Nematic Liquid Crystalline Materials Research Articles

Thermal and optical study of semiconducting CNTs-doped nematic liquid crystalline material

We report the thermal and spectroscopic analysis of the carbon nanotubes (CNTs)-doped nematic liquid crystal (NLC) material. The CNTs have been oriented in the p-ethoxybenzylidene p-butylaniline NLC. The thermal study of the CNTs doped nematic mixtures shows a significant decrease in the isotropic to nematic phase transition temperature. However higher doping concentration of CNTs has led to the further increase in transition temperature. The UV-Visible spectroscopy has been attempted on the CNTs/NLC mixtures at room temperature. The investigated NLC present one absorption band corresponding to π–π* electronic transition. A red shift of λmax with the increasing concentration of CNTs in the mixture has been observed. The band gap of NLC has been found to decrease after the doping of CNTs. The absorbance was measured for the UV light, polarized parallel and perpendicular to the LC director in the planar aligned cell.

Photoluminescence: Design and Integration in Electro‐Optic Devices of Highly Efficient and Robust Red‐NIR Phosphorescent Nematic Hybrid Liquid Crystals Containing [Mo6I8(OCOCnF2n+1)6]2− (n = 1, 2, 3) Nanoclusters (Adv. Funct. Mater. 31/2015)

As shown by L. Dupont, Y. Molard, and co-workers on page 4966, the functionalization of [Mo6I8(CnF2n+1COO)6]2− (n = 1, 2, 3) red–near-infrared phosphorescent nanocluster units with specific organic cations leads to a nematic liquid crystalline material that can be homogeneously mixed with E7 commercial liquid crystal. Integration in electro-controlled devices modulates the cluster photoluminescence intensity, demonstrating the potential of these hybrid materials for photonic or optoelectronic applications.

Synthesis of novel fluorinated bis(biphenyl)diacetylene derivatives as highly birefringent nematic liquid crystalline materials

Three novel fluorinated bis(biphenyl)diacetylene (1,4-bis(biphenyl-4-yl)buta-1,3-diyne) derivatives, with alkoxy tails at the terminal positions, were prepared for use as high-birefringent nematic materials. To lower the thermal transition temperatures and determine the origin of the nematic phase of these compounds, fluorine atoms were introduced at lateral positions. Thermal transition temperatures and mesomorphic properties were investigated by differential scanning calorimetry and polarized optical microscopy. In addition, birefringence was measured using a microscopic method based on the interference method. Both the nematogenic properties and optical performances depended not only on the number of fluorine substituents, but also on the position of the substituents. The compound having the highest fluorine content exhibited the lowest birefringence, and the compound bearing fluorine atoms only on the inner benzene rings of the biphenyl cores exhibited an enantiotropic nematic phase with a maximum birefringence value of 0.48 at 550 nm and 140 °C.

A molecular nematic liquid crystalline material for high-performance organic photovoltaics.

Solution-processed organic photovoltaic cells (OPVs) hold great promise to enable roll-to-roll printing of environmentally friendly, mechanically flexible and cost-effective photovoltaic devices. Nevertheless, many high-performing systems show best power conversion efficiencies (PCEs) with a thin active layer (thickness is ~100 nm) that is difficult to translate to roll-to-roll processing with high reproducibility. Here we report a new molecular donor, benzodithiophene terthiophene rhodanine (BTR), which exhibits good processability, nematic liquid crystalline behaviour and excellent optoelectronic properties. A maximum PCE of 9.3% is achieved under AM 1.5G solar irradiation, with fill factor reaching 77%, rarely achieved in solution-processed OPVs. Particularly promising is the fact that BTR-based devices with active layer thicknesses up to 400 nm can still afford high fill factor of ~70% and high PCE of ~8%. Together, the results suggest, with better device architectures for longer device lifetime, BTR is an ideal candidate for mass production of OPVs.

Enhanced negative dielectric anisotropy and high electrical conductivity of the SWCNT doped nematic liquid crystalline material

We report a significant improvement in the dielectric anisotropy of single wall carbon nanotube (SWCNT) doped p-ethoxybenzylidene p-butylaniline (EBBA) nematic liquid crystal (NLC). The modification in physical parameters of the doped system shows strong dependence on the concentration of SWCNTs. The presence of SWCNTs changes the physical properties like splay elastic constant, rotational viscosity and electrical conductivity of the pristine nematic LC remarkably. The behavior of electrical conductivity was found to be considerably dependent on the orientation of CNTs in nematic matrix influenced by applied probing field. Enhanced dielectric anisotropy and conductivity of the doped system show its utility in the improvement of variety of liquid crystal devices.

Effect of alumina nanoparticles on dielectric permittivity, electrical conductivity, director relaxation frequency, threshold and switching voltages of a nematic liquid crystalline material

In the present work, alumina (Al2O3) nanoparticles (Al-NPs) were dispersed in a room temperature nematic liquid crystal 4-pentyl-4ʹ-cyanobiphenyl (5CB) at the concentrations of 0.2 and 0.6 wt%. Differential scanning calorimetry studies suggest decrease in clearing temperature of the composite materials as compared to the pure material. Dielectric studies show that the presence of the Al-NPs decreases the effective longitudinal component () of the dielectric permittivity while has no effect on the transverse component (). Ionic conductivity decreases for low concentration of the nanoparticles. In the case of homeotropic aligned samples, a dielectric relaxation mode has been detected in the MHz frequency region. Presence of the Al-NPs decreases the relaxation frequency corresponding to flip-flop motion of molecules about their short axes. From the frequency-dependent dielectric measurements, important dielectric parameters have been determined. Electro-optical experiments show that the threshold voltage decreases and the steepness of the transmission–voltage curve improves due to dispersion of Al-NPs.

Numerical simulation of flow and structure in nematic liquid crystalline materials between eccentric cylinders

In this paper the time transient isothermal flow of lyotropic nematic liquid crystals (LC) between two eccentric cylinders was studied numerically by applying the Landau-de Gennes (LdG) theory. The start-up flow induced by the steady rotation of the inner cylinder was used to model the lubrication problem inside a journal bearing. The ability of liquid crystalline materials to form ordered boundary layers with good load-carrying capacity, outstanding lubricating properties, has been widely demonstrated. The LdG theory for the microstructure of the molecules along with continuity and momentum equations were solved simultaneously using General PDE and Laminar Flow packages of the COMSOL Multiphysics. Flow properties and structure were investigated as a function of simulation time, the Reynolds number and the Energy ratio, R. Interconnection and impact of the texture and defects formation/evolution on velocity profile and pressure distribution was observed; nucleation and evolution of disclination line and defect points were detected and pursued over the simulation time. For high Reynolds numbers, the flow reached quasi-stationary condition while the microstructure evolution was at unsteady state. The influence of the Energy ratio and the Reynolds number on the formation and evolution of polydomains was studied. Indeed, it was found that coupling the evolution of liquid crystalline microstructure with the hydrodynamics has a quantitative and qualitative impact on the macro-scale attributes of the flow and on the structure. The standard procedure to classify defects has been applied, oblate and isotropic defects have been identified.

Tuning Fluidic Resistance via Liquid Crystal Microfluidics

Flow of molecularly ordered fluids, like liquid crystals, is inherently coupled with the average local orientation of the molecules, or the director. The anisotropic coupling—typically absent in isotropic fluids—bestows unique functionalities to the flowing matrix. In this work, we harness this anisotropy to pattern different pathways to tunable fluidic resistance within microfluidic devices. We use a nematic liquid crystalline material flowing in microchannels to demonstrate passive and active modulation of the flow resistance. While appropriate surface anchoring conditions—which imprint distinct fluidic resistances within microchannels under similar hydrodynamic parameters—act as passive cues, an external field, e.g., temperature, is used to actively modulate the flow resistance in the microfluidic device. We apply this simple concept to fabricate basic fluidic circuits, which can be hierarchically extended to create complex resistance networks, without any additional design or morphological patterning of the microchannels.

Flow modelling and rheological characterization of nematic liquid crystals between concentric cylinders

Liquid crystals (LCs) have good load-bearing properties and the ability to lower friction, wear and temperature between rubbing surfaces. The steady flow of thermotropic flow-aligning nematic liquid crystalline materials was studied numerically between two concentric cylinders with small gap sizes. The rheological behaviour and flow characterisation of LCs for different inner cylinder rotational velocities and anchoring angles at the boundaries were also investigated. The Leslie–Ericksen theory was implemented to model the LC microstructures. Continuity and momentum equations were simultaneously solved with the microstructure equation. Considering the nature of the governing equations for LCs, the relaxation method was selected to solve this set of nonlinear ordinary differential equations (ODEs). The orientation angle distribution and apparent viscosity were presented as a function of the rotational velocity and shear rate. Rheological characterisation of N-(4-Methoxybenzylidene)-4-butylaniline (MBBA) was performed using a rotary rheometer and the results were compared with those of numerical simulations. Furthermore, it was shown that the preferred orientation of the molecules of liquid crystalline materials in vicinity of solid surfaces gives them an advantage over isotropic Newtonian fluids by reducing the amount of resistance torque on the inner cylinder.

Liquid crystal-based dielectric loaded surface plasmon polariton optical switches

An optical switch based on liquid crystal dielectric loaded surface plasmon polariton waveguides is proposed and theoretically analyzed. The infiltration of the plasmonic structure with a nematic liquid crystalline material serving as the dielectric loading is shown to allow for extensive electrical tuning of its waveguiding characteristics. Both the electrical switching and optical properties of the proposed waveguide are investigated in the context of designing a directional coupler optical switch, which is found to combine efficient voltage control, low power consumption, high extinction ratio, and relatively low insertion losses.

Biaxial nematic phases

An overview over the current state of research in the field of biaxial nematic liquid crystalline materials is given. After a short introduction, providing some general aspects and summarizing the classical approaches, the main part focuses on recent developments of new concepts for designing biaxial nematics. First, the major theoretical concepts are outlined, including the classification to different symmetries, the importance of cooperativity and cluster formation for the development of biaxial order and the conditions for the establishment of field induced and spontaneous biaxiality in nematic phases. These new concepts also require the re-evaluation of the tools used for the identification of phase biaxiality, which are discussed briefly. In the second part, recent progress in the design of potential biaxial nematic materials, especially focussing on bent-core molecules with nematic phases, is reported and, finally, comparisons with phase biaxiality as observed in smectic liquid crystals are made.

Tunable Polarization Properties of Hybrid-Guiding Liquid-Crystal Photonic Crystal Fibers

Tunable polarization properties in hybrid-guiding photonic crystal fibers infiltrated with nematic liquid-crystalline materials are theoretically investigated, both in the case of uniform switching of the liquid crystal molecules and for realistic nematic director patterns. It is demonstrated that the bandgap formation mechanism and the modal transmission properties in such fibers offer the possibility for electrically switchable, wavelength-selective single-polarization and/or zero to extremely high-birefringence guidance.

SAFT−LC: An Equation of State for Predicting Liquid-Crystalline Phase Behavior in Carbonaceous Pitches

By adding an orientational free-energy term based on Maier−Saupe theory to the statistical associating fluid theory (SAFT) equation, we have developed a versatile equation of state for describing the phase behavior of carbonaceous pitches, a polydisperse class of discotic nematic liquid-crystalline materials that exhibits both thermotropic and lyotropic behavior. SAFT−liquid crystal (SAFT−LC) can be used to describe the phase behavior of carbonaceous pitches both at conditions where orientational effects are relatively minor (i.e., isotropic liquid−liquid equilibria in the presence of solvent) and at conditions where such effects dominate (e.g., the effect of pitch composition on the isotropic/nematic phase transition). SAFT−LC contains five well-behaved, adjustable parameters: three in SAFT for defining the binary interaction parameters and two in LC for describing the clearing temperature of a pitch pseudocomponent. Predictions of SAFT−LC for phase densities, product yields, and mesophase percentages o...

Structure Control of π-Conjugated Polymers for Enhanced Solid-State Luminescence: Synthesis and Liquid Crystalline and Photophysical Properties of New Bulky Poly(p-phenylenevinylene)s and Oligo(phenylenevinylene)s Bearing Tricyclodecane Pendants

A series of novel highly luminescent bulky oligo(phenylenevinylene)s (MTCD−OPV, BTCD−OPV, and MEH−OPV), bulky π-conjugated poly(p-phenylenevinylene)s (MTCD−PPV, BTCD−PPV), and symmetrically or unsymmetrically substituted bulky random PPV copolymers (BTCD-x and MTCD-x series, where x = 0−100 mol %) bearing tricylodecane (TCD) pendants were synthesized to trace the factors which control the molecular aggregation of π-conjugated materials. The 1H NMR signals of the OPVs were utilized to identify different types of repeating units in the copolymer backbone for determining their compositions. The thermal analysis revealed that all the polymers were highly amorphous, and the plots of Tg vs the composition of copolymers followed straight line Flory−Fox trend for a wider temperature ranging from 75 to 200 °C. The emission studies indicate that the copolymers with high rigidity (or high Tg) showed 4−5 times enhancement in photoluminescence (PL) intensity in the solid state and doubling of quantum yield in solution compared to MEH−PPV. The CPK space-filling model confirmed that bulky TCD substitution in the PPV induces strong steric hindrance and increases the intra- or interchain distances in the polymer backbone, which in turn control the π-stack-induced molecular aggregation for high PL intensity. The luminescence of oligo(phenylenevinylene)s (OPVs) was unique to their pendant groups, and their absolute solid-state quantum efficiency determined by diffuse reflectance techniques revealed that TCD-substituted molecules MTCD−OPV and BTCD−OPV showed enhancement in the quantum efficiency (82 and 64%, respectively) compared to the MEH−OPV (27%). The thermal analysis and PLM observation of the OPV molecules indicate that while cooling from melt the TCD unit in MTCD−OPV hinders the packing of the molecules and forces the entire matrix to trap into amorphous glassy domain. The glassy nature of the molecule contributes to the 82% quantum yield of MTCD−OPV, which is the first OPV in the literature with more than 80% solid-state luminescent quantum yield. The symmetrical substitution enhances lateral packing of molecules in BTCD−OPV, and it was found as a thermotropic nematic liquid crystalline material with 64% quantum yield. In a nutshell, here for the first time, we have successfully demonstrated that the chemical structure of the bulky π-conjugated materials plays a major role in the manipulation of solid-state luminescent intensity, quantum yield, liquid crystalline and amorphous glassy nature of PPVs, and its triad oligomers.

Tunable one-dimensional photonic crystal slabs based on preferential etching of silicon-on-insulator

We design and assess a one-dimensional photonic crystal slab fabricated by preferential etching of a silicon-on-insulator substrate. The etched grooves are considered to be infiltrated by a highly-birefringent nematic liquid crystalline material. A detailed analysis of the nematic director response within the grooves is presented. We investigate different configurations and demonstrate large band gap shifting when switching the liquid crystal with an applied voltage. Furthermore, we assess this type of device as an efficient alternative for compact refractometric optical sensing applications.

514 ハイブリッドセルを用いた液晶アクチュエータの提案(2)(OS5-4 機能性流体工学の先端融合化,OS5 機能性流体工学の先端融合化,オーガナイズドセッション)

As a fundamental research on the development of liquid crystalline micro actuators, experiments on the liquid crystalline actuator have been performed using a hybrid cell. The nematic liquid crystalline material, 5CB, is placed between two parallel glass plates. The lower glass plate is fixed, and the upper glass plate can move freely. Pulsed voltage of 10 V is applied between the plates, and the movement of the upper glass plate is observed under a microscope. The speeds of the upper glass plate are about 4.3, 26.5, and 54.5 μm/s for 1, 10, and 100 Hz of the pulsed voltage frequency, respectively. It is also found that the liquid crystalline actuator with the hybrid cell can be driven with lower applied voltage than that with the planar cell. Finally, it is concluded that the hybrid cell is available for the liquid crystalline micro actuators, although its efficiency is low in comparison with the planar cell.

L-7: Late-News Paper: Novel Polymerisable Liquid Crystalline Acrylates for the Manufacturing of Ultrathin Optical Films

Newly developed polymerisable nematic liquid crystalline materials in the Paliocolor® family with medium and high birefringence (0.16 and 0.20, respectively) are described. They enable the manufacturing of optical films, such as retardation films, brightness enhancement films and color filters, with superior quality and extremely reduced thickness by using coating techniques.

Optimizing the nematic liquid crystal relaxation speed by magnetic field

Nematic liquid crystalline materials have been widely used in electro-optical control of visible light. However, when IR light is considered, the thickness of the liquid crystal layer must be increased which causes the relaxation time of the material to be slower than required for many applications. In this paper we use a magnetic field to increase the speed of thick nematic devices. We show that above a particular magnetic field strength, thicker cells relax more quickly than thinner ones. Also, we find that there exists an optimal voltage range for devices of a particular thickness and with a particular applied magnetic field. Devices that allow a half-wave modulation of 1.55μm light in less than 5ms are shown to be possible with the use of a 2T magnetic field.

液晶アクチュエータの基礎実験(S23-4 複雑流体の流れとその応用(4),S23 複雑流体の流れとその応用)

Experimental Study on liquid crystal actuators is performed under a polarized microscope. The low molecular weight nematic liquid crystalline material is placed between parallel glass plates. The lower plate is fixed, and the upper can move freely. When 5V of electric field is imposed on the liquid crystalline material, the rod-like molecules re-align to the orientation direction perpendicular to the plates. During this re-alignment of the liquid crystalline molecules, the fluid flow, called back-flow, is induced. Because of the back-flow mechanism, the upper glass plate moves in a certain direction.

Low–frequency acoustic waves in nematic elastomers

We study the spectral and polarization properties of acoustic waves propagating in nematic liquid-crystalline rubber materials. We apply the viscoelastic theory of nematic elastomers in the low-frequency (hydrodynamic) limit. Dynamic soft elasticity, exhibited by ideal nematic elastomers under certain geometries of shear at low frequencies, leads to anomalous anisotropy of energy transfer and attenuation of transverse waves. The results suggest an application of this class of materials as an acoustic polarizer medium.