Cholesteric Liquid Crystalline Phase Research Articles

Fabrication of Polymer/Cholesteric Liquid Crystal Films and Fibers Using the Nonsolvent and Phase Separation Method.

In recent years, liquid crystal materials have drawn great interest because of their wide range of applications. Among various thermochromic materials, cholesteric liquid crystalline (CLC) materials have been well studied and reported. CLC materials have the advantages of ready manipulation and multiple color transitions. For the further development of smart clothing and wearable electronics, however, the incorporation of CLC materials into polymers still remains challenging. The difficulties lie in the prevention of leakage of CLC and retention of the cholesteric liquid crystalline phase. In this work, we demonstrate a versatile nonsolvent and phase separation method using polar solvents to incorporate CLC microspheres into polymer matrix. Poly(vinyl alcohol) (PVA), a water-soluble polymer, is chosen as the polymer because of its high transparency and ease to handle. Using spin-coating and wet spinning techniques, PVA/CLC films and fibers can be fabricated. The formation of CLC microspheres in the polymer matrix is characterized through optical and polarized microscopy. Compared with the CLC films, the PVA/CLC composites demonstrate superior thermal stability. Moreover, both PVA/CLC films and fibers exhibit good color stability from the electrical tests. This work provides an effective strategy to prepare polymer/CLC composites, paving a wide avenue toward applications in smart textiles, display technologies, and medical devices.

Pressure/Temperature Dual‐Responsive Cellulose Nanocrystal Hydrogels for On‐Demand Schemochrome Patterning

AbstractCellulose nanocrystal (CNC) based optical devices with adjustable schemochrome have attracted immense interest. However, most of the previously reported structural colored CNC‐based materials can only achieve simple stress‐induced color change, which have difficulty achieving multimode control of complex patterning that can be accurately identified. Here, inspired by the nanostructure‐based color‐changing mechanism of neon tetra, this study presents a pressure/temperature dual‐responsive CNC‐based schemochrome hydrogel with adjustable dynamic chiral nematic structure. By incorporating abundant interfacial noncovalent interactions, dynamic correlations between adjustable helical pitch of the vertically stacked cholesteric liquid crystalline (LC) phase and responsiveness of flexible thermosensitive substrate are established, which further enable wide‐range optical characteristic (12°–213° in HSV color model and 421–734 nm in the UV–Vis spectra) and identifiable visualized patterning. The resultant hydrogels are applied in proof‐of‐concept demonstrations of on‐demand schemochrome patterning, including customizable patterned dual‐encryption label, smart digital display, temperature monitor, and intelligent recognition/control system. This study envisages that the bioinspired construction of structural colored nanomaterials will have promising applications in smart responsive photonic equipment including smart display, anticounterfeiting, and intelligent control systems.

Achieving a full color palette with thickness, temperature, and humidity in cholesteric hydroxypropyl cellulose

Hydroxypropyl cellulose (HPC) is a sustainable, cost-efficient, and bio-compatible cellulose derivative that forms cholesteric liquid crystalline phases in highlyconcentrated water solutions that reflects colour in the visible range. While there have been studies exploiting HPC’s structural coloration and transferring the cholesteric order of the solutions into solid form via cross-linking, there is still lack of understanding on the thermotropic mechanisms that enable the transfer of the structural ordering of the pure HPC at higher temperatures. In this work, we demonstrate the balance between the temperature, humidity, and film thickness to achieve a full color palette of pure HPC. We reveal that at the early stages of the evaporation, formation of a dense skin over the lyotropic phase facilitates the thermal expansion of the HPC during the heat treatment. Increasing the thickness, applying higher drying temperatures, and exposing the samples to higher humidity during the evaporation all result with increased pitch values that cause a red-shift in coloration in the solid state. Our analysis of the HPC samples dried in controlled temperature and humidity conditions at a fixed thickness provided an understanding of the dominance of the thermal expansion which drives the final structural organization in the solid cholesteric phase. When the thickness of the films was varied against fixed temperature and humidity conditions, the color shift from red to violet follows the thickness gradient of the sample due to the change in the drying time required to reach the solid form.

Optical signals of uniaxial and biaxial cholesteric lyotropic phases

Lyotropic cholesteric liquid crystalline phases were prepared by doping the ternary mixture of potassium laurate (KL)/decanol (DeOH)/ deuterium oxide (D2O) with brucine sulphate, in two different concentrations. Using the variation of the average color in a heating cycle, with and without the insertion of compensator a gypsum plate (1 λ), in the light path, the optical signal of the cholesteric phases ChD-(ChC+) was confirmed as being negative(positive). When the biaxial cholesteric phase (ChB) is between two uniaxial phases (ChD-ChB-ChD) or (ChD-ChB-ChC), the sign of the ChB phase depends on the uniaxial phase in its vicinity. In the case (ChD--ChB--ChD-), the phase ChB has a negative optical signal (ChB-) across the domain. However, if the ChB phase is between uniaxial cholesteric phases of opposite optical signals, it cannot have just one signal, but must have a change at some point in the biaxial domain showing (ChD--ChB--ChB+-ChC+) phase sequence.

Cholesteric Liquid Crystalline Polyether with Broad Tunable Circularly Polarized Luminescence.

Strong circularly polarized luminescence (CPL) with high purity and broad tunability was achieved in a new type of polyether-based cholesteric liquid crystalline (CLC) copolymers comprising chiral cholesteryl, nematic mesogens, and cross-linkable moieties. The phase boundary diagram of the copolymers was constructed, wherein the CLC phase in a wide composition and temperature window down to room temperature was achieved. Furthermore, reflection colors across the infrared and visible light regions can be continuously tuned by altering composition or temperature, which can be further fixed in the flexible CLC elastomer by photo-cross-linking. Introducing achiral dyes in the CLC thin films can generate strong CPL with distinct handedness and high dissymmetry factors (glum). Particularly, the left-handed full-color CPL is obtained by selective circularly polarized scattering in the spectral region outside the band gap of the CLC thin film, and the right-handed CPL with glum up to -1.05 is achieved within the band gap of the CLC thin film following the selective circularly polarized reflection mechanism. This type of CPL active material is expected to have potential applications in liquid crystal display and photonics.

Hydroxypropylcellulose-based liquid crystal materials

Hydroxypropyl cellulose is regarded distinctive cellulose derivative due to its unique properties and could display a lyotropic cholesteric liquid-crystalline phase in a variety solvents. This work deals with studying the role of cellulose- and carbon-based nanoparticles on liquid crystal performance of modified hydroxypropyl cellulose (HPC) in lyotropic suspension and nanocomposite films in a comparison study. The lyotropic suspension of HPC and HPC-nanocomposites were characterized by rheological study and optical properties. The Fourier Transformed Infrared Spectroscopy (FTIR), X‐ray Diffractometry (XRD), Thermogravimetric Analysis (TGA), mechanical analysis, scanning electron microscope (SEM) and optical properties were the techniques used to characterize the nanocomposite films. The results showed that, introducing the cellulose and carbon nanoparticles lowered the critical concentration of lyotropic suspension of HPC from 30 wt.% to 26–28 wt.% and earlier appearance of anisotropic phases was occurred which evidenced by the rheological curves and optical properties. The FTIR, XRD and TGA of nanocomposite films showed no change in backbone chemical structure of HPC but the incorporation of nanoparticles, especially cellulose-based NPs, enhanced the crystallinity, thermal stability behaviour and mechanical properties. The behaviour of carbon-NPs in HPC film was unaccepted, where it deteriorated the mechanical properties. The nanocomposite films displayed good fingerprint texture (especially with cellulose nanocrystals) with moderated defined morphologies that promote the liquid crystal nanocomposite development.

Responsive Photonic Liquid Marbles.

Liquid marbles have potential to serve as mini‐reactors for fabricating new materials, but this has been exploited little and mostly for conventional chemical reactions. Here, we uncover the unparalleled capability of liquid marbles to act as platforms for controlling the self‐assembly of a bio‐derived polymer, hydroxypropyl cellulose, into a cholesteric liquid crystalline phase showing structural coloration by Bragg reflection. By adjusting the cholesteric pitch via quantitative water extraction, we achieve liquid marbles that we can tailor for structural color anywhere in the visible range. Liquid marbles respond with color change that can be detected by eye, to changes in temperature, exposure to toxic chemicals and mechanical deformation. Our concept demonstrates the advantages of using liquid marbles as a miniature platform for controlling the liquid crystal self‐assembly of bio‐derived polymers, and their exploitation to fabricate sustainable, responsive soft photonic objects.

Chiral Photonic Liquid Crystalline Polyethers with Widely Tunable Helical Superstructures.

Liquid crystalline polymers with tunable structures on the scale of visible wavelength are important in optical technology due to their enhanced mechanical stability, processability, and structural integrity. Herein, we report a series of cholesteric liquid crystalline (CLC) polyethers with a widely tunable pitch length and a broad CLC phase window through a bottom-up structural design. The well-defined multicomponent polyethers were successfully synthesized by utilizing monomer-activated anionic ring-opening polymerization. Through adjustment of the composition of chiral cholesteryl (Ch) and photochromic azobenzene (Az) mesogenic moieties, rich phase behaviors have been discovered, and a phase boundary diagram was constructed consequently, wherein cholesteric helical superstructures in a broad composition range and temperature window straight down to the glassy state at room temperature were achieved. Particularly, the planar oriented helical superstructures can exhibit widely tunable and switchable reflections over the entire visible range across red, green, and blue colors through temperature and light control, which are closely related to the extraordinary flexibility of the polyether backbone. Their thermo-light dual-responsive properties provide an alternative opportunity to fabricate smart and switchable polymeric LC materials for optical applications.

Electrically Switchable Amplified Spontaneous Emission from Liquid Crystalline Phase of an AIEE‐Active ESIPT Molecule

AbstractUse of organic molecules as lasing media has much potential to develop next‐generation optical devices as soft‐matter photonics with wideband tunability and large coherence area. Although mirrorless lasing was theoretically predicted and practically demonstrated in helical cholesteric liquid crystalline (LC) phases of organic compounds, recent studies on optical confinement have been much focused into hard‐crystalline phases of the molecules. Aggregation‐induced emission (AIE) and enhancement (AIEE) provides one of the optimal molecular systems for light amplification in condensed phases, and herein AIEE activity and excited‐state intramolecular proton transfer (ESIPT) are successfully coupled in a room temperature (RT) nematic LC. Suppressing the effect of concentration quenching/self‐absorption and attaining four‐level system for population inversion by the combination of AIEE and ESIPT in LC phases, lead to the amplified spontaneous emission (ASE) from a newly designed molecule: C5Ph‐HBT dispersed in RT LC matrix with the pumping energy threshold of 20 mJ cm–2. Moreover, the nematic LC allows the orientation of the molecular dipoles in response to external electric fields. Hence, the fluorescence as well as the ASE switching is attained at RT, demonstrating the potential of these composite materials as “switchable” ASE media for technological progress.

Redox-Responsive Chiral Dopant for Quick Electrochemical Color Modulation of Cholesteric Liquid Crystal.

Here, we report the first redox-active chiral dopant FcD, which electrically alters its helical twisting power (HTP) for a cholesteric liquid crystalline (LC) medium and quickly changes the reflection color. FcD is composed of an axially chiral binaphthyl unit in conjunction with a redox-active ferrocene unit. A cholesteric LC phase of 4'-pentyloxy-4-cyanobiphenyl, doped with FcD (3.0 mol %), developed a blue reflection color. When nitrosyl tetrafluoroborate, a one-electron oxidant, was added to this cholesteric LC phase, FcD was oxidized to decrease its original HTP value by 13%, so that a green reflection color was developed. In the presence of a supporting electrolyte, the reflection color was electrochemically modulated using a sandwich-type glass cell with indium tin oxide electrodes. In quick response to the applied voltage of +1.5 V, the reflection color changed from blue to green within 0.4 s. When 0 V was applied, the reflection color returned to its original blue color. The FcD-doped cholesteric LC is characterized by its fastest electrochemical response and lowest operating voltage among those reported for electrically driven cholesteric LC devices.

Revisiting (hydroxypropyl) cellulose (HPC)/water liquid crystalline system

ABSTRACTThe discovery of a cholesteric liquid crystalline phase in concentrated aqueous solutions of (hydroxypropyl)cellulose (HPC) dates back to the 1970s. Due to its biocompatibility, low cost and ease of conversion to fibres and films, HPC is still the target of numerous studies. In this article, some of the properties of the HPC/water lyotropic system are reviewed, and original results describing a humidity-generated instability on HPC fibres are presented. The use of HPC films which respond to external stimuli is highlighted. In particular, novel applications include the manufacture of soft motors driven by a difference of humidity, and the production of electro-optical cells when the solid films are used in conjunction with low molecular mass thermotropic mesogens.

(Invited) Multifunctional Materials from Dispersions of Single-Walled Carbon Nanotubes and Biomolecules

Dispersions of single-walled carbon nanotubes (SWNT) and biological molecules can enable multifunctional materials that combine SWNTs’ intrinsic mechanical and/or electrical properties with the properties of the biomolecule. For example, functionalization of SWNT and organic phosphate hydrolase can be used for pesticide detection, and dispersions of SWNT and laccase can be used to make anodes for electrochemical fuel cells. Some of the most intriguing biological SWNT dispersants are double stranded DNA (dsDNA) and enzymes and protein digests containing tryptophan residues. For example, dilute dispersions of dsDNA/SWNT and lysozyme/SWNT have been used to produce conductive antimicrobial films using layer-by layer assembly. Higher concentration dsDNA/SWNT dispersions form either cholesteric or nematic liquid crystalline phases which can be processed into tightly packed aligned films with controlled optical properties. Higher concentration lysozyme/SWNT dispersions can be used to produce mechanically robust antimicrobial films and properties. Finally, dispersion of SWNT in tryptic soy broth (TSB) containing casein digest has recently been shown to enable direct investigation of SWNTs’ microbiological properties in standard biological growth media.

Anion Bridging-Induced Structural Transformation of Cellulose Dissolved in Ionic Liquid

We performed structural investigations of cellulose mixed with 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) in the entire concentration range (0-100 mol %) by wide-angle X-ray scattering with the aid of quantum chemical calculations and 13C solid-state NMR spectroscopy. We particularly focused on a highly concentrated region (≥30 mol %), which has previously been overlooked. At concentrations of 15-30 mol %, a periodic peak corresponding to cellulose chain alignment emerged; this is associated with a lyotropic cholesteric liquid-crystalline phase. At concentrations of ≥30 mol %, the structure is transformed into ordered layers where OAc anions and Emim cations intercalate. This transformation is found to be driven by a change in the interaction between the IL anions and the OH groups of cellulose. At low concentrations, the anion mainly interacts with the OH group of cellulose in a 1:1 ratio, as previously reported; at high concentrations, the anions bridge the OH groups of two cellulose chains.

Ordering of double-stranded DNA molecules in a cholesteric liquid-crystalline phase and in dispersion particles of this phase

Ordering of double-stranded DNA molecules in a cholesteric liquid-crystalline phase and in dispersion particles of this phase

Rheology of lyotropic cholesteric liquid crystal forming single-wall carbon nanotube dispersions stabilized by double-stranded DNA

The rheology of lyotropic cholesteric liquid crystalline phase forming double-stranded DNA (dsDNA) stabilized single-wall carbon nanotube (SWCNT) dispersions was analyzed over a wide range of SWCNT concentrations. In the semidilute regime, supernatant dispersions of dsDNA-SWCNTs showed the characteristic rheological behavior of semidilute solutions of rods predicted by the Doi-Edwards theory. These cholesterogenic liquid crystal dispersions exhibited some of the key rheological behaviors associated with lyotropic liquid crystalline polymers (LLCPs) including a non-monotonic relationship in the viscosity and moduli versus concentration curves, and the empirical Cox-Merz not being obeyed. However, the dsDNA-SWCNT viscosity versus shear rate curves exhibited an inflection point instead of the three distinct regions associated with many LLCPs and nanocylinder dispersions. This could be related to orientation defects and inherent secondary interactions between the dsDNA and the SWCNTs. Linear viscoelasticity measurements also showed similarities to other lyotropic systems.

Structure and transformation of tactoids in cellulose nanocrystal suspensions

Cellulose nanocrystals obtained from natural sources are of great interest for many applications. In water, cellulose nanocrystals form a liquid crystalline phase whose hierarchical structure is retained in solid films after drying. Although tactoids, one of the most primitive components of liquid crystals, are thought to have a significant role in the evolution of this phase, they have evaded structural study of their internal organization. Here we report the capture of cellulose nanocrystal tactoids in a polymer matrix. This method allows us to visualize, for the first time, the arrangement of cellulose nanocrystals within individual tactoids by electron microscopy. Furthermore, we can follow the structural evolution of the liquid crystalline phase from tactoids to iridescent-layered films. Our insights into the early nucleation events of cellulose nanocrystals give important information about the growth of cholesteric liquid crystalline phases, especially for cellulose nanocrystals, and are crucial for preparing photonics-quality films.

О пространственной организации двухцепочечных молекул ДНК в холестерической жидкокристаллической фазе

О пространственной организации двухцепочечных молекул ДНК в холестерической жидкокристаллической фазе

Ordering of double-stranded DNA molecules in a cholesteric liquid-crystalline phase and in dispersion particles of this phase

The current notion of the organization of molecules in a cholesteric phase is fairly well substantiated in the case of low-molecular-weight compounds. However, this question is open to discussion in the case of double-stranded nucleic acids. In this work, an attempt to compare the well-known data on the structure of cholesteric phases formed by double-stranded DNA molecules and the results of experimental modeling obtained by the authors has been undertaken. The comparison brings leads to assumption regarding the high probability of the existence of both short-range (positional) and long-range (orientational) order in the arrangement of double-stranded DNA molecules in the liquid crystalline phase. The presence of the orientational order, i.e., the rotation of quasinematic layers of double-stranded DNA molecules through a small angle, determines the formation of a spatially twisted (cholesteric) structure with specific physical and chemical properties. In addition, these results prompt a suggestion on the mode of the ordering of dsDNA molecules in liquid-crystalline dispersion particles and allow these particles to be considered candidate biosensing units.

About the spatial organization of double-stranded DNA molecules in the cholesteric liquid-crystalline phase and dispersion particles of this phase

The answer to a question on the organization of molecules in a cholesteric phase is well enough proved in case of low molecular mass compounds. However, in case of double-stranded nucleic acids molecules the unequivocal answer to such question is a subject of discussions. In this work an attempt to generalize the well known literary data on the structure of the cholesteric phase formed by double-stranded DNA molecules was undertaken. Besides the experimental results of authors describing the packing of these molecules in the cholesteric liquid-crystalline dispersion particles are added to these data. Comparison of the results obtained offers the possibility to come out with an assumption of high probability of the existence of both the short-range positional and long-range orientational order in arrangement of double-stranded DNA molecules in a liquid-crystalline phase, and in the particles of dispersions of this phase generated under certain conditions. The occurrence of the orientational order, i.e. rotation of 'quasinematic' layers of double-stranded DNA molecules by a small angle, defines the formation of spatially twisted (cholesteric) structure with characteristic for it physical and chemical properties.

Chiroptical properties of cholesteric liquid crystals of chitosan phenylcarbamate in ionic liquids

Chitosan phenylcarbamate (CtsPC) samples were synthesized with different degrees of substitution (DS) ranging from 2.3 to 3.5, and their lyotropic liquid crystallinity in ionic liquids was investigated. 1-Ethyl-3-methylimidazolium dicyanamide ([C2Mim][N(CN)2]) was found to be a novel solvent for CtsPC, and the concentrated lyotropic system formed a cholesteric liquid-crystalline phase and mostly exhibited vivid colorations owing to selective reflection of visible light. Ultraviolet–visible–near-infrared spectroscopy and wide-angle X-ray diffractometry were used to evaluate the maximum wavelength of selective light reflection and the distance and rotational difference between adjacent nematic thin layers in the cholesteric mesophase, respectively. The cholesteric helical pitch increased with decreasing polymer concentration, increasing temperature or increasing DS. The helical sense remained right-handed under the adopted measurement conditions, although CtsPCs are known to form a left-handed cholesteric structure in polar organic solvents, such as N,N-dimethylformamide (DMF). Accordingly, an inversion of the cholesteric sense was achieved by combination of [C2Mim][N(CN)2] and DMF as solvent; circular dichroism and optical rotatory dispersion spectroscopy verified that the sense of CtsPC cholesterics changed from left-handed to right-handed with increasing [C2Mim][N(CN)2] content in the mixed solvent. Chitosan phenylcarbamate (CtsPC) samples were synthesized and their lyotropic liquid crystallinity in ionic liquids was investigated. 1-Ethyl-3-methylimidazolium dicyanamide ([C2Mim][N(CN)2]) was found to be a novel solvent for CtsPC, and the lyotropic system formed a cholesteric liquid-crystalline phase and exhibited vivid colorations owing to selective light reflection. The cholesteric helical pitch was controllable by polymer concentration, temperature or the degree of phenylcarbamoyl substitution. Although the helical sense of CtsPC/[C2Mim][N(CN)2] cholesterics remained right-handed under the adopted measurement conditions, an inversion of cholesteric sense was achieved by using a mixed solvent of [C2Mim][N(CN)2]/N,N-dimethylformamide.