Nematic Mixtures Research Articles

Effect of nanoparticle chain formation on dielectric anisotropy of nematic composites

A general theory of the dielectric constant of nematic liquid crystal mixtures is presented including the particular case of nematics doped with polar nanoparticles. The results are used to estimate the contribution of chains of polar nanoparticles to the static dielectric anisotropy and birefringence of the nematic composite taking into account contributions from chains of different lengths. The dependence of the dielectric anisotropy on the dipolar interaction strength is considered in detail and it is shown that formation of polar chains of nanoparticles enables one to explain a significant increase of the dielectric constant of the composite as observed experimentally.

Asymmetric director structures and flexoelectricity in nematic pi-cell devices

The sum of the flexoelectric coefficients in a liquid crystal material has been measured in nematic pi-cell devices, based on a method that exploits the asymmetry in the director configurations of the different states in a pi-cell, the uniform surface alignment polarities, and the influence of ions. A value of |e1 + e3| = 10 pC m−1 was measured from data-theory comparisons in the standard commercial eutectic E7 nematic liquid crystal mixture.

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

By combining [Mo6I8(C n F2n+1COO)6]2‐ (n = 1, 2, 3) nanocluster units with liquid crystalline ammonium cations, a new series of hybrid materials is developed that show a nematic liquid crystal phase, the most fluid of all LC phase, on a large range of temperatures including room temperature. The photophysical studies performed in the LC state show that these self‐assembled hybrid materials emit in the red‐NIR with absolute quantum yields up to 0.7 and show a very good photostability under continuous irradiation. They are further integrated up to 20 wt% in E7, a well‐known nematic commercial LC mixture. Mixtures are investigated in terms of homogeneity and stability to select the best suitable candidate for the design of electro‐controlled devices. Studies of optical switching, contrast, viscosity, and behavior toward an electrical stimulus demonstrate the high potential of these hybrid materials in the fields of photonic or optoelectronic.

‘Blue phases’ of highly chiral thermotropic liquid crystals with a wide range of near-room temperature

The phase transitions of pure cholesteric liquid crystals, based on right- or left-handed chiral dopants (ChDs) (enantiomers R-811 and S-811, Merck, Darmstadt, Germany) with various concentrations dissolving in the nematic liquid crystal mixture CHCA (ZhK-805, Russia, NIOPIK) were experimentally studied. For the first time, blue phases of cholesterics were found at the concentration range of ChDs 32–36 wt.%. Experimentally it was observed that during the cooling process the blue phase of cholesterics is stable over a wide temperature range about ~15°C including human body and near-room temperatures. Thermal phase transitions, spectral characteristics and electro-optical features of blue phases were examined. Planar and homeotropic alignment layers were used to study the influence of various boundary conditions on platelet textures of blue phases.

Properties of two-component nematic liquid crystal mixtures in the range of 0.3–3.0 THz

In this work, we present the results of terahertz measurements of five nematic liquid crystal materials. Optical properties like refractive indices, birefringence and absorption coefficients of liquid crystals (LCs) for ordinary and extraordinary polarisation were compared. The aim of this study was to check the impact of the structure of LC molecules on above properties. The compounds with different core structure and various number of laterally substituted fluorine atoms were chosen. A standard terahertz Time-Domain Spectrometer to measurements from 0.3 THz to 3 THz was used.

DSM1-DSM2 Transition Threshold in Turbulent Nematic Mixtures

When forced by a suitable electric field, calamitic nematics can undergo electro-hydrodynamic instabilities and become turbulent. In contrast to isotropic fluid, they present a transition at high forcing from a turbulent regime (DSM1) to a regime characterized by high density of topological defects (DSM2). We study the transition threshold in nematic mixtures with some percent of 5 CB in MBBA. The threshold decreases by increasing the 5 CB concentration, an opposite behavior to what is expected by the increase of dielectric anisotropy. We suggest that the threshold control parameter is the biaxial coherence length, that is of the order of defect size.

Effects of Photonic Band Gap of Cholesteric Liquid Crystal on Chemiluminescence

We have fabricated glass cells filled with cholesteric liquid crystalline materials (CLC reflectors), which are mixtures of a nematic liquid crystalline mixture, ZLI-2293 (Merck), and a chiral dopant, MLC-6248 (Merck). We reported the enhancement of the maximum emission intensity of luminol reaction by the photonic band gap (PBG) of the CLC reflectors. Here, we report the effect of the relative position of PBG of the CLC reflectors to the emission spectra of luminol reaction on the enhancement of the maximum emission intensity.

Aggregate‐induced emission in light‐emitting liquid crystal display technology

AbstractNowadays, liquid crystal displays (LCDs) with light‐emission are considered as energy‐efficient devices and are promising alternatives to conventional LCDs. To realize such possibility, strong fluorescent materials with a dichroic properties are required. Aggregate‐induced emission (AIE) is an unusual photophysical phenomenon shown by some luminogenic materials that will be highly emissive in their aggregate state. In this work, we studied the AIE effect of a luminescent liquid crystalline molecule TPE‐PPE in our LC system as a luminophore dopant. The result showed the excellent AIE effect that higher concentration of luminogen in the nematic LC host induced stronger luminescent intensity. Through exposure of a photoisomeriable alignment material sulfonic‐dye‐1, the photopatterning of a light‐emitting LC device was achieved with the use of the TPE‐PPE/nematic LC mixture.

Fast switching negative dielectric anisotropic multicomponent mixtures for vertically aligned liquid crystal displays

Negative dielectric anisotropic nematic multicomponent mixtures are particularly attractive in vertically aligned mode liquid crystal displays for achieving faster response time. In this work, we report the mesomorphic and physical properties (e.g. birefringence, dielectric anisotropy, bend elastic constant, relaxation time and rotational viscosity) of three nematic multicomponent mixtures based solely on laterally fluorinated aromatic rigid core structures. A nine component Base mixture was prepared and further modified into two different ten component mixtures using suitable laterlally fluorinated tolane dopants. These mixtures exhibit reasonably large values of dielectric anisotropy, low values of rotational viscosity as well as a high figure of merit which is desirable for reducing the device response time. The mixtures emerged to be promising candidate for their applicability in vertically aligned mode display devices.

Helical Twisting Power Dependence of Blue Phase Stability for Chiral Nematic Liquid Crystal

In this study, we investigated blue phase (BP) temperature range and helical twisting power (HTP) using various types of chiral nematic liquid crystal (LC) mixtures composed of two kinds of chiral dopants and four kinds of host nematic LCs with different core structures. We found that chiral nematic LC mixtures containing three-ring core structure host nematic LCs such as 5CCB and 5CT presented small HTP values compared to those containing two-ring core structure host nematic LCs such as 5CB and 5CH regardless of chiral dopant species. We also found that the BP was not present when the HTP was of a magnitude less than 35.64 μm−1, that the BP temperature range increased when the HTP was of a magnitude between 35.64–49.03 μm−1, and that it decreased when the HTP was of a magnitude between 51.60 –63.03 μm−1 in cyano homologue chiral LC nematic LC mixtures.

Evolution of Microwave Nematic Liquid Crystal Mixtures and Development of Continuously Tuneable Micro- and Millimetre Wave Components

This work represents the evolution of micro- and millimetre wave optimized nematic liquid crystal mixtures and their applications. Starting with the well-known liquid crystal mixture K15 used in the display technology, microwave optimized LC mixtures have been developed. For that, novel characterization setups have been developed, which play an essential factor in the optimization process for the micro-, millimetre wave and THz regime. By using a specialized LC simulation tool, different tuneable waveguide topologies are compared in terms of tuneability, transmission loss and tuning speed. Based on that, a phase shifter for W-band frequencies has been fabricated, which reached a measured figure of merit for passive phase shifters of up to 148° /dB.

Molecular and Intramolecular Motions in Dual-Frequency Nematic Liquid Crystal, at Extra Low Temperatures, Observed by Means of Dielectric Spectroscopy

New dual frequency nematic liquid crystal mixture has been prepared and investigated by means of dielectric spectroscopy in a wide temperature range (from 120°C to −100°C). The molecular structures of compounds used to prepare the final mixture and mixture's composition influence its dielectric properties. Wide temperature investigations show how molecular and intramolecular motions change with temperature. It is possible at low temperatures to slow down intramolecular motions so much that they are visible in dielectric spectroscopy. At −80°C molecular motions disappear from dielectric response. At −100°C all thermally activated motions are too slow to be detected by means of dielectric spectroscopy.

Asymmetric Director Structures and Ions in the Measurement of the Flexoelectric Sum (e1 + e3)

Hybrid Aligned Nematic (HAN) devices filled with highly ionic liquid crystal materials screen out the internal bias due to surface polarization, making it possible to determine the flexoelectric sum, e1 + e3. A transient change is observed in the device response due to differing polarities of an external offset applied to an AC signal, with a time constant similar to the characteristic ion relaxation time in the material. Assuming ions screen out the internal bias, data-theory fits in the low frequency, low voltage perturbative regime gives a value of /verte1 + e3/vert = 10 pCm−1 for the standard commercial eutectic E70 nematic liquid crystal mixture.

Ultra-low viscosity liquid crystal materials

We report five ultra-low viscosity nematic liquid crystal mixtures with birefringence around 0.1, dielectric anisotropy in the range of 3 to 6, and clearing temperature about 80°C. A big advantage of these low viscosity mixtures is low activation energy, which significantly suppresses the rising rate of viscosity at low temperatures. Using our mixture M3 as an example, the response time of a 3-μm cell at −20°C is only 30 ms. Widespread application of these materials for display devices demanding a fast response time, especially at low temperatures, is foreseeable.

Hybrid aligned nematic based measurement of the sum (e1+e3) of the flexoelectric coefficients

A new method has been established for the measurement of the sum of the flexoelectric coefficients e1+e3 in liquid crystals by exploiting the properties of highly ionic materials in order to screen out the internal bias due to the different surface alignment polarities in a Hybrid Aligned Nematic (HAN) liquid crystal device. It has been shown that responses to pulses are independent of the external offset of a signal applied to a HAN device filled with a highly ionic material. Driving the device with step changes in the offset leads to either a transient increase or transient decrease in the response, depending on the polarity of the offset, while the equilibrium response remains the same. The time constant of the transient effect is consistent with the relaxation time of the ions present in the material. Assuming these ions screen out the internal bias completely, the remaining response can be used as a measure of the flexoelectric effect. Based on this approach, a value of (10 ± 2) pC m−1 was found for the modulus of the flexoelectric sum in the standard commercial eutectic E70 nematic liquid crystal mixture.

Immunoassays for the cancer biomarker CA125 based on a large-birefringence nematic liquid-crystal mixture.

The use of fluorescence is ubiquitously found in the detection of immunoreaction; though with good sensitivity, this technique requires labeling as well as other time-consuming steps to perform the measurement. An alternative approach involving liquid crystals (LCs) was proposed, based on the fact that an immunocomplex can disturb the orientation of LCs, leading to an optical texture different from the case when only antigen or antibody exists. This method is label-free, easy to manipulate and low-cost. However, its sensitivity was low for practical usage. In this study, we adopted a high-birefringence liquid crystal (LC) to enhance the sensitivity for the immunodetection. Experiments were performed, targeting at the cancer biomarker CA125. We showed that the larger birefringence (Δn = 0.33 at 20 °C) amplifies the detected signal and, in turn, dramatically improves the detection limit. To avoid signal loss from conventional rinsing steps in immunodetection, CA125 antigen and antibody were reacted before immobilized on substrates. We studied the specific binding events and obtained a detection limit as low as 1 ng/ml. The valid temperature ranges were compared by using the typical single-compound LC 5CB and the high-birefringence LC mixture. We further investigated time dependency of the optical textures and affirmed the capability of LC-based immunodetection in distinguishing between specific and nonspecific antibodies.

All-optical measurement of elastic constants in nematic liquid crystals.

In this article we present a new all-optical method to measure elastic constants connected with twist and bend deformations. The method is based on the optical Freedericksz threshold effect induced by the linearly polarized electro-magnetic wave. In the experiment elastic constants are measured of commonly used liquid crystals 6CHBT and E7 and two new nematic mixtures with low birefringence. The proposed method is neither very sensitive on the variation of cell thickness, beam waist or the power of a light beam nor does it need any special design of a liquid crystal cell. The experimental results are in good agreement with the values obtain by other methods based on an electro-optical effect.

Interpretation of different molecular packing in the micelles of lyotropic nematic and cholesteric phases via mass density measurements

Lyotropic nematic and cholesteric mixtures were produced by dissolving amino acid based racemic surfactants, potassium N-dodecanoyl-dl-alaninate (dl-KDDA) and potassium N-dodecanoyl-dl-serinate (dl-KDDS), and their l-enantiomer surfactants (l-KDDA and l-KDDS) in a mixture of sodium sulfate (Na2SO4)/1-decanol (DeOH)/water, separately. Polarizing light microcopy (PLM) and mass density measurements were applied to each mixture. The textures of all phases were investigated by PLM. Mass density measurements showed that the cholesteric phases of both l-KDDA and l-KDDS exhibited larger mass density values than their dl-KDDA and dl-KDDS nematic phases, respectively. This result was supported by our previous small-angle X-ray scattering study. The difference between micelle volumes of cholesteric and nematic phases was explained by different molecular packing of dl- and l-forms of the same surfactant molecules in the achiral and chiral micelles, respectively. This indicated that mass density measurement may be a useful way to detect the different molecular packing in the micelles which causes the increase or decrease in the micelle size or volumes qualitatively as it is expected.

Structural, optical and dynamical properties of a high birefringence laterally fluorinated terphenyl isothiocyanate

A doubly fluorinated isothiocyanato butyl terphenyl compound exhibiting a broad range of the nematic and a narrow range of smectic phase has been investigated by X-ray, dielectric and optical methods and compared with the properties of singly and doubly fluorinated ethyl homologues to see the effect of fluorination and chain flexibility. Contrary to common perception, weak antiparallel correlation of molecules is observed in each compound both from X-ray and dielectric studies. Observed dielectric anisotropy is strong enough to ensure a driving voltage suitable for thin film transistor-based liquid crystal display. Measured splay elastic constant suggests that the present compound will exhibit the fastest response. Flip-flop mode relaxations are observed in a few hundred kHz to MHz range. All the compounds exhibit high birefringence (0.373–0.331). They also show high order parameters at low temperatures. These compounds are expected to be suitable for formulating high birefringence nematic mixtures for fast switching displays.

Spontaneous reorientation from planar to homeotropic alignment in dual-frequency mixture doped with chiral dopant

We investigate a dual-frequency (DF) nematic mixture by means of the dielectric spectroscopy. A chiral dopant has been added to the DF matrix. A pure DF matrix as well as three samples with increasing concentration of chiral dopant (DF*) have been prepared. Investigated samples have shown a selective reflection band, according to a helical pitch induced in proportion to the chiral dopant amount. Such system promises a great application potential as a selective light reflecting shutter or spectral filter due to nearly symmetrical switching times and defectless structures.When we put the non-doped DF mixture into homogenous (HG) oriented cell we found that planar orientation is permanent on both cooling and heating cycles. In the case of DF* chiral mixtures, the behavior changes substantially. One can observe that the oriented virgin planar structure of DF* within the HG cell starts to change its orientation to homeotropic one at different temperature. When heating is continued the homeotropic orientation in the HG cell is preserved up to the clearing temperature. Upon a subsequent cooling from the isotropic state, the homeotropic orientation is preserved within the whole mesogenic state. The effect of induction of the homeotropic state observed within HG cells is analyzed and discussed.