Dispersed Liquid Crystal Research Articles

Preparing a liquid crystalline dispersion of carbon nanotubes with high aspect ratio.

We successfully prepared a surfactant-assisted carbon nanotube (CNT) liquid crystal (LC) dispersion with double-walled CNTs (DWCNTs) having a high aspect ratio (≈1378). Compared to dispersions of single-walled CNTs (SWCNTs) with lower aspect ratio, the transition concentrations from isotropic phase to biphasic state, and from biphasic state to nematic phase are lowered, which is consistent with the predictions of the Onsager theory. An aligned DWCNT film was prepared from the DWCNT dispersion by a simple bar-coating method. Regardless of the higher aspect ratio, the order parameter obtained from the film is comparable to that from SWCNTs with lower aspect ratios. This finding implies that precise control of the film formation process, including a proper selection of substrate and deposition/drying steps, is crucial to maximize the CNT-LC utilization.

Effect of rotational shear on the dielectric dispersion of a nematic liquid crystal above the Freedericksz threshold field.

Rheo-dielectric studies of soft materials provide important information on the dynamic structure and electric polarization. We study the dielectric dispersion of a nematic liquid crystal by applying a high AC probe field without a DC bias and a low AC probe field with a high DC bias under steady rotational shear. The dielectric anisotropy of the nematic is positive and the applied electric field is parallel to the velocity gradient with a magnitude larger than the Freedericksz threshold field. We find that the dielectric dispersion and the relaxation frequencies are strongly shear rate dependent. The analysis of the results based on a simple physical model shows that the effective dielectric constant of the nematic with non-uniform director tilt in the shear plane can be modelled as a series combination of parallel and perpendicular components. Our experiments demonstrate changes in dielectric dispersion are due to molecular reorientation under the influence of the competing effects of hydrodynamic and dielectric torques.

Tuning Molecular Orientation Responses of Microfluidic Liquid Crystal Dispersions to Colloid and Polymer Flows.

An important approach to molecular diagnostics is integrating organized substances that provide complex molecular level responses to introduced chemical and biological agents with conditions that optimize and distinguish such responses. In this respect, liquid crystal dispersions are attractive components of molecular diagnostic tools. This paper analyzes a colloid system, containing a nematic liquid crystal as a dispersed phase, and aqueous surfactant and polymer solutions as the continuous phases. We applied a microfluidic approach for tuning orientation of liquid crystal molecules in picoliter droplets immobilized on microchannel walls. Introduction of surfactant to the aqueous phase was found to proportionally increase the order parameter of liquid crystal molecules in microdroplets. Infusion of polymer solutions into surfactant-mediated microfluidic liquid crystal dispersions increased the order parameter at much lower surfactant concentrations, while further infusion of surfactant solutions randomized the orientation of liquid crystal molecules. These effects were correlated with the adsorption of surfactant molecules on surfaces of microdroplets, stabilizing the effect of a polymer matrix on bound surfactant ions and the formation of insoluble polymer-colloid aggregates, respectively. The revealed molecular behavior of liquid crystal dispersions may contribute to optimized synthesis of responsive liquid crystal dispersions for in-flow molecular diagnostics of polymers and colloids, and the development of functional laboratory-on-chip prototypes.

Size effect of submicron barium titanate particles on dielectric properties of smectic A liquid crystal 4-nitrophenyl 4‘-decyloxy benzoate

ABSTRACTThe influence of barium titanate (BaTiO3) particles with sizes of 100, 200 and 400 nm on the dielectric properties of smectic A liquid crystal (LC) 4-nitrophenyl 4‘-decyloxy benzoate (10 NF) was investigated by low-frequency dielectric spectroscopy. It is shown that BaTiO3 particles increase the longitudinal component of the dielectric permittivity of the LC more strongly than the transversal component. In other words, the anisotropy of dielectric permittivity is increased, which is crucial for improving the performance of electro-optic effect in LC. This is the result of the coupling of the polar axis of ferroelectric particles with the director at the moment of formation of the smectic A phase from the isotropic phase. The effect is maximal for particles with a size of 200 nm, which may be related to the monodomain structure of these particles. BaTiO3 particles also affect the dielectric dispersion of LCs, which can be explained on the basis of the local field concept.

Luminescent DNA-origami nano-rods dispersed in a lyotropic chromonic liquid crystal

ABSTRACTAnisometric DNA nanoparticles with precisely tailored size and shape are prepared by three-dimensional folding of DNA – DNA origami – marked with a fluorescent intercalating dye (YOYO-1), stabilised with silica, and dispersed in a lyotropic chromonic liquid crystal (an aqueous solution of disodium cromoglycate). Observations by total internal reflection fluorescence microscopy indicate that the luminescent nanoparticles are stable in the liquid crystal dispersion and accumulate at the positions of point defects in the liquid crystal director field. The observations indicate that silica-stabilisation of the nanoparticles prevents the dye from leaking into the liquid crystal host and that the luminescent particles assemble at disclinations.

Supramolecular design of CO2-responsive lipid nanomaterials

HypothesisStimuli-responsive materials can innovate in various fields, including food and pharmaceutical sciences. Their response to a specific stimulus can be utilized to release loaded bioactive molecules or sense their presence. The biocompatibility and abundance of CO2 in the environment make it an exciting stimulus for such applications. We hypothesize the formation of CO2-responsive self-assemblies of oleyl-amidine in water. Their integration into glycerol-monooleate-based (GMO) dispersions is further thought to form CO2-switchable liquid crystalline nanoparticles. The switch from an non-charged acetamidine surfactant to its cationic amidinium form triggers curvature changes that ultimately induces phase transitions. ExperimentsThe CO2-switchable lipid (E)-N,N-dimethyl-N-((Z)-octadec-9-en1-yl)acetimidamide (OAm) is synthesized and formulated into emulsions and dispersed liquid crystals with GMO. The supramolecular structure and its response to CO2 are characterized using small angle X-ray scattering, dynamic light scattering, ζ-potential measurements and cryogenic transmission electron microscopy. FindingsDepending on the composition, OAm is discovered to self-assemble into a variety of CO2-responsive lyotropic liquid crystalline structures that can be dispersed in excess water. CO2-triggered colloidal transformations from unstructured OAm-in-water emulsions to direct micelles; dispersed inverse hexagonal phase to direct rod-like micelles, and sponge phase to vesicles are discovered. These structural changes are driven by the reaction of OAm’s amidine headgroup with CO2. The results provide a fundamental understanding of CO2-triggered functional nanomaterials and may guide their future design into delivery platforms and biosensors.

New Ceramide Capsule: Development of Ceramide-Containing Reversed Hexagonal Liquid Crystal Dispersion (Cer-Hexosome) and Evaluation of Its Effect on Skin Permeability and Barrier Function

New Ceramide Capsule: Development of Ceramide-Containing Reversed Hexagonal Liquid Crystal Dispersion (Cer-Hexosome) and Evaluation of Its Effect on Skin Permeability and Barrier Function

Development and Prospect of Viewing Angle Switchable Liquid Crystal Devices

As we move from the industrial age to the information age, nowadays, the opportunity to access personal information in public increases as personal computers (PCs), cell phones, automated teller machines (ATM) and other portable display devices have come into wider use, so it is well suited for these liquid crystal displays (LCDs) to switch between wide viewing angle (WVA) (share mode) and narrow viewing angle (NVA) (privacy mode). In this review, we have summarized structures, principles and characteristics of several devices that show great potential application in controllable anti-peeping displays in the eyesight of materials consist of pure liquid crystals (LCs), polymer dispersed LCs (PDLCs), polymer stabilized LCs (PSLCs) or polymer network LCs (PNLCs) and non-LCs, which provides systematic information for next-generation viewing angle-controllable LCDs with lower operating voltage, higher transmittance and good viewing angle controllable characteristics. Because LCs/polymer composite films have the advantages of long life, low power consumption and energy saving, they are regarded as the mainstream technology of next-generation viewing angle controllable displays.

Nematic twist-bend phase of a bent liquid crystal dimer: field-induced deformations of the helical structure and macroscopic polarization

The twist-bend nematic (Ntb) phase is a recent addition to the family of nematic (N) phases of liquid crystals (LCs). A net polar order in the Ntb phase under an external electric field is interesting and it was predicted in several recent theoretical studies. We investigated the field-induced polarization behaviour, dielectric, and electro-optic properties of a bent LC dimer CB7CB in the N and Ntb phases. A threshold-dependent polarization current response was obtained in both the phases under triangular and square-wave input electric fields, existing till frequencies as high as 150 Hz. The polarization switching times were found in ∼1 ms region, especially in the N phase. In the Ntb phase, electric field-induced deformation of the helical structure was observed, like ferroelectric LCs. Dielectric measurements revealed the presence of cybotactic clusters via collective relaxations. The dielectric anisotropy (Δϵ) is negative at the frequencies of polarization measurements. The net polarization resulted from field-induced reorientation of cybotactic clusters and additionally from the field-induced deformation of helical structures in the Ntb phase. We explored the possibility of ionic contributions to the net polarization by synthesizing TiO2 nanoparticles (NPs) dispersed CB7CB LC nanocomposite. Incorporation of the NPs resulted in reduction of the collective order, increase in the ionic impurity content and conductivity, but an extinction of the field-induced polarization response. Our results demonstrate that the net polarization has competing contributions from both ferroelectric-like and ionic origin (up to ∼10 Hz) in the LC phases, but it becomes dominantly ferroelectric-like at higher frequencies.

Exploration of nanofibre/nanoparticle/PDLC composite system

ABSTRACT The electro-optical properties of the nanofibre/nanoparticle/polymer dispersed liquid crystals (PDLC) composite system were investigated. The nanofiber network was prepared by electrostatic spinning and introduced into the PDLC system as a carrier of nanoparticles. The effects of nanofibers and the doping of CeO2 nanoparticles on the PDLC system were investigated. It was found that the nanofibers had a significant impact on the response time of PDLC, while the nanoparticles could improve the contrast of PDLC very well. The fiber morphology, and the polymer mesh morphology, were characterised by SEM; the distribution of nanoparticles on the fiber was characterized by EDS. The experimental results show that nanofiber/nanoparticle doping can optimise the performance of PDLC and achieve low response time as well as high contrast in display applications.

Carbon Nanotube Liquid Crystal Dispersion for Large-Area, Micropatterned Aligned Films

Aligned single-walled carbon nanotube (SWCNT) films are potential materials for devices with excellent electrical and optical properties. However, the fabrication of well-aligned SWCNT films is limited, along with practical issues such as the area size and mass production. In this study, we successfully developed a method to fabricate SWCNT films with high alignment by applying an SWCNT dispersion to a substrate using a general wet-coating technique. The prepared SWCNT film showed a high degree of orientation with a nematic order parameter of 0.86 and 0.56 based on Raman spectroscopy and near-infrared (NIR) absorption spectroscopy, respectively. An orientation film several tens of centimeters in length can be prepared on polyethylene terephthalate (PET) film by wet-coating and drying. Furthermore, using the photoresponsive property of the dispersant, micrometer-scale patterning is also possible. The proposed method paves the way for the development of SWCNT-aligned devices for electronic and optical applications, such as the NIR/infrared polarizer and the detector and the electrode.

Enhanced redox property of polymer blends containing liquid crystalline molecules and their application in electrochemical sensing

A series of thermoplastic polyester elastomer (TPEE) derivatives based on recycled polyethylene terephthalate (r-PET) and poly(tetramethylene glycol) together with differing amounts (n%, n = 0, 1, 3, and 5) of 1,6-hexanediamine (HDA) were prepared. Blending of the copolymer with a rod-like liquid crystal (4-cyano-3-fluorophenyl 4-ethylbenzoate (4CFE)), named TPEEn%-4CFE, was studied. Among them, the TPEE3%-4CFE mixture featured optimal blending, with liquid crystals well-connected with the HDA moieties in the TPEE framework through hydrogen-bonding interaction, according to X-ray and Fourier-transform infrared spectroscopy analyses. A packing diagram is proposed to illustrate the unique performance of TPEE3%-4CFE, attributed to the enhancement of π–π interactions. The polarized mesophase and morphology indicated favorable dispersion of liquid crystals in the TPEE matrix in TPEE3%-4CFE, which was applied successfully in electrochemical sensing for ascorbic acid for the first time. The TPEE3%-4CFE-modified carbon paste electrode sensor is a rapid-response sensor with high sensitivity of approximately 11.30 μA mM−1, a wide linear dynamic range of 0.181.26 μM, and a limit of detection of 0.4 mM–1.6 mM.

Interactions of charged microrods in chiral nematic liquid crystals.

We study the pair interaction of charged silica microrods in chiral nematic liquid crystals and show that the microrods with homeotropic surface anchoring form a bound state due to the competing effect of electrostatic (Coulomb) and elastic interactions. The robustness of the bound state is demonstrated by applying external electrical and mechanical forces that perturbs their equilibrium position as well as orientation. In the bound state we have measured the correlated thermal fluctuations of the position, using two-particle cross-correlation spectroscopy that uncovers their hydrodynamic interaction. These findings reveal unexplored aspects of liquid-crystal dispersions which are important for understanding the assembly and dynamics of nano- and microparticles in chiral nematic liquid crystals.

Features of dielectric properties of medical thermal indicators based on dispersions of cholesteric liquid crystals in the polymer matrix

Within the frequency range 10…106 Hz, the frequency dependences of the real (ε') and imaginary (ε") components of the complex dielectric permittivity of medical thermal indicators based on polyvinyl acetate and mixtures of cholesteric liquid crystals have been studied. In them, the selective reflection of electromagnetic waves visible to the human eye occurs at normal (36.6 °C) and elevated (38.2 °C) human body temperatures. Being based on the comparison of the ε' frequency dependences for the studied in this work dispersions of nematic liquid crystals prepared using the same technology and with the same polymer, it has been shown that, already on the basis of analysis of frequency dependences for ε', it is possible to ascertain the difference in characteristics for two types of thermal indicators. From the comparison of frequency dependences for ε", the main reasons of a difference between dielectric properties of the investigated medical thermal indicators for various temperatures of a human body have been ascertained.

Design and Characterization of Glyceryl Monooleate-Nanostructures Containing Doxorubicin Hydrochloride.

Glyceryl monooleate (GMO) is one of the most popular amphiphilic lipids, which, in the presence of different amounts of water and a proper amount of stabilizer, can promote the development of well defined, thermodynamically stable nanostructures, called lyotropic liquid crystal dispersions. The aim of this study is based on the design, characterization, and evaluation of the cytotoxicity of lyotropic liquid crystal nanostructures containing a model anticancer drug such as doxorubicin hydrochloride. The drug is efficiently retained by the GMO nanosystems by a remote loading approach. The nanostructures prepared with different non-ionic surfactants (poloxamers and polysorbates) are characterized by different physico-chemical features as a function of several parameters, i.e., serum stability, temperature, and different pH values, as well as the amount of cryoprotectants used to obtain suitable freeze-dried systems. The nanostructures prepared with poloxamer 407 used as a stabilizer show an increased toxicity of the entrapped drug on breast cancer cell lines (MCF-7 and MDA-MB-231) due to their ability to sensitize multidrug-resistant (MDR) tumor cells through the inhibition of specific drug efflux transporters. Moreover, the interaction between the nanostructures and the cells occurs after just a few hours, evidencing a huge cellular uptake of the nanosystems.

Electro-optical effects of organic N-benzyl-2-methyl-4-nitroaniline dispersion in nematic liquid crystals

The dispersion of organic N-benzyl-2-methyl-4-nitroaniline (BNA) in nematic liquid crystals (LCs) is studied. BNA doping decreases the threshold voltage of cell because of the reduced splay elastic constant and increased dielectric anisotropy of the LC mixture. When operated in the high voltage difference condition, the BNA-doped LC cell has a fall time that is five times faster than that of the pure one because of the decrements in the threshold voltage of the cell and rotational viscosity of the LC mixture. The additional restoring force induced by the BNA’s spontaneous polarization electric field (SPEF) also assists to decrease the fall time of the LC cell. The decreased viscosity can be deduced from the decrements in phase transition temperature and associated order parameter of the LC mixture. Density functional theory calculation demonstrates that the BNA dopant strengthens the absorbance for blue light, enhances the molecular interaction energy and dipole moment, decreases the molecular energy gap, and thus increases the permittivity of the LC mixture. The calculation also shows that the increased dipole moment, polarizability, and polarizability anisotropy increase the dielectric anisotropy of the LC mixture, which agrees with the experimental results well. BNA doping has a promising application to the fields of LC devices and displays.

Size-Dependent Liquid Crystal Behavior of Graphene Oxides for Preparation of Highly Ordered Graphene-Based Films

We report the preparation of a highly-oriented graphene-based film prepared from liquid crystal dispersion of graphene oxides (GOs). We observed that the liquid crystal behavior of GOs is highly affected by the lateral size of GO flakes: the critical concentration for liquid crystal formation decreased with the increase of the lateral size of GO flakes, which is in a good agreement with Onsager’s theory. As a result, we were able to obtain highly-ordered graphene assemblies with large-sized GO flakes (150 ± 29 μm) at relatively low concentrations. By applying the shear force, we were able to obtain highly-oriented films from liquid crystal GO flakes. After hydrogen iodide (HI) reduction, GO films showed excellent mechanical strength and electrical conductivity, which were 278% and 283% higher, respectively, than those of films made from smaller sized GO flakes (28 ± 24 μm).

The Influence of Single-Walled Carbon Nanotubes on the Dynamic Properties of Nematic Liquid Crystals in Magnetic Field.

This article aims to study the impact of carbon nanotube dispersions in liquid crystals. A theoretical model for the system’s dynamics is presented, considering the elastic continuum theory and a planar alignment of liquid crystal molecules on the nanotube’s surface. Experimental calculation of the relaxation times in the magnetic field was made for two cases: when the field was switched on (τon), and when it was switched off (τoff). The results indicate an increase of the relaxation time by about 25% when the magnetic field was switched off, and a smaller increase (about 10%) when the field was switched on, where both were in good agreement with the theoretical values.

Magnetic field dependence of the hexagonal to isotropic transition temperature of a single-walled carbon nanotubes dispersed lyotropic liquid crystal

ABSTRACTWe have carried out electrical conductivity studies on a single-walled carbon nanotubes dispersed lyotropic liquid crystal consisting of 50 wt.% TX-100 in water as a function of magnetic field and temperature. This system exhibits hexagonal and isotropic phases on heating. For all the applied magnetic fields, the temperature dependence of electrical conductivity of the carbon nanotubes dispersed lyotropic liquid crystal system exhibits a discontinuous change at the hexagonal to isotropic transition temperature. We find that the magnetic field dependence of the hexagonal to isotropic transition temperature is similar to that of the viscosity of the system. Using photo images of the sample, we find that the carbon nanotubes in the lyotropic liquid crystal form magnetic field dependent aggregates. We find spherical, rod and hook-like nanotube aggregates for low and high applied magnetic fields respectively. These nanotube aggregates alter the viscosity of our system which in turn alters the transition temperatures.

Electric Freedericksz transition in nematic liquid crystals with graphene quantum dot mixture

A theoretical model to explain the Freedericksz transition for a nematic liquid crystal with graphene quantum dots adding in electric field below the instability area is proposed. A mathematical formula for the critical voltage was obtained by elastic continuum theory combined with Burilov - Zakhlevnykh anchoring model, classical electrostatic principles and temperature dependence of elastic constant. We compared the results with the experimental data recorded for three LC samples and a good agreement between them was found encouraging us to propose this model as a tool for theoretical analysis of graphene quantum dot's dispersions in liquid crystals.