Liquid Crystal Mixture Research Articles

Molecular dynamics parameters of liquid crystal mixtures with different chemical structures

Results of molecular parameters studies using methods of dielectric spectroscopy, acoustic spectroscopy and viscometry are compared. An influence of dipole-dipole interactions on the relaxation time, the coefficients of molecular friction and rotational diffusion of molecules has been established. An analysis of modern methods for determining the coefficient of molecular friction and rotational diffusion for liquid crystal mixtures of different composition is carried out. It is shown that currently there is no theory that can describe qualitatively and quantitatively the behavior of the molecular relaxation parameters of the liquid crystal mixtures.

Studies on binary mixtures of nematic liquid crystals made of strongly polar molecules with identical cores and antagonistic orientation of permanent dipoles

We report experimental studies on optical (birefringence, Δn), dielectric (Δε) and bend-splay elastic anisotropies (ΔK = K33 − K11) of a few mixtures of two nematic liquid crystals, namely CCH-7 and CCN-47, made of highly polar molecules with identical cores and antagonistic orientation of permanent dipoles. In particular, the polar group (-CN) attached to the bicyclohexane core of CCH-7 is oriented along the longitudinal direction whereas, in CCN-47, it is oriented along the transverse direction. We show that apart from the significant contribution to the optical and dielectric anisotropies, the antagonistic orientation of strongly polar groups plays a crucial role in determining the bend-splay elastic anisotropy. The elastic properties are explained based on a model proposed by Priest, considering the effect of intermolecular association and the resulting length-to-width ratio of the molecules.

Effect of graphene oxide dispersion in antiferroelectric liquid crystal mixture in the verge of SmC* to SmCA* phase transition

The continuous growing demand for nanoscience applications and the improvement in the performance of liquid crystal based devices has been extensively required by the technological world. Recent progress in the field of liquid crystals has found its practical implementation in various display and non display devices which experiences obstacle due to impurity effects that reduces its performance. The dispersion of nanoparticles in liquid crystal medium helps in the reduction of impurity ions and thus improving the performance of liquid crystal based devices. The present work is based on the collective dielectric relaxation processes that have been observed in antiferroelectric liquid crystal (AFLC) mixture W1000 dispersed with 0.1% wt/wt and 0.3% wt/wt concentrations of graphene oxide. Graphene oxide itself favors vertical alignment and the coupling of AFLC W1000 mixture with graphene oxide affects its molecular ordering. This has been confirmed from the polarizing optical micrographs. The dielectric relaxation modes have been observed with and without the application of bias voltage in SmC* to SmCA* phase transition during cooling cycle. The appearance and disappearance of PL, PH and X modes have been observed and are explained on the basis of molecular interactions. Graphene oxide dispersed system favors homeotropic alignment (dark state) and the application of bias field will convert it into homogenous alignment (bright state). Graphene oxide dispersion find prospective applications in good contrast display devices, supercapacitors, electronic gadgets, rechargeable batteries. Electro optical results unveil the faster response time, decreased rotational viscosity and spontaneous polarization with no change in tilt angle for the dispersed system. These observations can be exploited in photonic switches with sub millisecond response time which are required for fabricating faster liquid crystal devices.

Small-scale structure of thermodynamic phase in Arctic mixed-phase clouds observed by airborne remote sensing during a cold air outbreak and a warm air advection event

Abstract. The combination of downward-looking airborne lidar, radar, microwave, and imaging spectrometer measurements was exploited to characterize the vertical and small-scale (down to 10 m) horizontal distribution of the thermodynamic phase of low-level Arctic mixed-layer clouds. Two cloud cases observed in a cold air outbreak and a warm air advection event observed during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign were investigated. Both cloud cases exhibited the typical vertical mixed-phase structure with mostly liquid water droplets at cloud top and ice crystals in lower layers. The horizontal, small-scale distribution of the thermodynamic phase as observed during the cold air outbreak is dominated by the liquid water close to the cloud top and shows no indication of ice in lower cloud layers. Contrastingly, the cloud top variability in the case observed during a warm air advection showed some ice in areas of low reflectivity or cloud holes. Radiative transfer simulations considering homogeneous mixtures of liquid water droplets and ice crystals were able to reproduce the horizontal variability in this warm air advection. Large eddy simulations (LESs) were performed to reconstruct the observed cloud properties, which were used subsequently as input for radiative transfer simulations. The LESs of the cloud case observed during the cold air outbreak, with mostly liquid water at cloud top, realistically reproduced the observations. For the warm air advection case, the simulated ice water content (IWC) was systematically lower than the measured IWC. Nevertheless, the LESs revealed the presence of ice particles close to the cloud top and confirmed the observed horizontal variability in the cloud field. It is concluded that the cloud top small-scale horizontal variability is directly linked to changes in the vertical distribution of the cloud thermodynamic phase. Passive satellite-borne imaging spectrometer observations with pixel sizes larger than 100 m miss the small-scale cloud top structures.

Using Diazotization Reaction to Develop Portable Liquid-Crystal-Based Sensors for Nitrite Detection.

A liquid-crystal (LC)-based sensor for detecting nitrite in aqueous solutions was developed using a diazotization reaction as the sensing mechanism. First, tetradecyl 4-aminobenzoate (14CBA) was synthesized and doped into a nematic LC, i.e., 4-cyano-4′-pentylbiphenyl (5CB). When the LC mixture was cast on a glass substrate and then immersed into an aqueous solution without nitrite, the orientation of LC was planar and the LC image was bright. In the presence of nitrite, it reacted with alkylanilines to give corresponding diazonium ions with a positive charge, which aligned at the LC/aqueous interface to cause homeotropic orientation of LC. As a result, a bright-to-dark transition of the LC image was observed. The limit of detection (LOD) of this system for nitrite is 25 μM with high selectivity. In addition, this system can work in environmental water samples such as tap water and pond water. Finally, we demonstrated that the optical signals of LC can be measured and recorded using a built-in digital camera of a smartphone, suggesting the portability of this system for on-site applications.

Temperature effects on optical parameters of liquid crystal binary mixtures exhibiting induced smectics phases

Temperature dependent optical properties like birefringence, temperature gradients of refractive indices, molecular polarizabilities, effective geometry parameter, order parameter and cross over temperatures have been investigated for nine different liquid crystal mixtures. These mixtures are prepared by mixing two pure nematic liquid crystal compounds, namely, 4-n-pentyl phenyl 4-n′-hexyloxybenzoate (ME6O.5) and 4-cyanophenyl 4′-n-pentyl benzoate (CPPB) where, ME6O.5 is a monotropic compound and CPPB is an enantiotropic compound. The mixtures however are found to be exhibit induced smectic phases. The theoretical fitting values obtained by using Hallers’ extrapolated method of birefringence values agree quite well with the experimental values. It is observed that the normalized polarizability behaves exactly as refractive indices when it is plotted against temperature. The effective geometry parameter (αeg) of the mixture having highest birefringence shows highest value of αeg at certain temperature. Finally, order parameters of the mixtures have been calculated using five different techniques and for all mixtures the order parameters shows characteristics range from 0.3–0.9. The abrupt changes in order parameter before nematic phases are due to the existence of the induced smectic phases in the liquid crystal mixtures.

Binary Liquid Crystal Mixtures Based on Schiff Base Derivatives with Oriented Lateral Substituents

Binary mixtures of the laterally substituted Schiff base/ester derivatives, namely 4-((2- or 3-) substituted phenyl imino methyl) phenyl-4”-alkoxy benzoates, Ia–d, were prepared and mesomorphically studied by differential scanning calorimetry (DSC) and their mesophases identified by polarized optical microscopy (POM). The lateral group (1-naphthyl, 2-F, 2-Br, 3-F in Ia–d, respectively) is attached to different positions of the phenyl Schiff moiety. The mixtures investigated were made from two differently shaped compounds that differ from each other in the polarity, size, orientation, and relative positions of the lateral group. The results revealed that the binary mixture Ia/Ib (bearing the naphthyl and 2-flouro substituents) exhibited the SmA phase, which covered the whole composition range. For the mixtures Ib/Id (2-F and 2-Br), the isomeric lateral F-group in compound Ib distributed the SmA arrangement of Id. In the Ic/Id mixture bearing two positionally and structurally different substituents, the addition of Ic to Id resulted in solid binary mixtures where its behavior may be attributed to the negligible steric effect of the small electronegative fluorine atom compared to the Br atom. Density functional theory (DFT) theoretical calculations were carried out to estimate the geometrical parameters of individual components and to show the effect of these parameters in the mesophase behavior of the binary system, where the higher dipole moment of Id (6 Debye) may be the reason for its high π–π molecular stacking, which influences its mesophase range and stability.

Thermochromic luminescence in dual-dye-doped liquid crystal mixture induced by varying the energy transfer rate

Chromism in luminescent materials have attracted increasing interest for their unique feature of controllable luminescent colours and their applicability in various fields. Here we report a novel thermochromic luminescent liquid crystals (LC) system containing two luminescent dyes which do not have chromic properties themselves. The luminescent colours of the dual dye-LC mixture can be reversibly and continuously controlled between orange and blue by applying thermal stimuli; the relative intensity ratio between the two luminescent peaks is continuously controlled. This unprecedented phenomenon originates mainly from varying efficiency of the Förster energy transfer rates between two dye dopants depending on the phases of host LCs. The energy transfer rate is the highest in the crystal phase, giving rise to orange emission, and it is lowest in the isotropic state, producing blue emission. In the middle phases (nematic and smectic), mixed colours of these two colours are obtained. The proposed methodology, which takes advantage of controllable energy transfer rates, can greatly broaden the range of affordable dye materials for chromic luminescence systems because common fluorescent dyes can be used instead of special chromic compounds.

Mesomorphic properties improved via lateral fluorine substituent on benzoxazole-terminated mesogenic compounds

ABSTRACT Here, laterally monofluorinated heterocyclic mesogenic compounds, 2-[4-[2-[4-alkoxy-phenyl]-ethynyl]-3-fluorophenyl]-benzoxazole derivatives (nPEFPBx) with hydrogen, methyl and nitro terminal substituents (coded as nPEFPBH, nPEFPBM and nPEFPBN, respectively) at 5-position of benzoxazole unit, are synthesised and characterised. They display enantiotropic nematic mesophase with mesophase ranges of 22–30°C and 32–39°C on heating and cooling for nPEFPBH, 67–92°C and 87–115°C for nPEFPBM, 31–84°C and 42–51°C for nPEFPBN (n < 7), respectively, which is better than nonfluorinated and laterally difluorinated analogous . Improved mesophase property is ascribed to reduced dipole moment resulting from introduction of lateral fluorine atom in opposite direction of polar benzoxazole unit. This indicates that monofluorine substituent contributes to improve nematic mesophase for benzoxazole-terminated mesogenic compounds. In addition, the compounds nPEFPBx have high birefringence of 0.513–0.650 because of large π-conjugated mesogenic core composed of benzene, ethynyl and benzoxazole groups, which suggests a possible application in liquid crystal mixture to enhance birefringence. Meanwhile, except nPEFPBN, the compounds nPEFPBx show intense photoluminescence emission at 406–407 nm in methylene chloride solution with exciting them at absorption maxima.

Optical modulation studies of multiwall carbon nanotube dispersed in hydrogen bonded ferroelectric liquid crystal mixture for electro-optic devices

Structural, optical and electrical properties of multiwall carbon nanotubes (MWCNT) dispersed liquid crystalline (LC) mixture is extensively studied for electro-optic devices. A small amount of (0.1%) MWCNT is dispersed in cholesteryl stearate (CHS) and 4-dodecyloxybenzoic acid (12OBA) and its mixture is further characterized. Hydrogen bond between the mesogenic compounds are confirmed using FT-IR. Surface morphology and MWCNT dispersion in hydrogen bonded ferroelectric liquid crystal (HBFLC) mixture is established by SEM studies. Optical studies of the mixture using polarizing optical microscope (POM) is recognized by the induced smectic blue phase and smectic A* mesogenic phases. Transition temperatures and its corresponding enthalpy values of induced mesophases are analyzed by differential scanning calorimetry (DSC). An interesting observation of optical modulation in the present MWCNT dispersed HBFLC mixture is reported using dielectric studies. Textural modification with respect to external stimulus (bias voltage) in the present HBFLC mixture is reported. Further the field induced phase transitions and improved dielectric permittivity due to the dispersion of MWCNT is also reported.

Fast-response liquid crystal lens with doping of organic N-benzyl-2-methyl-4-nitroaniline.

In this study, a large-aperture hole-patterned liquid crystal (LHLC) lens was prepared from a mixture of nematic liquid crystal (NLC, E7) and organic material (N-benzyl-2-methyl-4-nitroaniline, BNA). The electro-optic properties of doped and undoped samples were measured, compared, and analyzed. The doped sample exhibited a response time that was ∼6 times faster than that of the undoped sample because BNA doping decreased the rotational viscosity of the NLC. BNA dopant effectively suppressed the RMS error of LHLC lens addressed at the high voltage. Furthermore, the BNA dopant revealed a considerable absorbance for short wavelengths (< 450 nm), automatically providing the LHLC lens with a blue light filtering function for ophthalmic applications.

Thermal and Photophysical Studies of Binary Mixtures of Liquid Crystal with Different Geometrical Mesogens

Three binary systems were prepared by mixing of two different mesogenic derivatives, homologues, the first is azo/ester, namely 4-alkoxyphenylazo-4′-phenyl-4″-alkoxybenzoates (IIn+m) and the second is Schiff base/ester, namely 4-(arylideneamino)phenyl-4″-alkoxy benzoates (In+m). The two corresponding analogues from both series in the binary mixtures investigated are of the same terminal alkoxy chain length. Mesomorphic properties were investigated by differential scanning calorimetry (DSC) and phases identified by polarized optical microscope (POM). Photophysical studies were investigated by UV spectroscopy connected to a hot stage. Results were discussed based on constructed binary phase diagrams. All mixtures were found to exhibit eutectic compositions, with linear or slightly linear nematic and smectic A stability/composition dependences. Geometrical parameters were predicted applying density functional theory (DFT) calculations. Twist angle (θ), aspect ratio, dipole moment and the polarizability of the individual compounds were discussed and correlated with the experimental results to illustrate the enhanced the mesophase stability and the mesophase range of the mixture at the eutectic composition compared with those of their individual components.

Fast switching behaviour and dielectric parameters of two chiral ferroelectric mesogens

ABSTRACT Two chiral ferroelectric reactive mesogens showing paraelectric SmA* and ferroelectric SmC* phase have been explored by means of electro-optical and dielectric measurements. The variations of spontaneous polarisation, relaxation time, effective torsional bulk viscosity, dispersion and polarisation anchoring coefficients with respect to temperature have been reported in the entire mesomorphic range. Additionally, helical pitch, static permittivity, dielectric anisotropy, complex permittivity (both real and imaginary parts) and ACconductivity have been determined as a function of frequency and temperature. Both the reactive mesogens are found to possess negative dielectric anisotropy. Also, the AC conductivity varies with the input frequencies and it is maximum for high frequency of 100 kHz. The results obtained have been discussed in the light of the applicability of these fast switching and negative dielectric anisotropy materials as chiral dopants in ferroelectric and antiferroelectric liquid crystal mixtures.

Liquid crystal material with gold nanoparticles as optical sensors active medium for the amino acids detection

In the article, the possibility of using the cholesteric liquid crystal mixture doped by the gold nanoparticles as an active medium of optical amino acid sensors is shown. The BLO-62 and the 5CB were used as a cholesteric liquid crystal and a nematic liquid crystal correspondingly. This mixture was doped by the gold nanoparticles. The method of gold nanoparticles synthesis is considered and their parameters are determined. Studies on the spectral characteristics of the investigated materials have shown that the addition of gold nanoparticles to the system leads to a significant decreasing in the spectral characteristic steepness and give the possibility of expanding the measurement range of amino acid concentration.

Synthesis and properties of benzoxazole-based liquid crystals containing ethynyl group

ABSTRACT Series of heterocyclic compounds containing ethynyl group, 2-[4-[2-[4-(alkoxy)phenyl]ethynyl]phenyl] -benzoxazole derivatives (nPEPBx), are prepared and investigated. They display enantiotropic nematic and smectic mesophases with large mesophase ranges of 30–133°C (heating) and 31–141°C (cooling). Compared with reference analogues without ethynyl group, the compounds nPEPBx roughly give lower melting points and much higher nematic mesophase stability because of the introduction of triple carbon-carbon bond. Meanwhile, they show much higher birefringence (Δn, 0.50–0.65 and 0.55–0.68 for experimental and calculated values with density functional theory (DFT), respectively) than common tolane-based liquid crystals, which indicates that the introduction of benzoxazole unit is an effective method to enhance Δn values of the rod-like molecule. It is concluded that nPEPBx has a potential application in liquid crystal mixture as a dopant to enhance the performance of the host mixture.

Photoinduced properties of “T-type” polyimides with azobenzene or azopyridine moieties

In this work series of “T-type” polyimides with azobenzene or azopyridine moieties were investigated. It is the first report where the “T-type” polyimides with derivatives of azopyridine were investigated. The cis-trans isomerization in the dark in the solid-state was showed. Polyimides with azobenzene derivatives exhibited higher stability of cis-trans recovery than their azobenzene analogues. For the inscription of the surface relief gratings, two different intensities of light (10 and 45 mJ/cm2) and a number of pulses (10 and 100) were used. Polyimides showed the modulation of SRGs up to almost 330 nm. Our studies showed, that azopolyimides are able to orient the nematic liquid crystal molecules in cell-based on effect at the twisted nematic. The obtained maximum value of tON was 0.9 ms (1 kHz, 1.5 V/μm) for polyimide with azobenzene moieties. Azopolyimides may be successfully used in many photonic devices based on the alignment of the liquid crystal mixture i.e. LC diffraction gratings, Fresnel lens, Vortex, in a photo-pattering technology for creating radical and azimuthal orienting layers.

The Characterization and Application of Two Liquid Crystal Mixtures in the Low THz Region

In the previous work, two new nematic liquid crystal (NLC) mixtures, E7-2 and S200-2, were produced by adding eight LC monomers to two commercial LCs S200 and E7, respectively. At λ = 589 nm, the birefringence (Δn) characteristics of the two LC nematic mixtures E7-2 (Δn = 0.260) and S200-2 (Δn = 0.298) are greater than those of the commercial LC E7 (Δn = 0.224) and S200 (Δn = 0.266). The properties (TN-I, ε//, Δɛ, K11, and K33) of these four NLCs were measured. A double-layer metal loop arrays modulation structure based on metamaterial (MM) metal–dielectric–metal (MDM) was designed and fabricated for use in the THz frequency range. The results show that the LC mixtures E7-2 and S200-2 have greater modulation depth (MD) and less modulation insertion loss (IL) than E7 and S200 at THz frequencies. The results show that LC mixtures have significant potential for designing active tunable LC-based devices in the THz and visible light range.

Chiral Reaction Field with Thermally Invertible Helical Sense that Controls the Helicities of Conjugated Polymers.

A chiral reaction field with thermally invertible helical sense enables control of the helicity of the reaction product, which is a central challenge in asymmetric synthesis that has yet to be overcome. A novel chiral compound comprising two types of chiral moieties with opposite helicities and temperature dependences is synthesized; this compound is added as a chiral dopant to a mixture of nematic liquid crystals to prepare a chiral nematic liquid crystal (N*-LC). The N*-LC containing the chiral dopant exhibits thermally invertible helicity to yield left- and right-handed helical senses at low and high temperatures, respectively. Interfacial polymerization of acetylene is achieved in the N*-LC by modulating the temperature. Helical polyacetylenes (H-PAs) that are synthesized at low (-12 °C) and high (28 °C) temperature show right- and left-handedness, respectively, in terms of the fibrils, fibril bundles, and spiral morphology. In addition, the helical sense of H-PA is opposite that of the N*-LC because of the peculiar polymerization mechanism for acetylene in the N*-LC. The current N*-LC is the first chiral reaction field that has not only the thermally invertible helical sense but also the chemical functions and stability needed to serve as the medium for polymer reactions.

Electrical Behavior of Liquid Crystal Devices with Dielectric Nanoparticles

Nowadays, the development of new devices based on liquid crystal (LC) materials requires improved tuning characteristics according to the application. One approach for this improvement is the use of nanomaterials with the capability of modifying the effective properties of the doped LC mixture. In this work, we analyze the electrical behavior of a titanium dioxide (TiO2) nanoparticle-doped liquid crystal cell using an equivalent circuit. The circuit parameters have been obtained using the impedance spectroscopy technique and time response measurements. The particularity of the samples designed is that the nanoparticles are not dispersed in the LC. Instead of that, nanoparticles are randomly deposited on one of the electrodes. Measurements show that the presence of the nanoparticles increases the temperature sensitivity of the equivalent cell capacitance and the capacitance difference between switched and nonswitched states. These results could be quite useful in the design of novel liquid crystal electronic devices and sensors.

Finger-Temperature-Detecting Liquid Crystal Composite Film for Anti-Counterfeiting Labels.

The development of the economy has increased the demand for anti-counterfeiting technologies, and with the traditional ones becoming known to the public, new and more effective ones are needed. In this study, a series of liquid crystal mixtures containing side-chain liquid crystal polymers and small-molecular-weight liquid crystals (LCs) were designed and prepared. The phase transition behavior and self-assembling structures of the LC mixtures were investigated by a combination of differential scanning calorimetry, polarized optical microscopy, and small-angle X-ray diffraction. The optical properties of the mixture film were characterized with a UV/VIS/IR spectrum study. The results reveal that the obtained film exhibited different optical modes between transparent, scattering, and selective reflection under finger-temperature control. Therefore, by the introduction of a coexisting thermal- or optical-polymer-dispersed network, a liquid crystal composite film with an integration of apparent optical switching modes and enhanced strength and toughness was successfully demonstrated. This research provides a versatile strategy for the design and preparation of liquid crystal anti-counterfeiting materials for practical use. In this study, a prototype finger-temperature-detecting anti-counterfeiting label was prepared, and its temperature-response property was demonstrated.