Cellulose Nanocrystal Films with Chiral Nematic Structures.

The self-assembly process in cellulose nanocrystal (CNC) film formation was studied as a function of evaporation time. It is known that the total evaporation time of CNC dispersions affects the structure of the film obtained, but the extension of different phases of the evaporation has not been explored. By extending the evaporation time of CNC suspensions after the onset of liquid crystallinity, the homogeneity of the resulting films could be improved as observed by polarized optical microscopy and scanning electron microscopy. Here, we show that an intermediate stage of self-assembly, between phase separation and gel vitrification, called tactoid annealing, helps explain the discrepancies in order for chiral nematic CNC films dried at varying evaporation times. This intermediate stage of self-assembly may be useful for designing highly ordered and homogenous CNC-based materials.

Order and gelation of cellulose nanocrystal suspensions: an overview of some issues.

Cellulose nanocrystals (CNCs) are polydisperse rod-shaped particles of crystalline cellulose I, typically prepared by sulfuric acid hydrolysis of natural cellulose fibres to give aqueous colloidal suspensions stabilized by sulfate half-ester groups. Sufficiently dilute suspensions are isotropic fluids, but as the concentration of CNC in water is increased, a critical concentration is reached where a spontaneously ordered phase is observed. The (equilibrium) phase separation of the ordered chiral nematic phase is in competition with a tendency of the CNC suspension to form a gel. Qualitatively, factors that reduce the stability of the CNC suspension favour the onset of gelation. The chiral nematic structure is preserved, at least partially, when the suspension dries. Solid chiral nematic films of cellulose are of interest for their optical and templating properties, but the preparation of the films requires improvement. The processes that govern the formation of solid chiral nematic films from CNC suspensions include phase separation, gelation and also the effects of shear on CNC orientation during evaporation. Some insight into these processes is provided by polarized light microscopy, which indicates that the relaxation of shear-induced orientation to give a chiral nematic structure may occur via an intermediate twist-bend state.This article is part of a discussion meeting issue 'New horizons for cellulose nanotechnology'.

Bioinspired Mesoporous Chiral Nematic Graphitic Carbon Nitride Photocatalysts modulated by Polarized Light.

Endowing materials with chirality and exploring the responses of the material under circularly polarized light (CPL) can enable further insight into the physical and chemical properties of the semiconductors to be gained, thus expanding on optoelectronic applications. Herein a bioinspired mesoporous chiral nematic graphitic carbon nitride (g-C3 N4 ) for efficient hydrogen evolution with polarized light modulation based on chiral nematic cellulose nanocrystal films prepared through silica templating is described. The mesoporous nematic chiral g-C3 N4 exhibits an ultrahigh hydrogen evolution rate of 219.9 μmol h-1 (for 20 mg catalyst), corresponding to a high enhancement factor of 55 when compared to the bulk g-C3 N4 under λ>420 nm irradiation. Furthermore, the chiral g-C3 N4 material exhibits unique photocatalytic activity modulated by CPL within the absorption region. This CPL-assisted photocatalytic regulation strategy holds great promise for a wide range of applications including optical devices, asymmetric photocatalysis, and chiral recognition/separation.

Study of humidity-responsive behavior in chiral nematic cellulose nanocrystal films for colorimetric response

This study focuses on the preparation and characterization of an iridescent chiral nematic cellulose nanocrystal (CNC) film in relation to their use as a humidity-responsive material. A free-standing film with Bragg reflection was successfully prepared. Thermogravimetric analysis demonstrated gradual thermal transitions with two major weight loss regions, while the crystallinity index obtained from X-ray diffraction method was about 78%, implying the presence of disordered regions. A scanning electron microscope confirmed the orientation of CNCs in a chiral nematic pattern with a pitch in the wavelengths of visible light. A dynamic vapor sorption study showed that water vapor can penetrate into the CNC film, leading to an increase of overall mass of up to 20% at 95% RH. The moisture sorption and desorption rates of the CNC film were faster at higher humidity (80% RH). The exposure of CNC film to elevated humidity was simulated to investigate the effect of humidity on their optical semaphore. As the humidity increased, the complementary color was red-shifted upward by about 60–100 nm, which was observable to the naked eye. This is potentially explained by the swelling of the nanocrystals and the expansion of the interspace between the quasi-nematic layers of the CNCs.

Study of Langmuir-Blodgett films of chiral nematic liquid crystals prepared by a templating method

The ordered alignment of liquid crystal molecules plays a critical role in the applications of their optical properties. Langmuir-Blodgett (LB) monolayer films of chiral nematic liquid crystal, 4-cyano-4′-pentylbiphenyl(5CB), doped with a proprietary left-handed chiral additive were prepared using stearic acid (SA) as template. The Langmuir isotherms of the chiral nematic liquid crystal at the CdCl2 interface with SA were studied. In the surface pressure curves, there was one sharp peak at the transition from gas phase to liquid phase. The phenomenon is considered to be the phase-separation of liquid crystals from SA. The chiral nematic liquid crystal multilayers LB films were characterized by ultraviolet-visible spectroscopy, X-ray diffraction, and atomic force microscopy. The results show that the chiral nematic liquid crystals were homogeneously and regularly dispersed in the SA template.

Transparent Depolarizing Organic and Inorganic Films for Optics and Sensors.

Fabrication of novel organic and inorganic depolarizing films derived from quasinematic cellulose nanocrystal (CNC) organization is demonstrated. These films convert linearly polarized and circularly polarized light into unpolarized light over the entire visible region. Patterning of the quasinematic CNCs on top of a chiral nematic film gives latent images that are revealed only upon observation through the circularly polarizing filters.

Recent Advances in Chiral Nematic Structure and Iridescent Color of Cellulose Nanocrystal Films.

One unique property of cellulose nanocrystals (CNC) is their property of forming suspensions with chiral nematic order. This order can be preserved in films cast from the suspensions, raising the possibility of applications as photonic materials and templates. However, it has proved difficult to generate uniform, well-ordered chiral nematic materials from CNC. Recently, the importance of kinetic arrest due to gel formation in the later stages of evaporation has been recognized as a key step in film formation. In this brief review, recent developments regarding the structure of chiral nematic suspensions and films as monitored by polarized light microscopy are outlined, and attention is drawn to the importance of shear forces on the self-organization process.

Theory and Simulation of Cholesteric Film Formation Flows of Dilute Collagen Solutions.

Dilute isotropic collagen solutions are usually flow processed into monodomain chiral nematic thin films for obtaining highly ordered materials by a multistep process that starts with complex inhomogeneous flow kinematics. Here we present rigorous theory and simulation of the initial precursors during flow steps in cholesteric collagen film formation. We first extract the molecular shape parameter and rotational diffusivity from previously reported simple shear data of dilute collagen solutions, where the former leads the reactive parameter (tumbling function) which determines the net effect of vorticity and strain rate on the average orientation and where the latter establishes the intensity of strain required for flow-birefringence, both crucial quantities for controlled film formation flow. We find that the tumbling function is similar to those of rod-like lyotropic liquid crystalline polymers and hence it is predicted that they would tumble in the ordered high concentration state leading to flow-induced texturing. The previously reported experimental data is well fitted with rotational diffusivities whose order of magnitude is consistent to those of other biomacromolecules. We then investigate the response of the tensor order parameter to complex flow kinematics, ranging from pure vorticity, through simple shear, to extensional flow, as may arise in typical flow casting and film flows. The chosen control variable to produce precursor cholesteric films is the director or average orientation, since the nematic order is set close to typical values found in concentrated cholesteric type I collagen solutions. Using the efficient four-roll mill kinematics, we summarize the para-nematic structure-flow process diagram in terms of the director orientation and flow type. Using analysis and computation, we provide a parametric envelope that is necessary to eventually produce well-aligned cholesteric films. We conclude that extensional flow is an essential ingredient of well-ordered film precursors with required Deborah numbers on the order of unity.

Hard Photonic Glasses and Corundum Nanostructured Films from Aluminothermic Reduction of Helicoidal Mesoporous Silicas

We have investigated the aluminothermic reduction of photonic chiral nematic mesoporous silica films templated by cellulose nanocrystals. The mechanical hardness and ordered structure of aluminothermally reduced materials are found to be dependent on the surface contact between aluminum and silica reactants. By heating the silica films in close contact with aluminum, the silica films undergo a surface aluminothermic reduction to generate photonic mesoporous glasses with enhanced mechanical hardness. The aluminothermic reduction of the silica is significantly enhanced when the reactants are in direct contact, transforming the helicoidal mesoporous silica films into colorful, stiff alumina/silicon replicas. Iridescent mesoporous films of corundum and corundum/silicon nanostructures are obtained from the alumina/silicon composites by selective etching of impurities. Our findings demonstrate that the aluminothermic reduction of silica is a simple process to improve the mechanical strength of chiroptical mesoporous glasses and is also a new one-step hard templating of hierarchically mesoporous corundum-based nanomaterials.

Free-Standing Optically Switchable Chiral Plasmonic Photonic Crystal Based on Self-Assembled Cellulose Nanorods and Gold Nanoparticles.

Photonic crystals incorporating with plasmonic nanoparticles have recently attracted considerable attention due to their novel optical properties and potential applications in future subwavelength optics, biosensing and data storage device. Here we demonstrate a free-standing chiral plasmonic film composed of entropy-driven self-co-assembly of gold nanoparticles (GNPs) and rod-like cellulose nanocrystals (CNCs). The CNCs-GNPs composite films not only preserve the photonic ordering of the CNCs matrix but also retain the plasmonic resonance of GNPs, leading to a distinct plasmon-induced chiroptical activity and a strong resonant plasmonic-photonic coupling that is confirmed by the stationary and ultrafast transient optical response. Switchable optical activity can be obtained by either varying the incidence angle of lights, or by taking advantage of the responsive feature of the CNCs matrix. Notably, an angle-dependent plasmon resonance in chiral nematic hybrid film has been observed for the first time, which differs drastically from that of the GNPs embed in three-dimensional photonic crystals, revealing a close relation with the structure of the host matrix. The current approach for fabricating device-scale, macroscopic chiral plasmonic materials from abundant CNCs with robust chiral nematic matrix may enable the mass production of functional optical metamaterials.

Circular Dichroism of Chiral Nematic Films of Cellulose Nanocrystals Loaded with Plasmonic Nanoparticles.

In the search for induced chiral plasmonic activity, cholesteric films formed by cellulose nanocrystals have attracted great interest as potential hosts for plasmonic nanoparticles. Circular dichroism (CD) spectra of the composite films exhibit two peaks, one of which is ascribed to the cholesteric host and the other one to plasmonic chiroptical activity of the plasmonic nanoparticles. Here we report the results of comprehensive studies of extinction and CD properties of composite films formed by different types of cellulose nanocrystals and different types of plasmonic nanoparticles. We show that the second peak in the CD spectra acquired using CD spectrometers appears as the result of the local reduction of the CD signal of the host material, due to excessive absorption by the nanoparticles, and thus it cannot be interpreted as induced plasmonic chiroptical activity. Instead, we propose an alternative way to measure CD spectra of plasmonic cholesteric films by using Mueller matrix transmission ellipsometry. The results of this study are important for ongoing research in the field of chiral plasmonics and for the optical characterization of a broad range of chiral nematic nanostructured materials.

Polarized Phosphorescence of Isotropic and Metal‐Based Clustomesogens Dispersed into Chiral Nematic Liquid Crystalline Films

Polarized Phosphorescence of Isotropic and Metal‐Based Clustomesogens Dispersed into Chiral Nematic Liquid Crystalline Films

Photoinduced polymer-stabilised chiral nematic liquid crystal films reflecting both right- and left-circularly polarised light

A phototunable polymer-stabilised liquid crystal film reflecting both right- and left-circularly polarised light has been successfully fabricated by a washout/refill method. The film was obtained by prefabricating the polymer network with a left-handed helical structure and then refilling a light-driven chiral nematic liquid crystal with a right-handed helical structure into the network. Interestingly, the reflection wavelength and the reflectivity properties can be precisely tuned by UV-light irradiation. A hyper-reflective notch, the reflectivity of which approaches 100%, was achieved in a layer when the pitch lengths of the two opposite helical structures are almost the same. Moreover, their original state can be properly returned by visible-light irradiation.

Electrically induced red, green, and blue scattering in chiral-nematic thin films.

Cholesteric liquid-crystalline materials are abundant in nature such as condensed phases of DNA, plant cell walls, and chiral biopolymers. These self-organized helical structures produce unique optical properties, giving rise to the selective Bragg reflection of colorful light. In this Letter, we focus on the focal conic state of cholesteric liquid crystals and report on stable, tunable, and reversible color switching among red, green, and blue in polymer-stabilized cholesteric films. The experimental results indicate that, with appropriate voltage pulses, the electrically induced color switching of all six routes can be realized in a single cell reflecting green light. The scattered transmissive color persists at zero voltage due to the polymer stabilization.

Bionanocomposite Films from Resilin-CBD Bound to Cellulose Nanocrystals

This research explores the properties of bionanocomposite films prepared by binding recombinant resilin-like protein (res) consisting of the exon 1 resilin sequence from Drosophila melanogaster, engineered to include a cellulose binding domain (CBD), to cellulose nanocrystals (CNCs). The optimal binding of res-CBD to CNCs was 1:5 by mass, and the resulting res-CBD-CNCs remained colloidally stable in water. Res-CBD-CNCs were solventcast into transparent, free-standing films, which were more hydrophobic than neat CNC films, with water contact angles of 70–80° compared to 35–40° for the latter. In contrast to the multi-domain orientation typical of chiral nematic CNC films, res-CBD-CNC and CBD-CNC films exhibited long-range, uniaxial orientation that was apparently driven by the CBD moiety. Glycerol was studied as an additive in the films to determine whether the addition of a wet component to solvate the recombinant protein improved the mechanical properties of the res-CBD-CNC films. In comparison to the other films, res-CBD-CNC films were more elastic with added glycerol, in the range of 0.5–5 wt% (i.e., the films responded more elastically to a given strain and/or were less plastically deformed by a given mechanical load), but became less elastic at 25 wt% glycerol. Overall, films made of res-CBD-CNCs plus 0.5 wt% glycerol displayed improved mechanical properties compared to neat CNC films, with an increase in toughness of 150% and in elasticity of 100%.

Chiral electronic transitions of YVO4:Eu3+ nanoparticles in cellulose based photonic materials with circularly polarized excitation

Hierarchical chiroptical activity of emissive YVO4:Eu3+ nanoparticles in chiral nematic cellulose host matrix.

Tuning the iridescence of chiral nematic cellulose nanocrystal films with a vacuum-assisted self-assembly technique.

Iridescent films composed of the chiral nematic liquid crystal phase of cellulose nanocrystals (CNC) have attracted significant interest due to their fascinating optical properties. However, the current fabrication method, i.e., solution casting with a subsequent evaporation process, has significant limitations and therefore hinders the application of CNC iridescent films. In the present study, we demonstrate, for the first time, that vacuum-assisted self-assembly (VASA) can be used to fabricate highly oriented, large area, smooth, and structurally homogeneous CNC iridescent films. It was found that a long ultrasonic pretreatment is necessary for obtaining CNC iridescent films via VASA. Furthermore, it was also found that the iridescent color of the CNC films can be tuned by the sonication time, suspension volume, and degree of vacuum. By combining CD spectroscopy, SEM, and WAXD techniques, the internal structure of CNC iridescent films prepared by VASA has been investigated in detail. Moreover, the origin of the ultrasonic pretreatment effect on the self-assembly behavior of CNCs is also discussed.