Liquid Crystal Mixture Research Articles

Low-Temperature Metallomesogen Model Structures and Mixtures as Potential Materials for Application in Commercial Liquid Crystal Devices

The present work was the preliminary study of phase diagrams and miscibilities of low-temperature metallomesogen (MOM) model structures based on rod-like palladium (Pd) alkyl/alkoxy-azobenzene metal complexes and their mixtures with commercial liquid crystal materials for potential application. The initial results indicated the accessible temperature range and mesgenic miscibility between parent ligand, MOMs and commercial liquid crystal mixtures. The eutectic ligand/MOM composition with other MOMs and commercial nematic liquid crystal materials exhibited complete mesogenic miscibility and wide low-temperature mesogenic stability for eventual utilization in commercial liquid crystal devices.

Widely tunable reflection by a hybrid ion-doped positive-negative chiral nematic liquid crystal under an electric field.

In this study, a new methodology, in which the cation and chiral agent are doped in positive and negative nematic liquid crystal mixture to form cation-doped positive-negative chiral nematic liquid crystals (PN-CNLCs), is proposed. These PN-CNLCs are capable of achieving a wide wavelength tuning range close to 90 nm under the action of a high-frequency electric field. The dielectric interaction between these two liquid crystals with different dielectric anisotropies in the electric field allows for the effective change of the pitch of the CLCs. We discovered the tilting of the pitch in the PN-CNLCs by a series of well-designed validation experiments. The experimental results prove the symmetrical helix is formed along the alignment direction in PN-CNLCs. Furthermore, the experiments demonstrated that an increase in voltage does not directly affect the pitch, but it increases the tilt angle of the helix axis with respect to the normal of the LC cell. This study offers, to our knowledge, a novel perspective on the tuning performance of CLCs under electric field conditions, by integrating the properties of positive and negative liquid crystals. This finding not only contributes to the understanding of the electric field response mechanism of CLCs but also provides new avenues for the application of CLCs in fields such as wavelength tuning.

Induced blue phase and twist grain boundary phase in binary mixtures of chiral calamitic and achiral hockey stick-shaped liquid crystals

Frustrated phases in chiral liquid crystalline systems, such as the blue phase (BP) and the twist grain boundary (TGB) phase, can create new opportunities in liquid crystal research. These frustrated phases have unique optical properties which can be used for specific applications. A new binary mixture system composed of a chiral rod-like ferroelectric liquid crystal and an achiral hockey stick-shaped compound was designed. This binary system exhibits induced frustration, resulting in the emergence of the BP-III and TGB*A phases in addition to the chiral ferroelectric Smectic-C* phase. We report a comprehensive study on several binary mixtures (xH-22.5= 0.162, 0.25, 0.396, 0.562) designed of a chiral ferroelectric liquid crystal derived from the lactic acid, namely 4-(octyloxy)phenyl 3-methoxy-4-((4-((2-(pentyloxy)­propanoyl)­oxy)benzoyl)oxybenzoate and achiral hockey stick-shaped compound, namely (E)-4-((E-((3-(decyloxy)-2-methylphenyl)imino)methyl)-phenyl 3-(4-(dodecyloxy)phenyl)acrylate. These mixtures induced the blue phase (BP-III, ~2.5–6.2K) and the twist grain boundary Smectic-A*phase (TGB*A  ~1.8–6K). Polarizing optical microscopy, birefringence, and dielectric spectroscopy were employed to investigate the BP-III and  phases, as those techniques provide insights into their structural and dynamic properties. The ferroelectric behaviour of the SmC* mesophase was investigated by electro-optics. The obtained results are discussed in relation to the molecular structures of the used materials.

Pretilt angle modulation of liquid crystals based on single crystal rubrene with incorporation of homeotropic polyimides

The alignment control of liquid crystals (LCs) is critical for various practical applications. The pretilt angle modulation of LCs typically requires a mechanical rubbing on substrates to orient the LCs. This study presents a contact-free approach to achieve pretilt angle modulation of LCs. Initially, a single crystal rubrene (SCR) film is deposited on the substrate. Subsequently, a mixture of nematic LCs and homeotropic polyimides (HPIs) is introduced between two SCR substrates via capillary action. During capillary filling, the synergy of capillary flow and the interaction between LCs and SCR ensures specific LC orientation. Subsequently, HPI dopants migrate toward and organize on the SCR through vertical phase separation, enhancing surface hydrophobicity and thereby increasing the pretilt angle of LCs. The pretilt angle of LCs can be continuously adjusted over a wide range from 2° to 90° by varying the HPI concentration. The contact-free process preserves against electrostatic charges, dust contamination, and surface damage typical of rubbed LC cells. This believed to be novel technique shows promise for developing no-bias-bend and bistable bend-splay LC displays.

Chiral liquid crystal dimers with smectic phases for stabilization of anticlinic state in surface-stabilised geometry

The growing demand for enhanced optical quality and faster electro-optical response in liquid crystal (LC) photonic devices has expanded the search for advanced materials capable of improving the properties of LCs beyond conventional ones. Among these compounds are LC dimers with ferroelectric and antiferroelectric properties which can be incorporated into LC mixtures to enhance performance. However, existing dimers often feature high melting temperatures, enthalpies, and other physicochemical characteristics that render them unsuitable as dopants in antiferroelectric liquid crystal (AFLC) mixtures targeted to modern photonic devices. Here, we show the design and physicochemical properties of novel dimers which are characterized by low melting points, high tilt angles and spontaneous polarizations, temperature-stable helical structures, and reasonably broad temperature ranges of ferroelectric or antiferroelectric phases. These features have facilitated the effective use of one dimer as a dopant in AFLCs. This eliminates the primary drawback of the electro-optical effect based on the surface-stabilised geometry of AFLCs by strongly promoting the anticlinic state in this effect. Importantly, for the first time, the above can be achieved without affecting any other physicochemical or optical properties of AFLCs. Thus, the obtained results represent a pivotal link between the longstanding concept of developing AFLCs that meet the stringent requirements for electro-optical effects based on surface-stabilised geometry and the realization of novel photonic devices demonstrating all the benefits of this effect.

Broad temperature range ferrielectric liquid crystal: Temperature dependencies of dielectric and electro-optical properties

Broad temperature range ferrielectric liquid crystal mixture FerriLCM-1 was previously developed in 2022 (Pozhidaev et al. 2022). In that work dielectric and electro-optical properties of the mixture were studied at room temperature. Electrically induced birefringence index reached 0.01 at a wavelength of 532 nm with response times of 300 μs. In this paper we continue the FerriLCM-1 properties investigation in a wide temperature range from the room ones till the isotropic phase transition temperature. It is found that one of the critical electric fields is almost temperature independent, while the Kerr coefficient of quadratic electro-optical effect in deformed helix ferroelectric (DHF) mode grows with temperature increase. This leads to an increase in the depth of phase modulation. Another critical electric field decreases smoothly with temperature rising. At 47–50 °C two critical electric fields merge, while temperature dependencies of all other dielectric and electro-optical parameters of the mixture are continuous and smooth functions. At about 50 °C the maximum electrically induced birefringence in DHF-mode is ΔneffE=0.022 under the applied electric field E=0.4 V/μm and with 80 μs response time. FerriLCM-1 mixture operating in the DHF-mode exhibits a giant Kerr coefficient of 1700 nm/V2 (or even up to 2900 nm/V2 at λ=400nm) at the temperature of 85 °C combined with a 50 μs response time. This is the highest value of Kkerr coefficient among liquid crystals. All these properties make ferrielectric liquid crystal mixture FerriLCM-1 suitable for solving various problems of phase modulation of light and use in low-voltage devices.

New graphene oxide doped polymer dispersed liquid crystal nanocomposites targeted to eco-friendly and energy-efficient smart windows

Graphene oxide-polymer dispersed liquid crystal (GO-PDLC) composites are proposed as a solution for energy-efficient switchable windows. These composites were prepared by incorporating varying concentrations of graphene oxide (GO) into a mixture of nematic liquid crystal (E7) and a UV–Vis curable resin using the polymerization-induced phase separation technique. The results indicate that the dispersion of GO at an optimum concentration of 0.005 wt% significantly affects the morphology, phase transition temperatures, and electro-optic properties of polymer dispersed liquid crystal (PDLC). In particular, the GO-PDLC composites exhibit a transition from an opaque to a transparent state at a remarkably low voltage (∼11.5 V) compared to pure PDLC cell. Dielectric analysis reveals that adding GO increases the permittivity, conductivity, and dielectric strength of PDLC composites. Moreover, the Cole-Cole plot indicates a non-Debye type relaxation in a higher frequency region. Furthermore, a blue shift in emission wavelength and enhanced photoluminescence intensity suggest alterations in electronic transitions within the composites. This comprehensive study provides valuable insights into optimizing the properties of PDLCs through the dispersion of GO, thereby offering promising prospects for their utilization in various optoelectronic applications.

In-situ homeotropic alignment of dye doped liquid crystal molecules on multilayered self assembled nanoparticles in confined cells for next generation display devices

The in-situ homeotropic alignment (HA) of pure liquid crystal (LC) and dye doped LC molecules was prepared on the multilayered self assembled silica nanoparticles (SNPs) deposited on the substrates in a confined empty ITO cell. The LC and azo dye mixtures with (0.0625–0.5 wt%) concentrations show the uniform in-situ HA of nematic LC (NLC) over SNPs coated surface. The surface morphology with mapping of elements and functionality of the SNPs coated ITO glass substrate were confirmed by “field emission scanning electron microscope (FESEM)” and “X-ray photoelectron spectroscopy (XPS)”. Polarized optical microscopic textures and conoscopic view of pure and dye doped LC mixtures in SNPs coated confined cells, confirmed the HA of LC molecules and exhibited the uniform black textures under crossed polarizers. The electro-optic behaviour of dye doped homeotropically aligned LC (HALC) cells was compared to pure SNPs coated HALC cell. The results showed that on increasing the azo dye concentration (0.0625–0.25 wt%), the threshold (Vth) and operating (Vo) voltages were found decreased. However, on further increase of dye concentration to 0.5 wt%, Vth and Vo were observed relatively increased. The contrast ratio (CR) was found improved in dye doped HALCs with maximum CR at lowest dye concentration (0.0625 wt%). The UV results exhibit higher absorbance of the dye doped HALC cells with increasing the dye concentration. Also, the band gap values showed that the addition of even small amount of dye, it reduced the value of band gap in HALC cell.

Effect of SiO2-doped nanoparticle/electrostatic spinning system on the electro-optical properties of polymer dispersed liquid crystals for smart windows

ABSTRACT Polymer Dispersed Liquid Crystal (PDLC) films are attracting increasing attention as versatile and innovative material for use in electrical switching and flexible devices. This paper investigates the effect of a Poly(methyl methacrylate (PMMA) nanospun membrane system doped with SiO2 nanoparticles on the electro-optical properties and transmittance/voltage slopes of thiol- and acrylate-based PDLC films. Experimental samples were prepared for testing by adding mixtures of liquid crystals and crosslinkers in different proportions to liquid crystal cassettes doped with PMMA/SiO2 nanospun membranes. The size of the polymer mesh was found to influence all parameters of PDLC. The phase separation process, which determines the morphology and electro-optical properties of PDLC films, is directly affected by the doping of different components of PMMA/SiO2. The doping of the PMMA/SiO2 system resulted in improved optoelectronic properties of PDLC films, significantly enhanced weathering resistance, and increased the degree of intelligent regulation of the transmittance of PDLC films. Thus, by developing PDLC-doped reticulated nanofibre films with faster response times and better electro-optical properties, the potential for technological applications of PDLC-based devices in optical windows, displays, energy storage and flexible devices can be significantly enhanced.

31‐1: Invited Paper: C‐PS‐VA and SA‐VA – Technologies for Next Generation TV LCDs

The Chiral Polymer Stabilized Vertical Alignment (C‐PS‐VA) technology enables up to 10% increase in transmittance leading to potentially lower power consumption of final devices and energy saving compared to conventional Polymer Stabilized Vertical Alignment (PS‐VA) technology. Self‐Aligned Vertical Alignment (SA‐VA) enables the simplification of the process to achieve energy saving by eliminating high‐temperature baking. This paper summarizes the recent developments of liquid crystal mixtures for C‐PS‐VA & SA‐VA technologies in terms of performance and reliability.

Binary Nematic Liquid Crystals Mixture with Enhanced Electro-Optics Properties for Photonic Applications

Purpose: In this work, we mix two simple nematic liquid crystals (NLCs) and investigated the binaryNLCs mixtures of 7CB/PCH5 of different mixing ratios. Methodology: The pure liquid crystals 7CB and PCH5 and binary mixtures of them of high temperature stability were thermally analyzed by differential scanning calorimetry. The mixture 7CB/PCH5:30/70 wt% has the highest thermal stability with a nematic-isotropic (N-I) transition temperature at 50oC. The electrooptic properties of 7CB, PCH5, and the mixture 7CB/PCH5:30/70 wt% at room temperature were also investigated using an amplitude modulated electric signal (1 kHz - 100 Hz) by increasing diving peak voltage from 0 V to 10 V. The threshold volage is relatively reduced for the binary mixture in comparison to that value for PCH5. In comparison to the pure LCs, the mixture 7CB/PCH5:30/70 wt% has the fastest response times of values 2.36 ms total time response, 0.41 ms rise time, and 1.95 ms fall time. It has also the highest contrast ratio. Moreover, it has a maximum measured transmission that is higher than those for PCH5 and 7CB by about 17 % and 8%, respectively, at a field strength of 2V/mm. Findings: The obtained results indicate that the electrooptic properties of PCH5 was improved when mixed with a proper ratio of 7CB, of lower cost, more stablity , and higher potential for photonic applications. Unique Contriburibution to Theory, Practice and Policy: This expermental study shows that simply by mixing two relatively low cost NLCs materials, one of high thermal stability and low electro-optic properties with other one of low thermal stability and better electro-optic properties; this would improve the stability, response, and transmition of the binary mixture. If the a suitable driving method is applied, without doping with other orgnic or inorganic matrial.

Dielectric anisotropy in liquid crystal mixtures with nematic and smectic phases

Abstract The modulation of dielectric anisotropy (△ε) is pivotal for elucidating molecular interactions and directing the alignment of liquid crystals. In this study, we combine liquid crystals with opposing dielectric anisotropies to explore the impact of varying concentrations on their properties. We report the sign-reversal of △ε in both the nematic and smectic A phases of these mixed liquid crystals, alongside a dual-frequency behaviour across a broad temperature spectrum. Our research further quantifies the influence of mixture ratios under various temperatures and electric field frequencies. This exploration may pave the way for the discovery of new physical phenomena.

A flexible optically driven liquid crystal display based on RM257 doping

With advances in technology, requirements for the performance of liquid crystal displays (LCDs) are gradually increasing, especially for flexible displays, which involve many new challenges. Here, we propose a new method for implementing optically driven LCD flexible displays. Upon doping an E7 nematic liquid crystal (LC) with RM257 liquid crystal monomer and polymerizing it using ultraviolet light, a polymer network forms as the spacer between the two substrates of the LC, maintaining the surrounding E7 LC in a relatively stable state and improving the mechanical properties of the LC. This also endows the LC mixture with a limited capacity for deformation recovery and enables uniform deformation under stress, thereby achieving flexible displays. Applying an electric field to the two ends of the LC cell, the viscosity coefficient of the LC molecules is improved and the rewriting time can be optimized. This type of LC cell can be programmed through light control, achieving low power consumption for a wearable display device.

Polymer Network Film with Double Reflection Bands Prepared Using a Thermochromic Cholesteric Liquid Crystal Mixture.

Cholesteric liquid crystal polymer network (CLCN) films with a single reflection band have found applications for decoration and anticounterfeiting. The CLCN films with double reflection bands were more suitable for these applications. Herein, they were prepared by using thermochromic cholesteric liquid crystals (CLCs) through a two-step photopolymerization approach. At the first step, due to oxygen inhibition, the CLC monomers near the substrate surface were polymerized at a certain temperature. At the second step, those near the air were polymerized at another temperature. The wavelengths of these two reflection bands of the CLCN film were dominated by the two polymerization temperatures. Based on this approach, patterns with composite colors were prepared, which were suitably applied for decoration. Moreover, a double-layered CLCN film with a broad reflection band was prepared that could potentially be applied for displays.

Estimation of Ionic Impurities in Poly(propylene Glycol) Diacrylate Monomers/Liquid Crystal E7 Mixtures Using Dielectric Spectroscopy

The study investigated the effect of the molecular weight of three difunctional poly(propylene glycol) diacrylates on the temperature-dependent ionic conductivity of these monomers and their blends with an eutectic nematic liquid crystal mixture (E7). The results revealed two distinct regions. At low temperatures, ionic conduction can be described by the Vogel–Tamman–Fulcher (VTF) equation, while at high temperatures, the conductivity data follow the prediction of the Arrhenius model. The Arrhenius and VTF parameters and their corresponding activation energies were determined using the least squares method. In addition, a conductivity analysis based on an ionic hopping model is proposed. Estimates of ion concentrations and diffusion constants were calculated. It was found that both the ionic concentration and the diffusion constant decrease with the increase in the molecular weight of the monomers. The static dielectric permittivity decreases in the following order: TPGDA, PPGDA540, and PPGDA900. This can be explained by the higher dipole moment of TPGDA, which is caused by an enhanced volume density of carbonyl groups.

Size-dependent optical and dielectric anisotropy of CdS nanowire doped high-birefringent nematic liquid crystalline compound

Highly birefringent liquid crystals have grabbed ample attention because of their potential applications as an active medium for devices in visible, infrared and other spectral regions. Here, we have attempted to study the effect of the size and concentration of CdS nanowires on a nematic liquid crystal. Anisotropy in liquid crystals is one of those distinguishing traits that gain their superiority over other materials. We hereby have analyzed how optical and dielectric anisotropy respond to variations in the size and concentration of the dopant. The study is of significant scientific value because there is very little literature available on how dopant size can alter liquid crystalline properties and also introduces a method to tune the properties of pure liquid crystals (LCs) by varying the size and concentration of the dopant. The study explains how the size of the nanowires changes the analyzed parameters and up to what extent. The obtained results yield a significant increase in both optical and dielectric anisotropy by doping CdS nanowires making them an excellent choice for dopant selection. CdS-doped nematic LC mixtures have proved to be highly birefringent and possess a high positive dielectric anisotropy as well. Such a compound can be of great application in device fabrications such as laser beam steerers, electronic lenses, attenuators and light shutters, etc.

Induced blue phase of cholesteric-nematic mixtures under the action of acetone vapors

Liquid crystals can exhibit structural orientational order. The creation of a mixture with the addition of chiral molecules to the nematic liquid crystal induces helical twisting, the axis of which is directed perpendicular to the cell surfaces. When some cholesteric mixtures with a sufficiently short spiral pitch (up to 400-500 nm) are heated to a temperature close to, but still lower than, the temperature of the main transition to the isotropic state, in some cases the so-called blue phases can be formed.
 We carried out a study on the detection of structural manifestations of the blue phase under the action of vapors of chemical substances, in particular acetone. The main dependences of effect of acetone on the liquid crystal mixture depending on the concentration were also revealed. The two-stage phase transformation from cholesteric liquid crystal to the isotropic liquid via the intermediary blue phase could be clearly recorded by changes in optical transmission. Possible applications are discussed.

Collective molecular relaxations and electro-optical switching response of a wide room temperature antiferroelectric liquid crystal mixture

Antiferroelectric liquid crystals (AFLCs) are advanced smart materials for displays with unique properties of tristate switching. Thermodynamic, dielectric and switching characteristics of a room temperature bicomponent AFLC mixture at eutectic composition was investigated in the present work. The emergence of various phases in the eutectic composition was identified on the basis of observed dielectric relaxation modes and other parameters. Critical electric field (Ec) to unwind the helix, spontaneous polarization (Ps), torsional viscosity (γt) and switching time (ts) have been determined in the ferroelectric (SmC*) and the antiferroelectric (SmCA*) phases from switching experiments. Spontaneous polarization follows the equation, Ps = Po(1-T/Tc)γ in the temperature range of tilted chiral smectic phases, with values of Tc = 91.8 ± 0.1 °C and γ = 0.410 ± 0.005. Value of γ indicates that the SmA* to SmC* transition is of the first order as observed by the enthalpy measurements from the thermodynamic studies. In the AFLC phase of interest, ts is ∼2–3 ms and Ps∼200 nC/cm2. These parameters suggest that the mixture is appropriate base material for electro-optical applications.

Ultra-low temperature-responsive liquid crystal elastomers with tunable drive temperature range

Liquid crystal elastomers (LCEs) have recently gained significant attention in the field of smart materials due to their ability to respond to external stimuli, offering substantial potential for practical applications in soft robotics and bioengineering where actuation and sensing are crucial. However, conventional LCEs typically have high driving thresholds, significantly limiting their practical utility. Thus, reducing the driving temperature of LCEs remains a critical challenge in current research. In contrast to complex chemical molecular modifications aimed at lowering the driving threshold, our work employs a straightforward one-step cross-linking method to synthesize bisacrylate LCEs with varying compositions. These LCEs exhibit remarkably low driving temperatures and improved actuation performance. The monofunctional and difunctional acrylate liquid crystal mixtures employed in our experiments possess different ranges of anisotropy and can be effectively incorporated into the liquid crystal elastomers. Following polymerization, these LCEs demonstrate actuation capabilities at temperatures as low as −18 °C while maintaining excellent performance even at elevated temperatures of up to 60 °C. Moreover, in a series of 50 repeat experiments, the LCEs exhibited exceptional repeatability with a minimal error rate of 2.0 %. Additionally, the introduction of carbon nanotubes into the LCEs resulted in achieving a significant angular deflection of up to 100° within a narrow range of low light intensities, specifically between 26.19 mW cm−2 and 32.38 mW cm−2, highlighting the continued effectiveness of these materials even with this modification.

The influence of cholesteric helical ordering on the dynamics of liquid crystal microdroplet coalescence

ABSTRACT The dynamics of liquid crystal droplet coalescence in an isotropic melt was studied using polarised optical microscopy. Droplets of both nematic and cholesteric phases, formed from a mixture of nematic liquid crystal and chiral-photochromic dopant, were investigated. It was found that the characteristic coalescence times were significantly increased in the cholesteric phase compared to the nematic phase due to the presence of cholesteric helical ordering. In addition, these characteristic times increased as the cholesteric pitch decreased.