Physical Properties Of Liquid Crystals Research Articles

Synthesis and examination of alkoxycyanobiphenyl mesogens with a single fluorine atom at specific locations in the tail

ABSTRACT Selective fluorination is a well-established technique to tailor and improve many of the physical properties of liquid crystals. Cyanobiphenyls are among the best-known and understood thermotropic liquid crystals and have served admirably as a testbed for structural modifications that can eventually be extended to other classes of liquid crystals. As such, and expanding on our earlier work, in the present study of the alkoxycyanobiphenyl M series, the individual tail sites of both 4’-(butoxy)[1,1’-biphenyl]-4-carbonitrile (M12) and 4’-(pentyloxy)[1,1’-biphenyl]-4-carbonitrile (M15) were selectively monofluorinated in order to examine the influence on the phase behavior. These fluorinated compounds were synthesized using epoxidation, regioselective epoxide opening and deoxyfluorination chemistry. In all the fluorinated compounds in both series a monotropic nematic phase was found in the cooling cycle. It was observed that as the fluorine atom on the tail moves closer to the core, the compounds exhibited a metastable room temperature nematic mesophase, but recrystallization invariably occurred. Both enantiomers of 4’-(2-fluorobutoxy)[1,1’-biphenyl]-4-carbonitrile were synthesized in high enantiomeric purity by regioselective opening of chiral epoxide intermediates followed by deoxyfluorination, as established by chiral high-performance liquid chromatography, and their phase behaviors were determined. Moreover, the average dipole moments of the synthesized fluorine substituted derivatives were also calculated.

Determination of order parameter and birefringence depending on temperature of nematic calamitic mesophase formed by TTAB + water binary system using polarized microscopy

In the present study, the temperature-dependent optical transmittances of lyotropic nematic calamitic mesophase formed by TTAB-water binary system were measured for certain wavelengths under polarized microscopy to obtain macroscopic order parameter with birefringence. Because birefringence in lyotropics is quite small (order of 10-3), it is very difficult to make sensitive measurements of birefringence and to obtain order parameter. The study easily achieved the order parameter depending on temperature for different wavelengths only using polarized microscopy. Therefore, the parameter could be more easily determined in liquid crystal application and the physical properties of liquid crystals, especially for lyotropics, can be better understood.

Physical properties of liquid crystals doped with CsPbBr3 quantum dots

ABSTRACT CsPbBr3/SiO2 perovskite quantum dots (PQDs), which exhibit good stability in water, air, and light, are synthesised using the water-trigger method. Due to the interaction between the long alkyl ligands of the PQDs and the liquid crystal, the liquid-crystal molecules prefer to align perpendicular to the surface of the PQDs, which disturbs the liquid crystal order and decreases its clearing point. The effect of CsPbBr3/SiO2 PQDs on the electro-optical properties and physical parameters of a nematic liquid crystal are investigated. The dielectric anisotropy (Δε) and splay elastic constant (K11) decrease substantially with increasing concentration of PQDs. PQDs, which are doped into the liquid crystal, can effectively reduce the threshold voltage (V th), which opens possibilities to reduce energy consumption in many optoelectronic devices.

Synthesis and properties of 5,6-dihydro-4H-cyclopenta[b]thiophene-based nematic liquid crystals: A new access to mesogens with high birefringence and large dielectric anisotropy

A novel liquid crystal core unit 5,6-dihydro-4H-cyclopenta[b]thiophene was designed and synthesized in four steps and three mesogens containing terminal cyano group were then obtained. The effects of this skeleton on the mesophase and physical properties of liquid crystals were then investigated by compared with two series of reference compounds based on 1,4-disubstituted phenylene and disubstituted phenylcyclohexane and found this unit can benefit the formation of the nematic phase. Besides, both the birefringence (Δn) and dielectric anisotropy (Δε) values were increased after the introduction of this core unit. Density function theory calculations suggested that the combination of thiophene and cyclopentane had increased the height/width ratio and linearity of the mesogens which can stabilise the liquid crystal phase. Five different liquid crystals based on this new core were synthesized at last to further illustrate the usefulness of this core in constructing high birefringence and large dielectric anisotropy liquid crystals with wide nematic phase interval.

Learning physical properties of liquid crystals with deep convolutional neural networks

Machine learning algorithms have been available since the 1990s, but it is much more recently that they have come into use also in the physical sciences. While these algorithms have already proven to be useful in uncovering new properties of materials and in simplifying experimental protocols, their usage in liquid crystals research is still limited. This is surprising because optical imaging techniques are often applied in this line of research, and it is precisely with images that machine learning algorithms have achieved major breakthroughs in recent years. Here we use convolutional neural networks to probe several properties of liquid crystals directly from their optical images and without using manual feature engineering. By optimizing simple architectures, we find that convolutional neural networks can predict physical properties of liquid crystals with exceptional accuracy. We show that these deep neural networks identify liquid crystal phases and predict the order parameter of simulated nematic liquid crystals almost perfectly. We also show that convolutional neural networks identify the pitch length of simulated samples of cholesteric liquid crystals and the sample temperature of an experimental liquid crystal with very high precision.

Order parameter of dual-frequency nematic liquid crystals measured over a wide range of thicknesses

ABSTRACT Basic physical properties of liquid crystals strongly depend on the orientational order parameter. Given a fundamental importance of this parameter, it was measured for a variety of liquid crystals by means of various experimental techniques. At the same time, the reported results were limited to a relatively narrow range of thicknesses of the studied liquid crystal cells (typically, within a 1–20 µm interval). This paper presents the values of the orientational order parameter of dual-frequency nematic liquid crystals measured over a wide range of thicknesses (from 5 to 200 µm). The non-monotonous dependence of the order parameter S on the cell thickness obtained for relatively thin layers (< 60 µm) of dual-frequency nematics is followed by a rapid monotonous decay of S for thicker (60–200 µm) liquid crystal samples. The obtained results are important for the development of emerging, non-display, applications of liquid crystals utilising thick cells and fast switching materials such as dual-frequency nematics. In particular, they provide a basis for understanding recently discovered two regimes of electro-optical switching of thin and thick layers of dual-frequency nematics (Crystals, 9(6), pp. 314, 2019).

Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience

The research field of liquid crystals and their applications is recently changing from being largely focused on display applications and optical shutter elements in various fields, to quite novel and diverse applications in the area of nanotechnology and nanoscience. Functional nanoparticles have recently been used to a significant extent to modify the physical properties of liquid crystals by the addition of ferroelectric and magnetic particles of different shapes, such as arbitrary and spherical, rods, wires and discs. Also, particles influencing optical properties are increasingly popular, such as quantum dots, plasmonic, semiconductors and metamaterials. The self-organization of liquid crystals is exploited to order templates and orient nanoparticles. Similarly, nanoparticles such as rods, nanotubes and graphene oxide are shown to form lyotropic liquid crystal phases in the presence of isotropic host solvents. These effects lead to a wealth of novel applications, many of which will be reviewed in this publication.

Estimating physical properties from liquid crystal textures via machine learning and complexity-entropy methods.

Imaging techniques are essential tools for inquiring a number of properties from different materials. Liquid crystals are often investigated via optical and image processing methods. In spite of that, considerably less attention has been paid to the problem of extracting physical properties of liquid crystals directly from textures images of these materials. Here we present an approach that combines two physics-inspired image quantifiers (permutation entropy and statistical complexity) with machine learning techniques for extracting physical properties of nematic and cholesteric liquid crystals directly from their textures images. We demonstrate the usefulness and accuracy of our approach in a series of applications involving simulated and experimental textures, in which physical properties of these materials (namely: average order parameter, sample temperature, and cholesteric pitch length) are predicted with significant precision. Finally, we believe our approach can be useful in more complex liquid crystal experiments as well as for probing physical properties of other materials that are investigated via imaging techniques.

The effect of phenyl ring on the physical properties of liquid crystals containing 4-pyridyl terminal group

ABSTRACTLiquid crystals with 4-pyridyl end group containing one triple bond and phenylenes have been synthesised. Their structures were confirmed by traditional spectroscopic methods. The properties were measured by differential scanning calorimetry (DSC), polarising optical microscopy (POM), Abbe refractometer and an electrical constants instrument. The results show that all of the compounds exhibit nematic phase. The addition of a phenylene to the molecular skeleton brings about an obvious increase of the mesophase intervals (from 8.8 to 95.5) and birefringence (up to 0.33), as well as minus or plus effect of dielectric anisotropies depending on the position of the phenylene. Density Functional Theory (DFT) calculations of molecular conformation, polarisability and dipole moment were used to further study the molecular structure–property relationship.

Ultralow Friction Between Steel Surfaces Achieved by Lubricating with Liquid Crystal After a Running-in Process with Acetylacetone

A suitable running-in process is advantageous for reducing friction. The aim of the present work was to study the influence of the running-in with acetylacetone on tribological performance of 4-Cyano-4’-pentylbiphenyl (5CB) liquid crystal. Friction tests were performed between steel surfaces in a ball-on-disk sliding system. After a running-in period of 240 s, the COF of 5CB was measured to be 0.013, which is about a quarter of the value (0.055) without running-in. The reduced contact pressure, caused in running-in process, does not directly lead to a drop in COF. The generation of tris(acetylacetonato) iron(III) induced by the tribochemical reactions between acetylacetone and steel surfaces, and the unique physical properties of liquid crystal are assumed to be reasons for the ultralow COF. Surface analysis was performed to correlate COF with the topography of wear surfaces. An evenly distributed specific grooved structure observed on wear area of the ball may have a beneficial effect on COF as well. We believe our findings can provide an effective and simple solution to reduce COF of liquid crystal between steel surfaces. A better understanding of the tribological behavior is needed for the development of this tribological system and for the possible future applications.

45.3: Invited paper: Problem Solving about Image Sticking through Order Parameter

Order Parameter (OP) controls almost every physical properties of liquid crystal including Tni, △n and △ε…. Moreover, we found that order parameter of LC material is strongly related to image sticking performance. In another words, the image sticking performance can be predicted and optimized by using the index of order parameter. Hereinafter, further on in this document about the fundamental mechanism in between order parameter and image sticking performance and also how to control order parameter will be introduced. It should be pointed out that order parameter only representing the capability of LC material itself against image sticking issue, and ignore the influence factors from material collocation such as LC/PI.

Segregation of liquid crystal mixtures in topological defects

The structure and physical properties of liquid crystal (LC) mixtures are a function of composition, and small changes can have pronounced effects on observables, such as phase-transition temperatures. Traditionally, LC mixtures have been assumed to be compositionally homogenous. The results of chemically detailed simulations presented here show that this is not the case; pronounced deviations of the local order from that observed in the bulk at defects and interfaces lead to significant compositional segregation effects. More specifically, two disclination lines are stabilized in this work by introducing into a nematic liquid crystal mixture a cylindrical body that exhibits perpendicular anchoring. It is found that the local composition deviates considerably from that of the bulk at the interface with the cylinder and in the defects, thereby suggesting new assembly and synthetic strategies that may capitalize on the unusual molecular environment provided by liquid crystal mixtures.

Freedericksz transition in smectic-A liquid crystals doped by ferroelectric nanoparticles

The structure, orientation and physical properties of liquid crystals is significantly affected by doping of ferroic (ferromagnetic and ferroelectric) nanoparticles into liquid crystals. Behavior of these suspensions in external fields depends on anchoring energy, volume fraction of doped nanoparticles, and sign of dielectric anisotropy of liquid crystals as well as relative orientation of director and local polarization of nanoparticles. Here, the threshold voltage for molecular reorientation (Freedericksz transition) is estimated theoretically in smectic-A liquid crystals doped by spherical ferroelectric nanoparticles, near the smectic-A to smectic-C transition temperature taking into account local electric field effects. It is shown that the threshold voltage is decreased by doping ferroelectric nanoparticles into liquid crystals.

Nematic mesophase enhanced via lateral monofluorine substitution on benzoxazole-liquid crystals

ABSTRACTFluorine is widely used as a lateral substituent to modify the physical properties of liquid crystals. Here, laterally monofluorinated compounds, 2-(4ʹ-alkoxy-2-fluorobiphenyl-4-yl)-benzoxazole derivatives (nPPF(2)Bx) bearing different substituents (H, CH3, NO2, coded as nPPF(2)BH, nPPF(2)BM and nPPF(2)BN, respectively) at 5-position, were synthesised and characterised. It is interesting to note that these only display enantiotropic nematic mesophases with mesophase ranges of 12–28°C and 13–45°C on heating and cooling for nPPF(2)BH, 46–97°C and 62–120°C for nPPF(2)BM and 82–108°C and 87–113°C for nPPF(2)BN, which are very different from the corresponding monofluorine-substituted analogue (compounds I) with enantiotropic smectic or smectic/nematic mesophases. The enhanced nematic mesophase is attributed to the reduced π–π interaction/conjugation resulting from the twisted structure of the molecule caused by the introduction of a fluorine atom into the inter-ring of the biphenyl unit. These results suggest that modification of the monofluorine substituent position is an effective method to improve the nematic mesophase in benzoxazole-liquid crystals.

Design of liquid crystals: from a nematogen to thiophene-based π-conjugated mesogens

It is fundamentally important to study the relationships between structures and properties of liquid crystals. Professor G.W. Gray developed new rod-like nematic liquid crystals, 4-alkyl- and 4-alkoxy-4ʹ-cyanobiphenyls, which act as materials for stable informational liquid crystal displays. This molecule was invented based on his deep understanding of the relationships of chemical structure and physical properties of liquid crystals. After the discovery of nematogenic liquid crystals for display application, the advent of the molecular design and synthesis of liquid crystals has produced a variety of new family of functional liquid crystals. For example, as a new twist of rigid rod mesogens, oligothiophene rings have emerged recently. Due to their extended π-conjugated systems, oligothiophene-based liquid crystals exhibit various electronic and optical functions. In this review, we describe the impact of Professor Gray’s nematic liquid crystals as well as the structures and functions of oligothiophene-based liquid crystals.

Influence of the size and the attached organic tail of modified detonation nanodiamond on the physical properties of liquid crystals

For the functionalisation of the detonation nanodiamond (DND), we attached via grafting carboxylate groups, leading to only one of such groups per 5000–6000 carbon units. Activation of the COOH-surface functionalised groups allowed the attachment of various organic tails. Dielectric and electro-optical properties of nematic liquid crystalline mixtures (LCMs) doped with modified DND (MDND) have been investigated. It is established that the effect of MDND on the dielectric properties of the LCMs depends on the size of the nanoparticles and the type of rod-like elongated organic molecules attached to the MDND. It was found that nanoparticles of small size (4–5 nm) do not significantly affect the parameters of LCMs. At the same time, the conglomerates based on MDND of about 50 nm or about 100 nm in diameter can increase or decrease the dielectric anisotropy and the response time of LCMs to about 1.5–2.5 times, depending on the polarity of the tails.

Abnormal switching behavior of nanoparticle composite systems

We have investigated the properties of liquid crystals (LCs) doped with ZnO (8% Cu doped) nanoparticles. The electro-optic properties of LCs have changed with varying concentration of ZnO nanoparticles. The dielectric anisotropy obtained from the values of dielectric permittivity at 5 kHz in the nematic and smectic phases was found to increase with increasing concentration of nanoparticles in LCs. It has been established that the effect of nanoparticles on the dielectric anisotropy depends on the physical properties of LCs; the nanoparticle disturbs the orientation ordering of LC molecules. The nanoparticle also influences the switching behavior, splay elastic constant, rotational viscosity and threshold voltage of pure LCs. A small quantity of nanoparticles causes slight reduction of the splay elastic constant and rotational viscosity of LC cells.

Physical Properties of Liquid Crystals in Athabasca Bitumen Fractions

Naturally occurring amphotropic liquid-crystals were recently identified in unreacted hydrocarbon resources and resource fractions from around the world, including Athabasca bitumen. Liquid crystal forming constituents are present in both asphaltene and maltene fractions and appear to be an important class of materials that is missed entirely during conventional hydrocarbon characterization, i.e.: SIMDIST or SARA analysis. In this contribution, some physical properties of liquid crystals in Athabasca asphaltenes and maltenes identified using experimental methods as diverse as polarized light microscopy, differential scanning calorimetry (DSC), and mid- and near-infrared photoacoustic spectroscopy with depth profiling are reported. Liquid crystals, comprising materials with an aromaticity between that of maltenes and asphaltenes, form irreversibly on the surface of both asphaltene particles and maltene drops on heating. At higher temperatures the liquid crystals become isotropic but remain on particle surf...

フェロセンを含む液晶物質の合成と物性 ( XXVI ) 1,3-二置換フェロセン誘導体の液晶相転移

フェロセンを含む液晶物質の合成と物性 ( XXVI ) 1,3-二置換フェロセン誘導体の液晶相転移

フェロセンを含む液晶物質の合成と物性 (XXVII) ビフェニル基をメソゲン基に含む一置換フェロセン誘導体の合成と物性

フェロセンを含む液晶物質の合成と物性 (XXVII) ビフェニル基をメソゲン基に含む一置換フェロセン誘導体の合成と物性