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Graphene oxide is hydrophilic due to its oxygen functionality, making it easy to disperse in liquid crystalline media. The nanocomposites of graphene oxide and reduced graphene oxide have been prepared in a nematic liquid crystal. Results reveal rising transition temperatures, enthalpies, and order parameters for graphene oxide dispersed nanocomposites. UV-Vis spectroscopy results show an increase of the optical band gap for graphene oxide whereas decrease for reduced graphene oxide dispersed system. Impedance spectroscopy is used for the determination of various parameters such as permittivity, dielectric anisotropy, conductivity, relaxation frequency and activation energy. Results show that the dielectric anisotropy is increased for graphene oxide dispersed nanocomposite, however, reduced graphene oxide dispersed system demonstrates the opposite property. Relaxation frequency has increased for graphene oxide dispersed system while unchanged for reduced graphene oxide dispersed system. Conductivity of reduced graphene oxide dispersed nanocomposite has increased by three orders of magnitude but for graphene oxide dispersed system, it has decreased as compared to that of the pure sample. We observed decrease in threshold voltage, elastic constant, and rotational viscosity for both GO/rGO dispersed systems. The outcome for the graphene oxide dispersed nanocomposite looks to be more effective due to the system's enhanced molecular polarisability. [ABSTRACT FROM AUTHOR]

Embedding of nanoparticles in nematic liquid crystalline materials has received huge attention in recent years. In most of the work, nanoparticles decrease nematic-isotropic transition temperature along with an undesired increase in conductivity. Interestingly, there are few reports where nematic-isotropic transition temperature has increased along with improvement in other relevant properties due to presence of ferroelectric and/or functionalised nanoparticles of small size. Here, we are reporting enhanced nematic-isotropic transition temperature of 4-pentyl-4′-cyanobiphenyl (5CB) due to dispersion of metallic copper nanoparticles (CuNPs). Low concentrations (0.2 and 0.5 weight%) of CuNPs have been dispersed in 5CB to prepare nanocomposites. Thermodynamic results on unaligned samples and dielectric results on planar and homeotropic aligned samples show enhancement of isotropic to nematic transition temperature of 5CB in the presence of CuNPs without any substantial change of conductivity. Optical studies show a marginal decrease in the band gap of the dispersed samples as compared to those of the pure 5CB which validates conductivity results. The relaxation frequency corresponding to the rotation of molecules about their short axes has increased due to the presence of CuNPs in the nematic matrix of 5CB. These observed results are useful for the electro-optical applications as well as for fundamental aspects. [ABSTRACT FROM AUTHOR]

Herein, a small concentration (0.05 wt %) of reduced graphene oxide is dispersed in the liquid crystalline media. Thermodynamic study shows the reduction of the transition temperature, enthalpy and entropy of the reduced graphene oxide dispersed nanocomposite. Optical study suggests the decrease of the optical bandgap of the dispersed nanocomposite as compared to that of the pure sample. Dielectric spectroscopic measurement is done for measuring the permittivity and conductivity of the pure and the dispersed samples. The value of dielectric anisotropy has decreased for the dispersed nanocomposite whereas the value of ionic (DC) conductivity of the dispersed nanocomposite has increased by 3 orders of the magnitude as compared to that of the pristine sample. The activation energy for DC conductivity has reduced in the dispersed nanocomposite which is responsible for the increment in DC conductivity of the system. The cut-off region of DC conductivity has increased in the dispersed nanocomposite. Electro-optical studies suggest the reduction of the threshold voltage for the Freedericksz transition, splay elastic constant and rotational viscosity for the dispersed nanocomposite. [ABSTRACT FROM AUTHOR]

A room temperature nematic liquid crystal (N-3017) mixed with surface-stabilised gold nanoparticles was examined for its thermodynamic, dielectric, and electro-optical properties. The isotropic to nematic transition temperature decreases in both the nanocomposites as compared to that of the pure nematic material, according to thermodynamic findings. In the frequency range of 1 Hz to 40 MHz, dielectric data of both the dispersed nanocomposites in homeotropic and planar aligned samples are collected. The dispersion of capped GNPs enhanced the dielectric anisotropy of the dispersed nanocomposites, according to the dielectric results. As the quantity of capped GNPs in the host matrix increases, the threshold voltage for LC reorientation decreases due to the decreased elastic constant and increased dielectric anisotropy. The dielectric study also signifies a prominent relaxation mode due to the flip-flop motion of the nematic molecules about their short axes. To analyse this mode, experimental data have been fitted with the modified Cole–Cole equation. This removes the parasitic effects in the low- and high-frequency domains. [ABSTRACT FROM AUTHOR]

Colloidal suspension of 8CB dispersed with graphene oxide has been prepared. Differential scanning calorimetry scans have been performed for both pure and graphene oxide dispersed samples. The Cr-SmA, SmA-N and N-I transition temperature have been reduced in graphene oxide dispersed sample. Textures have also been recorded in nematic and smectic phases for pure and dispersed samples. Optical studies have been carried out with UV-Visible spectrophotometer in order to determine optical band gap of the samples. Increased optical band gap for nanocomposite has been observed. Increased threshold voltage, splay elastic constant and rotational viscosities have been observed for dispersed nanocomposite. Furthermore, dielectric studies have been carried out for both homeotropic and planar alignment of molecules. Dielectric anisotropy has been found to be increased in the dispersed sample. Observed value of conductivity for graphene oxide dispersed nanocomposite is lower in the smectic phase while it is higher in the nematic phase. Relaxation phenomenon shifts to low frequency for graphene oxide dispersed nanocomposite as compared to the pure sample. [ABSTRACT FROM AUTHOR]

In the present work, we have prepared a liquid crystal-carbon nanotube (LC-CNT) composite by dispersing a low concentration of carboxylic group (-COOH) functionalised multi-walled CNTs (MWCNTs) in a liquid-crystal 6CHBT and studied in detail the alterations in thermal and electrical parameters. Thermodynamic studies reveal that transition processes are becoming fast due to the dispersion of MWCNTs. The change in the behaviour of transitions is credited to the reduction in ionic impurities owing to the ion trapping characteristic of CNTs. The isotropic liquid to nematic transition temperature of the composite system is increasing for the exothermic cycle as compared to that of the pure sample. The average transition enthalpies and entropies for various transitions of the composite system are increasing as compared to those of the virgin sample. Longitudinal component of the permittivity and dielectric anisotropy of the composite system is slightly reduced as compared to those of the virgin sample. Relaxation frequencies of an observed relaxation mechanism corresponding to the rotation of molecules about their short axes of the composite system are enhanced as compared to the undispersed sample. Total ionic conductivity of the composite system is boosted as compared to those of the pure sample. [ABSTRACT FROM AUTHOR]

We have prepared the composites of a room temperature nematic liquid crystal mixture by dispersing small fractions of Multi-Walled Carbon Nanotubes into its matrix. The electro-optical and dielectric properties of the prepared samples were investigated. The effect of Multi-Walled Carbon Nanotubes on various display parameters of host nematic liquid crystal mixture was observed. The variations in some of the display parameters like dielectric anisotropy, threshold voltage, splay elastic constant and activation energy have been observed for composite systems with respect to pure nematic liquid crystal mixture. Dispersion of Multi-Walled Carbon Nanotubes enhanced the nematic order of liquid crystal host, resulting in the improvement of dielectric and electro-optical parameters of studied composite systems. [ABSTRACT FROM AUTHOR]

Dielectric spectroscopic measurements of five homologous series of orthoconic antiferroelectric liquid crystalline (OAFLC) esters (S)-MHPOBC ((S)-4-(1-methylheptyloxy) carbonyl phenyl 4'-octyloxy-4-biphenylcarboxylate) has been investigated. The investigated compound contains SmC*, SmCA* and SmA phase with SmCA* phase at a broad temperature range. Parameters such as absorption curve, dispersion curve, Cole-Cole plot, relaxation frequency, relaxation time, dielectric strength and electrical conductivity has been discussed. The structure property correlations of these compounds are also highlighted in the light of the dielectric measurements. Compound S1.6 (FF) and S2.6 (FH) has dielectric strength of around 250 in the SmC* phase due to the presence of Goldstone mode. [ABSTRACT FROM AUTHOR]

Incorporation of magnesium oxide nano particles (MgONPs) in a nematic liquid crystal display material namely 4-pentyl-4'-cyanobiphenyl (5CB) have been analyzed by differential scanning calorimeter, polarizing light microscopy and dielectric spectroscopy. MgONPs have been dispersed in two different concentrations in pure 5CB viz. 0.2 and 0.6 wt%. It has been experimentally noticed that with the increase of MgONPs concentrations in 5CB, the nematic to isotropic transition temperatures decreases and dielectric anisotropy increases. Relaxation frequency of an observed relaxation mode corresponding to molecular rotation about the short axis is almost unchanged after dispersion of MgONPs. The ionic conductivity has increased by approximately one order of magnitude for 0.6 wt% MgONPs and decreased for 0.2 wt%. [ABSTRACT FROM AUTHOR]

In this article, we present the Raman study of two laterally fluorinated liquid crystal compounds, namely, C3PP(23F)P-NCS and C3PP(23F)PC3. The two compounds have different terminal groups, one with NCS which is more polar than the other which has an alkyl group. The Raman bands were deconvoluted with Lorentzian profiles to get the values of spectral parameters. The main feature of the work is the temperature-dependent behaviour of the spectra profiles of the C–H bending mode, C–F stretching modes and aromatic C–C stretching mode in the vicinity of the phase transitions. The effect on the charge distribution over different bonds of the molecule and the vibrational energy of the bonds due to the presence of the more polar group on the terminal chain is well observed. The contrasting behaviours of the spectral profiles at the Cr–N and N–I transitions were also observed which we have ascribed to some pre-transitional effect associated with the breaking of the orientational order near the N–I transition. Density functional theory using B3LYP exchange correlation with the 6-31G (d,p) basis set was also used to generate a theoretical spectra which were found to be matching with the experimental spectra. [ABSTRACT FROM AUTHOR]

A vibrational spectroscopy study of a laterally fluorinated terphenyl liquid crystalline (LC) compound is presented in this paper. The study consists of temperature dependent Raman study and room temperature FTIR spectroscopy. The molecular structure was optimized using Density Functional Theory (DFT) with B3LYP functional and 6-31G (d, p) basis set and some structural properties have been noted. From the detailed study of the temperature dependent Raman spectra, the nature of the phase transition of the LC compound was investigated. It was found that the quadrant stretching modes and the CH bending modes of the phenyl rings display significant changes at Cr-N transition and N-I transition. This study therefore throws some light on the nature of transitions. From the FTIR study, it was found that the CC vibrations of the rings and NCS vibrations have large absorptions while the CH bending modes have less absorption. [ABSTRACT FROM AUTHOR]

The temperature dependence Raman spectra of two liquid crystalline compounds defined by the chemical formula of 3,5-difluoro-4ʹ-(4-pentylcyclohexyl)-(1,1ʹ-biphenyl)-4-carbonitrile and 3,4,5-trifluoro-4ʹ-(4-pentylcyclohexyl)-1,1ʹ-biphenyl is being first reported in this study. These compounds are bent-core monomers and their bent nature has been verified by the Density Functional Theory (DFT). The temperature-dependent Raman spectroscopy has been widely used in understanding the effects of temperature-based phase transitions on the molecular vibrations. The same spectroscopic technique; helps to understand various phase transitions temperature in the liquid crystalline compounds (LC) and also their molecular arrangements during the phase transitions. This study has successfully revealed the nature of intermolecular interactions between the investigated compounds during the phase transitions and the correlation between the observed Raman spectra and the measurement temperature. The contributions of different types of chemical bonds in the investigated LC compounds to their recorded Raman spectra have also been discussed in detail. In predicting the observed Raman spectra, the theoretical Raman spectra obtained from the DFT calculation was used as a reliable tool. In the light of the calculated data, the peak position, line width, and integral intensity data for each band in the observed Raman spectra were reported. [ABSTRACT FROM AUTHOR]

Thermodynamic and electro-optical characterization of a ferroelectric liquid crystalline material, namely ((S) (+) 4-(1-methylheptyloxy) phenyl 4′-octyloxybiphenyl-4-carboxylate) possessing paraelectric SmA*, ferroelectric SmC*, hexatic SmBh* and SmI* phases has been carried out. Phase identification has been done by optical and thermodynamic studies. Switching parameters viz. spontaneous polarization, switching time and rotational viscosity have been determined. The spontaneous polarization has been found to increase with decreasing temperature in SmC* phase. The switching time is found of the order of few milliseconds. [ABSTRACT FROM AUTHOR]

Optical and X-ray studies have been conducted on a four component liquid crystal mixture (code name 2013) with cyano-end groups in all the four components and a lateral substitution of fluorine atom in one of them. The variations in the optical birefringence with temperature, have been investigated and the thermal dependence of polarisability anisotropy and orientational order parameter have been evaluated. X-ray studies have been conducted to determine the effective molecular characteristics such as apparent molecular length and inter molecular distance and their variations with temperature. From nature of molecular alignment as envisaged from X-ray photographs the order parameter variation with temperature has been determined and compared with macroscopic order parameter variation as determined from birefringence studies. The results have been compared with these obtained for another structurally similar four-component liquid crystal mixture (code name 2014) with isothiocyanato end group in place of cyano. [ABSTRACT FROM AUTHOR]

Series of low-birefringence liquid crystal mixtures composed of alkyl alkylbicyclohexyl carbonates and modified mixtures with other compounds have been formulated, their refractive indices and electric permittivity measured upon temperature. They exhibit the ordinary refraction indexnolower than the refractive index of silica glass in a different range of temperature. This enables to observe in photonic liquid crystal fibres (PLCFs) a change in the light propagation mechanism from photonic band gap guiding to modified total internal reflection at different temperatures. Selected applications of PLCFs are also discussed. [ABSTRACT FROM AUTHOR]

In this work, X-ray and optical studies have been carried out on a four-component isothiocyanato liquid crystal mixture with laterally substituted fluorine atoms. Molecular characteristics, such as effective inter molecular distances and apparent molecular length of the average-simulated molecule have been determined from X-ray studies at various temperatures in the liquid crystalline phase. From the nature of molecular alignment as envisaged from X-ray photographs, the orientational order parameter and its variation with temperature have been determined. Optical studies have also been conducted at various temperatures to investigate the thermal variations of birefringence. Macroscopic orientational order parameter and its thermal variation have been determined from these studies. The nature of order parameter variation obtained from the two studies, X-ray and optical have been compared. [ABSTRACT FROM PUBLISHER]

Three partially fluorinated ferroelectric liquid crystals (4F4R, 4F5R and 7F3R) with biphenylyl benzoate rigid core have been investigated by frequency-dependent dielectric spectroscopy and electro-optic methods. Molecular structures of the compounds differ only by the length of the carbon chain and the number of oligomethylene spacers. 4F4R and 4F5R exhibit ferroelectric SmC* phase over a considerable temperature range and directly melts into isotropic phase, 4F4R also shows a ferroelectric typesubphase. Only Goldstone mode (GM) relaxation is observed in these compounds, but no soft mode (SM). 7F3R exhibits both the SmC* and SmA* phases, has higher stability of SmC*. Both the GM and SM relaxations are observed in this case, signifying that SM relaxation is possible only when SmC* is formed on cooling from SmA*. Increase of dielectric strength and critical frequency with temperature, in all cases, has been explained in the light of generalised Landau model. Spontaneous polarisation is found to decrease with increasing flexibility, and optical tilt depends more on fluorination than on chain length. [ABSTRACT FROM AUTHOR]

The display parameters and the stability of the various mesophases of a newly synthesised racemic mixture of anti-ferroelectric liquid crystals (AFLCs) have been boosted with the help of the electron beam irradiation. The mixture has phase sequence of isotropic–smectic C–smectic CAcrystal. The effective macroscopic polarisation and transition temperatures of the various mesophases of the irradiated mixture have increased as compared with the pure mixture. The macroscopic polarisation of the mixture has increased from 21 to 27 nC/cm2due to the irradiation. From various studies, we have observed that the temperature range of the smectic CAphase has increased by 20.6°C with marginal decrease in the temperature range of the smectic C phase. [ABSTRACT FROM AUTHOR]

Dielectric behaviour of a new ferroelectric liquid crystal (CLF08) having large spontaneous polarisation (120 nC/cm2at 110 Hz), short pitch and high purity, is studied in the frequency range 5 Hz to 1 MHz, in the smectic C* and smectic A phases using a 7-micron-thick cell in planar alignment. Dielectric permittivity, polarisation and optical measurements are investigated that shows complex ferroelectric behaviour. These experiments evidence the Goldstone and Soft mode relaxations far from the SmC*to SmA transition. The density of dechiralization lines is observed under polarised light microscope. Its behaviour under electric field exhibits a remarkable accordance with the dielectric response. These results are discussed in connection with a possible onset of ferrielectricity in the dechiralization lines system. [ABSTRACT FROM AUTHOR]

Thermodynamical, dielectric, optical and electro-optical characterisation of pure 8CB and its composites with gold and silver nanoparticles have been studied. Thermodynamical studies suggest a decrease in clearing temperature of the nanocomposite systems as compared to the pure system. Dielectric parameters of pure nematic liquid crystal and nanocomposites in the homeotropic and planar aligned samples have been measured in the frequency range of 1–35 MHz. Ionic conductivity increases significantly in nematic and smectic Ad(SmAd) phases, whereas dielectric anisotropy is almost unchanged for both the nanocomposites. Threshold voltage for Freederick transition, switching voltage and splay elastic constant have decreased in the case of nanocomposite systems. Relaxation frequency and activation energy of an observed relaxation mode corresponding to molecular rotation about the short axis increase in the SmAdphases of both the nanocomposites. The optical study suggests that due to dispersion of nanoparticles, the optical band gap has decreased. [ABSTRACT FROM PUBLISHER]

Titled ferroelectric liquid crystalline material (in short, 4H3R) exhibits cholestric (N*), ferroelectric (SmC*) and hexatic (SmB*) phases, when it is cooled from isotropic liquid phase to crystal phase. Dielectric studies have been carried out in the frequency range of 1 Hz to 35 MHz under planar anchoring conditions of the molecules. The soft mode relaxation due to the amplitude fluctuation of the polarisation vector has appeared in the N* phase, whose relaxation frequencies decrease but dielectric strength marginally increases (~0.30–0.54) with decrease in the temperature. The Goldstone relaxation mode has appeared in SmC* phase with a constant relaxation frequency. The relaxation frequency of hexatic SmB* phase follows Arrhenius behaviour. The conductivity of the material is also measured in all the exhibited phases. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

Thermodynamic, dielectric, optical and switching parameters of a single-phase antiferroelectric (AF) liquid crystalline material (S)-(+)-4-(1-methylheptyloxycarbonyl)-2,3-difluorophenyl 4′-[3-(2,2,3,3,4,4,4-heptafluorobutoxy)prop-1-oxy]biphenyl-4-carboxylate have been studied. These studies show wide temperature range (~97.8°C–25.3°C) of AF SmC*Aphase in the material. The dielectric studies have been carried out in the frequency range of 1 Hz–35 MHz under planar anchoring conditions of the molecules. The dielectric spectrum of the SmC*Aphase exhibits three relaxation modes due to the collective as well as individual molecular processes. Relaxation frequencies of these modes lie in the range of kHz–MHz regions. Relative permittivity of the material (at 10 kHz) varies from ~8.8 at 98.8°C to 9.9 at 41.0°C. Maximum tilt of the molecule in the SmC*Aphase is ~43°C. Spontaneous polarisation, switching time and rotational viscosity have also been determined. The maximum value ofPSis ~439 nC/cm2and switching time is the order of 1–5 millisecond, whereas viscosity is moderate. [ABSTRACT FROM PUBLISHER]

Frequency- and temperature-dependent dielectric and switching parameters of a room temperature tri-component antiferroelectric liquid crystal mixture W-287 have been determined. Dielectric, optical texture and thermodynamic studies show wide room temperature range antiferroelectric SmC*a(˜91.1°C to <–25°C) phase in addition to high temperature paraelectric SmA* (˜2.6°C) and ferroelectric SmC* (˜4.4°C) phases. The dielectric studies carried out in the frequency range of 1–35 MHz under planar anchoring condition of the molecules show five different relaxation modes appearing in the SmA*, SmC* and SmC*aphases. Using Curie–Weiss law fit, ferroelectric SmC* to paraelectric SmA* transition temperature has been found to be 91.8°C. The dielectric response of SmC*aphase exhibits unusually three relaxation modes due to collective as well as individual molecular processes in addition to phason mode in the SmC* phase and amplitudon mode in the SmA* phase. Spontaneous polarisation, switching time and rotational viscosity have also been determined. The maximum value of PSis ˜300 nC/cm2, whereas viscosity is moderate. Switching time is of the order of few milli seconds. [ABSTRACT FROM AUTHOR]

In this article, optical properties of four fluoro-substituted 4-propyl-4′-[(4-ethylphenyl)ethynyl] biphenyls and liquid crystal mixture A are presented in the terahertz (THz) range. Birefringence, refractive indices and absorption coefficients for ordinary and extraordinary ray of liquid crystals are described in the range of 0.3 to 3.0 THz. It shows that the measured parameters are dependent on the number and placement of fluorine atoms in the molecules. Measurements have been performed using time-pulsed spectroscopy. [ABSTRACT FROM AUTHOR]

Dielectric and electro-optical response of the title compound (abbreviated as (S) 7F6Bi) have been investigated. The dielectric studies have been carried out in the frequency range of 1 Hz to 35 MHz under planar anchoring conditions of the molecules. Material exhibits paraelectric SmA*, ferroelectric SmC* and a wide temperature range (∼83°C) antiferroelectricphases. The SmA* phase shows soft mode relaxation due to tiltfluctuation of molecules in the MHz region. Its relaxation frequencies decrease with decrease in the temperature. Dielectric strength increases (∼0.2–3.4) with decrease in temperature and follows Curie–Weiss law. The SmC* phase shows Goldstone mode relaxation due to phase fluctuation of molecules with relaxation frequency ∼10 kHz. Only one mode of relaxation has been observed in thephase presumably due to antiferroelectric ordering of the molecules. Its dielectric strength varies from ∼0.55 at 104.3°C to 2.20 at 37.6°C. Switching parameters, namely spontaneous polarisation, switching time and rotational viscosity, have also been determined. [ABSTRACT FROM AUTHOR]

A new pentaphenyl liquid crystalline (LC) compounds with 2,3-difluorobenzene moiety exhibiting wide nematic range have been prepared. The key stage is the condensation of the corresponding 3-dimethylaminopropanoyl-4-pentyl benzene hydrochloride with substituted methyl benzyl ketone in the presence of base leading to 3,6-disubstituted cyclohexenones. Methylation with methyl magnesium iodide followed by oxidative aromatization gives three-ring bromides, which are used in the Suzuki cross-coupling reaction. Pentaphenyl LC compounds possessing lateral methyl group have moderate melting points, improved solubility, and miscibility with another LC components. The new compounds could be useful in LC compositions with the high optical anisotropy. [ABSTRACT FROM AUTHOR]

The aim of this present paper is to compare the effects of conducting polymer, poly (3,4-ethylenedioxythiophene) nanotubes and carbon nanotubes, on the electro-optical properties of a nematic liquid crystal mixture (1770-2). From the optical transmission and capacitance behavior it has been proved that the conducting polymer - nematic liquid crystal system possesses much lower threshold voltage and driving voltage than the carbon nanotube - nematic liquid crystal system which might be very useful in display applications. The effective elastic constant has been also significantly reduced in the conducting polymer - nematic liquid crystal system. [ABSTRACT FROM AUTHOR]

We have studied a compound (S)-(+)-4-(1-methylheptyl) 4-[4-(3-hexanoyloxyprop-1-oxy)benzoyloxy]biphenylate by means of differential scanning calorimeter, polarizing microscope and dielectric spectroscopy. This compound shows exceptionally a wide temperature range of [image omitted] phase ∼9°C together with a paraelectric SmA* phase. A relaxation mechanism in the high kHz region has been detected for the [image omitted] phase under planar orientation of the molecules. This mechanism has been characterized with temperature and bias electric field. Recent theoretical advances predict two different nature of dielectric relaxation mode in the [image omitted] phase (Phys. Rev. E, 69, 031709, 2004). Our experimental results confirm two different behaviors of this relaxation mode in different temperature range of the phase. [ABSTRACT FROM AUTHOR]

In this paper a tunable broad-band polarizer based on the photonic crystal fiber (PCF) filled with liquid crystal (LC) is presented. Host PCF is made of glass with 1.518 refractive index and ordinary refractive index of the LCs is lower than glass index. Numerical simulations have proven that by changing LC orientation from planar to transverse a broad-band single-polarization guidance is possible: for planar orientation of LC molecules both polarizations are index-guided, whereas for transverse orientation only one polarization is index-guided. Also some initial experimental results are included showing that polarization dependent losses and phase birefringence can be tuned with transverse electric field. [ABSTRACT FROM AUTHOR]

Switching and dielectric relaxation phenomena were investigated for an antiferroelectric liquid crystal, 4,4-(1-methyloctyloxycarbonyl)phenyl]-4'-[3-(butanoyloxy)prop-1-oxy]biphenyl carboxylate, exhibiting chiral smectic A (SmA*), smectic C (SmC*) and antiferroelectric (SmCA*) phases. Spontaneous polarisations, rotational viscosities, relaxation frequencies, dielectric strengths and distribution parameters were determined as a function of temperature. The electric field required for saturation of the spontaneous polarisation increased with a decrease in temperature. In the SmA* phase, only one relaxation mechanism was observed that behaves as soft mode. Two relaxation processes were detected in the SmC* phase. A high-frequency relaxation process invariant at 2.2 kHz was due to a Goldstone mode, but the origin of low-frequency relaxation process (1-20 Hz) is unclear; however, it may belong to an X-mode. The dielectric spectrum of the SmCA* phase exhibits two absorption peaks separated by two decades of frequency. The low-frequency peak is related to the antiferroelectric Goldstone mode, whereas the high-frequency peak originates from the anti-phase fluctuation of the directors in the anti-tilt pairs of the SmCA* phase. [ABSTRACT FROM AUTHOR]

We have designed, synthesized, and evaluated the physical properties of some high birefringence (Δn) isothiocyanato phenyl-tolane liquid crystals. These compounds exhibit a Δn ≃ 0.48-0.52 at room temperature and wavelength λ = 633 nm. Laterally substituted short alkyl chains and fluorine atom both eliminate smectic phase formation and lower the melting temperature. Both the moderate melting temperature and very high clearing temperature make those compounds attractive for eutectic mixture formulation. Examples of the mixtures containing the described components are presented. [ABSTRACT FROM AUTHOR]

High birefringence (Δn > 0.4) nematic liquid crystals are particularly attractive for infrared applications because they enable a thinner cell gap to be used for achieving fast response time. In this paper, we report the mesomorphic and physical properties of several fluorinated NCS terphenyl single compounds and mixtures based solely on laterally fluorinated aromatic rigid core structures. The single compounds have Δn ∼ 0.35 in the visible spectral region and relatively low viscosity. In terms of applications, these compounds can be used as a dopant for enhancing the figure-of-merit of commercial mixtures for display applications. Or we can formulate high birefringence eutectic mixtures using solely NCS terphenyl compounds for near IR laser beam steering applications. [ABSTRACT FROM AUTHOR]

We have designed, synthesised and evaluated the physical properties of high-birefringence (Δn) isothiocyanato phenyl-tolane and terphenyl liquid crystals. These compounds exhibit a Δn∼0.36-0.52 at room temperature and wavelength λ = 633 nm. A different mesomorphic sequence was observed depending on the terminal alkyl chain length. Compounds with a longer chain show smectic and nematic phases, whereas short-chain homologues only exhibit a nematic phase. In the vicinity of the nematic-to-smectic transition, the viscoelastic coefficient decreases, instead of increases, as the temperature decreases. The observed change is as large as 75% for the 4'-(3,5-difluoro-4-isothiocyanatophenylethynyl)-4-pentylbiphenyl compound studied. This behaviour is useful for improving the response time of a liquid crystal device, if the operating temperature can be controlled accurately. [ABSTRACT FROM AUTHOR]

The homologous series of mesogenic (S)-4''[n-(perfluoroalkanoyloxy)hexyl-1-oxy]-4-(1-metylheptyloxy-carbonyl)terphenyl compounds exhibiting low temperature, wide range, near orthoconic antiferroelectric phase has been synthesized [1]. Compounds of the homologous series differ from each other by a position of a single fluorine atom substituted at the phenyl ring of the molecular rigid core. This affects the mesogenic behavior. Moreover, the molecular dipole moment of the compound as well as the physical parameters of the antiferroelectric phase are changed. The importance of the molecular dipole moment orientation and magnitude for the mesogenic behavior and physical parameters of the orthoconic antiferroelectric phase is discussed. [ABSTRACT FROM AUTHOR]

High birefringence (Δn>0.4) nematic liquid crystals are particularly attractive for infrared applications because they enable a thinner cell gap to be used for achieving a fast response time. In this experiment, we evaluate the mesomorphic and physical properties of several fluorinated NCS terphenyl compounds. These compounds have Δn∼0.35 in the visible spectral region and relatively low viscosity. In terms of applications, these compounds can be used as a dopant to enhance the figure-of-merit of commercial mixtures for display applications. We can also formulate high birefringence eutectic mixtures using solely NCS terphenyl compounds for near-IR laser beam steering applications. [ABSTRACT FROM AUTHOR]

Horizontally surface-stabilized orthoconic antiferroelectric liquid crystals (OAFLCs) provide excellent contrast electrooptic switching. However, in very thin cells the bulk relaxation to the anticlinic state tends to be slow because the synclinic state is upheld by the surfaces. In cells thin enough to accomplish a surface-stabilized state of today's short-pitch OAFLC mixtures, the field-induced synclinic (ferroelectric) states often become metastable making the corresponding bright electrooptic state prevail for long times after the electric field is switched off. Fast relaxation is then achieved by specially designed waveforms.By introducing a polymer network formed by in-situ photopolymerization of a small concentration of a commercial reactive monomer dissolved in the OAFLC, we have succeded to drastically reduce the time for relaxation from the synclinic to the anticlinic state after switching off the field down to about 3 milliseconds at room temperature, without the use of any tailored electronic waveforms. The relaxation time of the polymer stabilized OAFLCs (PS-OAFLCs) is thus shorter than in todays nematic liquid crystal displays, which make PS-OAFLCs attractive for use in future high-contrast, blur-free displays. [ABSTRACT FROM AUTHOR]

We have developed some high birefringence (Δ n &nvsim; 0.3 at λ = 633 nm) and relatively low viscosity nematic mixtures using isothiocyanato tolanes. These mixtures exhibit a high figure-of-merit (FoM) which is desirable for reducing the device response time. Using such a high FoM liquid crystal mixture in a 2-μm homogeneous cell, we demonstrate a 1π phase modulator at λ = 633 nm with submillisecond response time. [ABSTRACT FROM AUTHOR]

High birefringence and low viscosity isothiocyanate liquid crystal single compounds, and eutectic mixtures based solely on unsaturated rigid core structures, are reported. Extraordinarily high values of figure-of-merit were observed at room temperature for the formulated nematic mixtures. Potential applications of such mixtures for laser beam steering at λ=1.55 mm using optical phased arrays are emphasized. [ABSTRACT FROM AUTHOR]

We have investigated the static and dynamic electrooptical effects of adding an alignment layer derived from a chiral liquid crystal having SmC* and N* phases to the standard polymer alignment layer in antiferroelectric liquid crystal cells. Our results show that the added layer reduces the relaxation time of the liquid crystal, the threshold voltage and the dynamic range required to generate a full greyscale. The electrooptical response of materials was tested using a 7 level waveform, and the relaxation was studied using a novel memory test. [ABSTRACT FROM AUTHOR]

Results of dielectric studies for three tolane derivatives containing the isothiocyanato (NCS) and alkyl or alkoxy groups in the terminal positions are presented. A relaxation process associated with molecular rotations around the short axes has been observed in the crystal-like smectic phases (B, E, K). The K phase was characterized for the first time by dielectric spectroscopy. The results are discussed by taking into account the dielectric relaxation data obtained recently for the biphenyl analogues in the E phase. The available X-ray data allow us to calculate the so-called packing parameter which characterizes the free volume of the molecules in the unit cells. [ABSTRACT FROM AUTHOR]

We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in this material. The optical birefringence increases strongly below the transition to the tilted phases, showing that the onset of tilt is coupled with a considerable increase in orientational order. The layer spacing in the smectic A* (SmA*) phase is notably smaller than the extended length of the molecules constituting the mixture, and the shrinkage in smectic C* (SmC*) and smectic C a * (SmC a *) is much less than the optical tilt angle would predict. These observations indicate that the tilting transition in W107 to a large extent follows the asymmetric de Vries diffuse cone model. The molecules are on average considerably tilted with respect to the layer normal already in the SmA* phase but the tilting directions are there randomly distributed, giving the phase its uniaxial characteristics. At the transition to the SmC* phase, the distribution is biased such that the molecular tilt already present in SmA* now gives a contribution to the macroscopic tilt angle. In addition, there is a certain increase of the average tilt angle, leading to a slightly smaller layer thickness in the tilted phases. Analysis of the wide angle scattering data show that the molecular tilt in SmC a * is about 20° larger than in SmA*. The large optical tilt (45°) in the SmC a * phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations. [ABSTRACT FROM AUTHOR]

The use of antiferroelectric liquid crystals(AFLCs)on practical applications is often precluded by their low contrast. Orthoconic AFLCs, in principle, may overcome this limitation, for they behave as isotropic plates at normal incidence. A number of orthoconic AFLC materials have been evaluated. Multiplexed seven-level driving schemes have been employed for dynamic analysis. The most relevant results include average static contrast ratio over 200:1 and dynamic contrast ratio over 130:1. On the other hand, hysteresis often shows slight voltage shifts when the cells are driven with low frequency AC signals. This feature impairs symmetry of the transmission maxima on the positive and negative lobes of the hysteresis curve. The effect is attributed to the extremely low pitch of actual orthoconic materials–usually below 1&ThickSpace;μm. The distinctive properties of orthoconic materials make them suitable for photonic applications requiring moderate response time such as routing or beam steering applications. [ABSTRACT FROM AUTHOR]

Polymer-dispersed liquid crystals containing orthoconic antiferroelectric smectics were prepared by photopolymerization-induced phase separation. Mean droplet size allowed formation of antiferroelectric helix inside droplets. Bias electric field applied during phase separation aligned helical axes parallel to the cell plane. Obtained ellipsoidal droplets were optically uniaxial for perpendicular light beam. The cells were switched by square or triangle driving signal unwinding the helix. V-shape switching was found instead of threshold one observed in conventional cells of the same mixture. The switching time was crucially depended on temperature. Those effects were caused by anchoring conditions inside polymer cavities. [ABSTRACT FROM AUTHOR]

Addressing waveforms for reflective LCOS devices filled with V-shape chiral smectic liquid crystals, have been tested and optimised. Analogue greyscale generation has been tested on passive devices mimicking the behaviour in active matrix based devices. A comparative study of a number of experimental chiral smectic mixtures showing V-shape response has been carried out. Specific driving waveforms have been designed and optimised in order to improve the material's electro-optic response. This procedure is lengthy since every voltage level of the waveform ought to be separately optimised, and interdependences between the different waveform fragments lead to an iterative optimisation process. The effect of a short blanking pulse named well pulse has been particularly remarkable, thus forcing a thorough tune-up as a function of working temperature and video rate. [ABSTRACT FROM AUTHOR]

The temperature dependence of density, refractive indices, apparent molecular length and intermolecular distance of 11CHBT, are reported, together with the static dielectric permittivities, ε || , ε ⊥ , ε iso of 11CHBT and 12CHBT; the effect of chain length on various physical properties of the CHBT series is also discussed. All the temperatures T m , T NI and T NCr show an odd-even (OE) effect, but transition enthalpies Δ H m and Δ H C show no such behaviour. T m , T NI , T NCr and Δ H m are found to increase with chain length, while Δ H C shows a maxima at n = 6. At the common reduced temperature T * = 0.98 T NI , the density of the compounds are found to decrease with increasing chain length while the polarizability parameters α e , α o and α increase. However, the volume expansion coefficient, polarizability anisotropy and normalized polarizability anisotropy show a distinct OE effect. Similarly, n e decreases smoothly with chain length but n o and Δ n show an OE effect like ε ⊥ and Δε. The ratio ln

/ln

remains closer to Faber theory than MS theory. Effective dipole moments are found to be approximately constant in the series; antiparallel association is observed in all the members. Kirkwood correlation factors g ⊥ and Δ g are found to exhibit an OE effect like ε ⊥ and Δε, but g || and ε || show no such effect. The effects of substitution in the core and end polar group on these properties are also discussed. [ABSTRACT FROM AUTHOR]

Results reported on the dielectric properties, spontaneous polarization (Ps) and switching time of a FLC material and a mixture of the FLC material after having doped with a small quantity of optical adhesive polymer. The mixture has been prepared by dissolving NOA65 prepolymer and the FLC compound in chloroform. A homogenous mixture of FLC and polymer formed after chloroform is being evaporated. Dielectric and Ps measurements of FLC-Polymer (PSFLC) mixture have been performed in a 10 μm ITO coated cell after the cell being irradiated with UV radiation for Photo-polymerization in situ. It has been observed that the spontaneous polarization of PSFLC mixture is unexpectedly reduced in polymer network formed as a result of photo polymerization. Similarly the Goldstone mode dielectric strength is also significantly reduced and switching time has been increased. The Soft mode dielectric strength however remains unchanged. The reduction of spontaneous polarization and Goldstone mode dielectric strength of the PSFLC mixture is not the accordance with the mixing ratio. The results have been discussed. [ABSTRACT FROM AUTHOR]

The synthesis and chemical transformations of 3-aryl- or 3-(trans-4-alkylcyclohexyl)- 6-arylcyclohex-2-enones into liquid crystalline compounds are discussed. These 3,6-disubstituted cyclohex-2-enones were prepared by the condensation of appropriate Mannich salts or trans-4-alkylcyclohexyl 2-bromoethyl ketones with 4-substituted benzyl methyl ketones. [ABSTRACT FROM AUTHOR]