Nematic Liquid Crystal Mixture Research Articles

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.

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.

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.

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.

Physical Unclonable Functions Employing Circularly Polarized Light Emission from Nematic Liquid Crystal Ordering Directed by Helical Nanofilaments.

This study proposes the use of physical unclonable functions employing circularly polarized light emission (CPLE) from nematic liquid crystal (NLC) ordering directed by helical nanofilaments in a mixed system composed of a calamitic NLC mixture and a bent-core molecule. To achieve this, an intrinsically nonemissive NLC is blended with a high concentration of a luminescent rod-like dye, which is miscible up to 10 wt % in the calamitic NLC without a significant decrease in the degree of alignment. The luminescence dissymmetry factor of CPLEs in the mixed system strongly depends on the degree of alignment of the dye-doped NLCs. Furthermore, the mixed system prepared in this study exhibits two randomly generated chiral domains with CPLEs of opposite signs. These chiral domains are characterized not only by their CPLE performances but also by their ability to generate random patterns up to several millimeters, making them promising candidates for high-performance secure authentication applications.

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.

Optical and thermodynamic studies of binary mixtures of nematic liquid crystals from homologous members of alkyloxybenzoic acid

Optical and thermodynamic studies of binary mixtures of nematic liquid crystals from homologous members of alkyloxybenzoic acid

Relaxation processes in nematic liquid crystals in the nonstationary regime of a rotation of magnetic field

The prospects of using the nonstationary regime of motion of the director for studying the relaxation properties of liquid crystals in the low-temperature interval of the nematic phase are shown. In the nonstationary regime, the phase shift between the direction and the wave vector is 45 and does not depend on the angular velocity of rotation of the magnetic field. A eutectic mixture of nematic liquid crystals with a wider temperature range of the nematic phase compared to individual components has been studied. A decrease in the anisotropy of the absorption coefficient in the nonstationary regime is associated with a partial violation of the homogeneous orientation of the sample. The nature of the dependence of the low-frequency component of the phase characteristic on the thermodynamic parameters of the state is established. Within the framework of the hydrodynamics of the nematic phase, relations were obtained that allow calculating the dissipative coefficients responsible for the performance of liquid crystal technical devices.

The novel n-channel liquid crystal organic field effect transistor (LC-n-OFET): A promising technology for low-power electronics

In this study, a novel n-channel liquid crystal organic field effect transistor (LC-n-OFET) was fabricated using a mixture of E7 nematic liquid crystal (NLC) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in an OFET cell. The LC-n-OFET performance was investigated regarding dielectric properties, UV illumination sensitivity, and channel length dependence. Output and transfer current-voltage (I–V) characteristics were measured to evaluate the electrical properties in dark and under UV illumination for varying channel lengths. Additionally, the dielectric characteristics of the LC-n-OFET were determined along with the capacitance-frequency response (Ci-f), threshold voltage (VTh), field-effect mobility (μFET), and on/off current ratio (Ion/off) as a function of UV illumination intensity and channel length changes. Electrical measurements showed the Ion/off current ratio is inversely proportional to channel length but directly proportional to UV intensity. Meanwhile, the threshold voltage (VTh) and mobility (μFET) increased with longer channel length and higher light intensity. Overall, the novel LC-n-OFET exhibited promising performance with low VTh, high μFET, and large Ion/off.

Dielectric characterization of polymer dispersed liquid crystal film with chitosan biopolymer

Polymer Dispersed Liquid Crystals (PDLC) systems were obtained using chitosan as a biopolymer matrix and the nematic liquid crystal (LC) mixture E7 in various concentrations (30, 40 and 50% E7 in chitosan). Several techniques were used to analyze the obtained films: polarizing optical microscopy (POM), IR spectroscopy (FTIR), differential scanning calorimetry (DSC) and dielectric spectroscopy (DS). FTIR spectroscopy revealed minor changes for all PDLC films, suggesting weak interactions between LC and chitosan matrix. For each sample a clear segregation of the nematic droplets could be observed by POM. DSC measurements show that the phase transitions have shifted to lower temperature values for PDLC samples with approximately 2.7 °C compare to pure E7. The values of α (fraction of LC in the droplets) for compositions of 30, 40 and 50 wt% E7 in Chitosan were 0.48, 0.55 and 0.61 respectively, that are lower than for E7/PMMA system.The qualitative and quantitative analysis of the permittivity spectra obtained by DS reveals the fact that there are no significant changes in the molecular dynamics attributed to pure LC E7 compared to the PDLC composites, leading to the conclusion of the existence of a weak interaction between the two components, fact confirmed by the findings from the FTIR results.

Optical properties and zeta potential of polyvinyl pyrrolidone capped gold nanoparticles dispersed nematic liquid crystal mixture E7

The effect of dispersion of polyvinyl pyrrolidone (PVP) capped gold nanoparticles in nematic mixture E7 on the optical properties (absorbance and photoluminescence, fluorescence life time and Raman imaging) and zeta potential is studied. The gold nanoparticles of 0.25, 0.5 and 1.0 wt% concentrations are dispersed into E7. It has been found that the absorbance and photoluminescence intensity of dispersed systems are increased and depend on the GNPs concentration. These increase correspond to the coupling between the radiation field (electromagnetic waves of the light source) and the GNPs low-energy phonons and constructive coupling of emissions between the E7 and GNPs, respectively. The change in the fluorescence lifetime of pristine and GNPs dispersed E7 systems has confirmed the energy transfer between E7 and GNPs molecules. The study of Raman spectrum shows the variations in the intensity of the Raman peaks with different orientations. Due to dispersion of GNPs in E7 the values of zeta potential are increased. This ensures the good stability of the dispersed systems. The conductivity of dispersed system has increased drastically. In conclusion, the dispersion of GNPs in host nematic mixture E7 has influenced significantly its properties, thereby making the dispersed material more promising for the potential applications.

Thermal Degradation Studies of Poly(2-ethyl hexyl acrylate) in the Presence of Nematic Liquid Crystals.

The thermal degradation behavior of Poly(2-ethyl hexyl hcrylate) (Poly(2-EHA)), blended with a commercially available nematic liquid crystal (LC) mixture, was investigated by thermal gravimetric analysis (TGA). Different heating rates, ranging from 5 to 200 °C/min, were applied under an inert atmosphere. Based on the TGA results, activation energies (Eα) at different conversion rates (α) were determined using three integral isoconversion methods: Flynn-Wall-Ozawa (FWO), Tang, and Kissinger-Akahira-Sunose (KAS). It can be noticed that the global evolution of these activation energies was the same for the three models. The coefficient of determination R2 presented values generally higher than 0.97. Using these models, the Eα value for the LC remains constant at 64 kJ/mol for all conversions rates. For the polymer Poly(2-EHA), applying the Tang and FWO models, the activation energy presents a variation ranging from 80 kJ/mol, for conversion α = 0.1, to 170 kJ/mol, for α = 0.9. For the third model (KAS), this energy varies between 80 and 220 kJ/mol in the same range of α.

Enhanced electro-optical properties of nematic liquid crystal doped with Nitrogen doped graphene quantum dots

The first step of this study was to prepare mixtures that contained different concentrations of N doped graphene quantum dots (NGQDs) into nematic liquid crystals (NLCs). Then, through the Z-scan technology, the optical measurements in the nonlinear regime were conducted with a continuous wave (CW) at 632 nm wavelength and a low power laser beam. The nonlinear absorption (NLA) coefficient, nonlinear refractive (NLR) index, and third-order nonlinear susceptibility of NLCs doped with different concentrations of the NGQDs at distinctive laser powers and diverse applied voltages of external electric field was experimentally measured. The Z-scan results demonstrate that prepared cells have two photon absorption (2PA) and self-focusing effects. As the result, the concentration of the NGQDs in E7 and power of laser were optimum at 1% wt. and 11 mW, respectively. The effect of a DC electric field on optical nonlinearity was also investigated in the mixtures of NLCs and NGQDs at the concentration of 1% wt. Based on these findings, this sample can be used in both nonlinear optics and ultrafast broadband photonics.

Impact of red emissive ZnCdTeS quantum dots on the electro-optic switching, dielectric and electrochemical features of nematic liquid crystal: Towards tunable optoelectronic systems

In the present study, the concentration-dependent dielectric, electro-optical, and electrochemical properties of ZnCdTeS quantum dots (QDs) doped E7 nematic liquid crystal (NLC) mixtures were investigated. The dielectric permittivity components (ε‖ and ϵꓕ) and dielectric anisotropy (Δϵ = ε‖ ˗ ϵꓕ) of NLC samples containing varied concentrations of ZnCdTeS QDs (i. e. 0.10, 0.25, 0.50, 0.75, and 1 wt%) were measured at various temperatures. In the nematic phase, the results demonstrated that ε‖ increases much more than ϵꓕ upon an increase in the concentration of ZnCdTeS QDs. Δϵ enhanced as the concentration of QDs increased, reaching a maximum at 0.50 wt%, then decreased with further addition. Dielectric measurements revealed the formation of self-aligned QD arrays along the nematic director, which act similarly to multiple parallel capacitors in the NLC system. Moreover, electro-optical studies illustrated the significant effect of QDs doping on lowering the threshold voltage and response time. Interestingly, the optical switching-off time of NLC containing 0.50 wt% of the QDs decreased by ∼50% compared to that of the pure E7 sample. The reduced screening effect resulting from the QDs ion-capturing mechanism, enhanced effective intermolecular interactions, and increased dielectric anisotropy in the NLC system are the major factors responsible for the improved electro-optical characteristics. The impedance behavior of NLC cells was studied in the frequency range of 0.1 Hz–100 kHz. It indicated that the addition of ZnCdTeS QDs results in a remarkable increase of 96% in the electrical conductivity of the NLC system. Furthermore, the QDs doping significantly improved the NLC device's charge capacitance. Such studies would undoubtedly be beneficial for designing next-generation tunable optoelectronic systems since QDs can be utilized for tuning the dielectric anisotropy, electro-optical characteristics, charge capacitance, and conductivity of NLCs.

Composite of DNA-Stabilized Multiwalled Carbon Nanotube and Nematic Liquid Crystal for Display Performance Optimization

We have prepared composites of a nematic liquid crystal (NLC) mixture to improve display performance by doping multiwalled carbon nanotubes (MWCNTs) and single-stranded-DNA-wrapped MWCNTs. Polarized optical microscopy images indicated the uniformity of the composite. Infrared absorption spectra showed there existed an interaction between MWCNTs and 4-pentyl-4′-cyanobiphenyl (5CB, a practical solvent-type monomer NLC) due to π–π stacking and charge transfer, indicating the anchoring of 5CB molecules on the MWCNT surface. The electro-optical property proved the incorporation of MWCNTs and DNA-wrapped MWCNTs reduced the threshold voltage and response time, thus following the changed splay elastic constant and rotational viscosity coefficient of LC, reflecting that DNA can improve the stability of MWCNTs in 5CB. Moreover, dielectric measurement showed the enhanced value to dielectric anisotropy and shift in relaxation frequency, including the changes of dielectric permittivity and loss, revealing the electrically “inertness” MWCNTs at high frequencies and the DNA-induced broadened frequency range of low dielectric loss. Then electrical conductivity tests further proved the cooperative orientation of MWCNTs with 5CB under the applied voltage and the contribution of MWCNTs’ high conducting along the long axis. This study revealed that MWCNT doping could improve the physical properties of NLCs, and DNA wrapping could pave a promising way toward stabilized MWCNT/NLC composites for display performance optimization.

Characterising nematic liquid crystals using a circular patch resonator

ABSTRACT Reconfigurable microwave material is a promising candidate for designing and manufacturing tunable microwave components. Nematic liquid crystals (NLC) are such materials since their permittivity can be tuned by an external electric field. However, many NLC mixtures were not properly characterised at higher frequency bands due to requiring a complex measurement setup. In this work, a novel method using circular patch resonator (CPR) is developed to measure the dielectric constant and loss tangent of NLCs at microwave frequencies. In addition to using the cavity model for the preliminary design and analysing the fringing effect for a better accuracy, full-wave simulations are employed to confirm the final design and aid the characteristic analysis. Three prototypes were fabricated and measured to reduce uncertainty from manufacturing defects. To avoid the possible damage when higher voltage is required for a large range tuning, a coupling mechanism is proposed between the microstrip line and coplanar waveguides (CPWs) to replace connection through vias. A high accuracy with an uncertainty of 0.02 for relative permittivity estimate has been demonstrated with experiment verification, approximate 80% improvement than other typical methods. The simple design and PCB-based manufacturing techniques can be widely employed to characterise the properties of newly-developed LC mixtures.

Investigations of axioms of twist grain boundary phases (TGBPs) in binary mixture of liquid crystals

In this paper, the investigated mixture of liquid crystals shows a spacious range of twist grain boundary phases (TGBPs). Thermo dynamical, optical and dielectric measurements confirmed the presence of TGBPs in the mixtures of nematic and cholesteric liquid crystals. In the heating cycle, differential scanning calorimetry (DSC) thermogram shows the mixed different TGBPs at the high scanning rate. Cylindrical and cone-like textures with brushes of TGBA* is observed under the optical investigation. The grain boundaries and blocks of smectic phases are twisting two times around an axis. Wide-ranging dielectric investigation of nematic and cholesteric liquid crystal mixture have been performed in the 50 Hz–10 MHz range of frequency for the homogeneous conditions. In the absence of direct bias, TGBA* phase exhibits two dielectric relaxation processes, soft mode (SM) and process in the grain boundaries (PGBs). In the presence of electric field, grain boundaries steadily demolished the phases. Whereas the weak dielectric relaxation processes have been distinguished in the presence of the electric field in the TGBPs.

Acoplanarity, Aromaticity, Chirality, and Helical Twisting Power of Chlorin e6 13(N)-Methylamide-15,17-dimethyl Ester Complexes: Effect of a Metal

The experimental and theoretical study of the influence of metal complexing on geometry, aromaticity, chirality, and the ability to twist the nematic phase by complexes based on modified natural chlorin e6 was carried out. The geometry optimization of the chlorin e6 13(N)-methylamide-15,17-dimethyl ester (MADMECl) and its Zn, Cu, and Ni complexes by DFT (CAM-B3LYP/6–31 G(d,p) functional) method was performed. Based on these calculations, the acoplanarity degree of the macrocyclic ligand and the distortion energy of its dianion were estimated, which allowed the arrangement of the MADMECl complexes in the series Ni > Cu > Zn. Aromaticity was evaluated using the NICS criterion (nuclear independent chemical shift). An increase in the degree of aromaticity of the macrocycle upon complex formation was established. At the same time, the aromaticity of the inner conjugation contour corresponds to the same series as the acoplanarity, while the outer π-delocalization is characterized by the reverse sequence. An experimental evaluation of the electron circular dichroism of the Soret and the Q-bands, as well as the g-factor of dissymmetry, was carried out. The growth of these parameters with an increase in the degree of acoplanarity and aromaticity of the internal conjugation contour was determined. The induction of helical phases in mixtures of nematic liquid crystals (LCs) based on cyanobiphenyls and MADMECl macrocyclic metal complexes was studied by polarization microscopy, and the clearance temperatures and helix pitch of the mesophases were measured. A strong effect of the metal on the phase transition temperature and helical twisting power was established.

Self-Folding Liquid Crystal Network Filaments Patterned with Vertically Aligned Mesogens.

Fibrous soft actuators with high molecular anisotropy are of interest for shape morphing from 1D to 2D and 3D in response to external stimuli with high actuation efficiency. Nevertheless, few have fabricated fibrous actuators with controlled molecular orientations and stiffness. Here, we fabricate filaments from liquid crystal networks (LCNs) with segmental crosslinking density and gradient porosity from a mixture of di-acrylate mesogenic monomers and small-molecule nematic or smectic liquid crystals (LCs) filled in a capillary. During photopolymerization, phase separation between the small-molecule LCs and LCN occurs, making one side of the filament considerably denser than the other side. To direct its folding mode (bending or twisting), we control the alignment of LC molecules within the capillary, either along or perpendicular to the filament long axis. We show that the direction of UV exposure can determine the direction of phase separation, which in turn direct the deformation of the filament after removal of the small-molecule LCs. We find that the vertical alignment of LCs within the filament is essential to efficiently direct bending deformation. By photopatterning the filament with segmental crosslinking density, we can induce a reversible folding/unfolding into 2D and 3D geometries triggered by deswelling/swelling in an organic solvent. Moreover, by taking advantage of the large elastic modulus of LCNs and large contrast of the modulus before and after swelling, we show that the self-folded LCP filament could act as a strong gripper.