Splay Elastic Constants Research Articles

Improvement of orientational order and display parameters of liquid crystalline material dispersed with single-wall carbon nanotubes

Composites were prepared by inserting Single-Walled Carbon Nanotubes (SWCNTs) in the Nematic Liquid Crystal (NLC) to fasten the reorientation dynamics of the system under the electric field. Dispersion of SWCNTs in NLC medium enhanced the orientational order and consequently the display parameters of composite systems improved. Various display parameters such as orientational order, dielectric anisotropy, threshold voltage and splay elastic constant were explored for pure and composite systems.

Investigating of dielectric anisotropy and birefringence of binary liquid crystal mixtures

Absorbance, capacitance, conductance, dielectric anisotropy and birefringence properties and splay elastic coefficient of 4’-Hexyl-4-biphenylcarbonitrile (6CB) and its mixture with 4’-Octyl-4-biphenylcarbonitrile (8CB) were investigated. The absorbance was studied using the UV-visible spectrophotometry. Capacitance, conduction and dielectric measurements for 6CB and its mixture were performed in 0-1000 kHz frequency range at the room temperature. Capacitance-voltage and capacitance-frequency changes were investigated for the mixture. Threshold voltage was determined as 2.1 V. It was seen that the capacitance suddenly damped at this voltage. Capacitance is large at lower frequencies and small at higher frequencies than 100 kHz. The conductance-voltage change was obtained for the mixture. The conduction suddenly rose at the threshold voltage. The conduction is small at lower frequencies and large at higher frequencies. Dielectric, birefringence index and splay elastic constant were investigated for 6CB/8CB mixture. The measured values of the mixture are bigger than 6CB values up to the certain frequency.

Transition of vertically aligned liquid crystal driven by fan-shaped electric field

Interdigital electrodes are implemented in many commercial and novel liquid crystal devices to align molecules. Although many empirical principles and patents apply to electrode design, only a few numerical simulations of alignment have been conducted. Why and how the molecules align in an ordered manner has never been adequately explained. Hence, this investigation addresses the Fréedericksz transition of vertically aligned liquid crystal that is driven by fishbone electrodes, and thereafter identifies the mechanism of liquid crystal alignment. Theoretical calculations suggest that the periodic deformation that is caused by the fan-shaped fringe field minimizes the free energy in the liquid crystal cell, and the optimal alignment can be obtained when the cell parameters satisfy the relation p/2d=k11/k33, where p is the spatial period of the strips of the electrode; d denotes the cell gap; and k11 and k33 are the splay and bend elastic constants of the liquid crystal, respectively. Polymer-stabilized vertical alignment test cells with various p values and spacings between the electrodes were fabricated, and the process of liquid crystal alignment was observed under an optical microscope. The degree of alignment was evaluated by measuring the transmittance of the test cell. The experimental results were consistent with the theoretical predictions. The principle of design, p/2d=k11/k33, greatly improves the uniformity and stability of the aligned liquid crystal. The methods that are presented here can be further applied to cholesteric liquid crystal and other self-assembled soft materials.

Elastic and dielectric properties of ferroelectric nanoparticles/bent-core nematic liquid crystal blend.

Bent-core liquid crystals present the first evidence of forming polar superstructures from achiral molecules. The nematic phase is the newest member of the bent-core family and turns out to be extremely interesting owing to its distinct features compared to its calamitic counterpart. Here the investigation of one achiral unsymmetrical 2-methyl-3-amino-benzoic acid (2,6-substituted toluene)-derived four-ring bent-core nematic (BCN) liquid crystals (11-2M-F) is presented after nanodispersion. Ferroelectric nanoparticles significantly affect the phase transition temperature, threshold voltage, dielectric permittivity, elastic constants and splay viscosity of the pristine BCN. In most bent-core nematic liquid crystals the bent elastic constant (K33) is usually lower than the splay elastic constant (K11) owing to the presence of short-range smectic-C-like correlations in the nematic phase. Thus the elastic anisotropy ([Formula: see text]) is usually negative in bent-core nematics unlike in rod-like nematic liquid crystals where K33 is always greater than K11. Here we report a short-core bent-shaped nematic liquid crystal whose negative elastic anisotropy was turned to positive by minute addition of ferroelectric nanoparticles.

Study of dielectric anisotropy and other related parametersof a liquid crystal compound 7OAOB

ABSTRACTTemperature-dependent dielectric anisotropy has been measured for the compound 4,4′-di-n-heptyloxy-azoxy benzene (7OAOB) by using a digital LCR bridge (HIOKI 3532-50 LCR HiTESTER). The compound 7OAOB is found to exhibit negative dielectric anisotropy in the nematic and smectic-C phase. Frequency-dependent electrical conductivity has been measured for this compound. The dc conductivity has also been obtained from frequency-dependent total conductivity studies by using universal power law equation. The activation energy of the dc conduction process and relaxation time has been determined in different mesophases. The behavior of splay elastic constant (K11) with temperature has been examined for this compound. From the studies of UV-visible spectra, the values of energy band gap have been evaluated.

Bipolar to toroidal configuration transition in liquid crystal droplets

ABSTRACTLiquid crystal (LC) droplets dispersed in isotropic media are of significant interest for scientific community and great importance for industries. The confinement can generate many fascinating LC director configurations and enable important practical applications. With tangential anchoring condition at the droplet surface, theoretically there are several possible configurations: bipolar, twisted bipolar, escaped toroidal, toroidal and so on. Bipolar configuration is usually observed in droplets made from common LCs while the toroidal configuration is rarely observed and it is hard to create especially in thermotropic LCs. Their realisations depend on the splay, bend and twist elastic constants ratio, and anchoring condition of the LC and polymer interface. We constructed thermotropic LCs with abnormally small bend elastic constants, with which stable toroidal configuration were successfully created. We provide a brand new method to create toroidal droplet by simply varying the bend elastic constant. We observed the transition from bipolar configuration to toroidal configuration. We performed a detailed study of the texture of toroidal droplets.

Studies of temperature and frequency dependence of dielectric permittivities of two Schiff's base liquid crystal compounds

Abstract Effect of temperature and frequency on the dielectric permittivities of two Schiff's base liquid crystal compounds p-ethoxybenzylidine p-heptylaniline(EBHA) and p-butoxybenzylidine p-heptylaniline(BBHA) have been studied by using a digital LCR bridge (HIOKI 3532-50 LCR HiTESTER). Due to the changes of CH 2 groups at the oxygen containing flexible side chain between these two compounds, some changes occur in the behavior of their temperature dependent physical properties such as dielectric anisotropy, splay elastic constant, relaxation time, which has been studied extensively. The frequency dependent electrical conductivity of these two compounds has also been carried out. At the nematic mesophase the values of activation energy have been estimated from the temperature dependent DC and AC conductivity curves. The optical band gap of EBHA and BBHA has been calculated from UV spectra.

Influence of fluorination and chain length on the static dielectric properties of nematogenic bi-cyclohexyl phenyl derivatives

ABSTRACTEffects of fluorination and chain length of six bi-cyclohexyl phenyl compounds (3ccp-f, 3ccp-ff, 3ccp-fff, 5ccp-f, 5ccp-ff, 5ccp-fff) on their physical properties have been investigated. All the compounds exhibit nematic phase over a wide range of temperatures. Molecular mechanics calculation reveals that dipole moments of the molecules increases (1.93D to 3.37D) with fluorination in both propyl- and pentyl-based systems; increment in f to ff derivatives is more than in ff to fff derivatives. However, no change in dipole moment is observed with increased chain length. In the nematic phase, average value of dielectric constant (ϵavg) is less than the extrapolated values of ϵiso in isotropic phase. Increased fluorination also decreases splay elastic constant, and thus faster response is expected in triply fluorinated compounds.

Sign-inversion of elastic anisotropy in a bent-core nematic liquid crystal doped with carbon nanodots

In most bent-core nematic liquid crystals the bent elastic constant (K33) is usually lower than splay elastic constant (K11) owing to the presence of short range smectic-C like correlations in nematic phase. Thus the elastic anisotropy (K33−K11) is usually negative in bent-core nematics unlike rod-like nematic liquid crystals where K33 is always greater than K11. Here we report a short-core bent-shaped nematic liquid crystal whose negative elastic anisotropy was turned to positive by minute addition of carbon nanodots (C-dots). The experimental results demonstrate significant crossover of dielectric anisotropy and splay-bent elastic constant values at ~125°C. This effect is attributed to gradual formation of chain-like structures of C-dots as the mixture is cooled from isotropic temperature. Threshold voltage reduces significantly at high temperature region owing to higher local electric field and reduced viscosity. Dielectric relaxations of nano-sized clusters become faster in nano-dispersed liquid crystal.

Polar interactions between bent–core molecules as a stabilising factor for inhomogeneous nematic phases with spontaneous bend deformations

ABSTRACTIt is generally accepted that the transition into the twist–bend nematic phase (NTB) is driven by an elastic instability related to the reduction of the bend elastic constant. Here we use a molecular–statistical theory to show that sufficiently strong polar interactions between bent-shaped molecules may lead to experimentally observed reduction of the bend elastic constant in the nematic phase even if electrostatic dipole–dipole interactions are not taken into account. We propose a simple model of bent–core particles and derive explicit analytical expressions which enable one to understand how polar molecular shape affects the elastic constants, and, in particular, the important role of the bend angle. Numerical graphs showing temperature variations of all elastic constants are also presented including the variation of the bend and splay elastic constants before and after the renormalisation determined by local polar order of molecular steric dipoles and the corresponding polar correction to the one-particle distribution function.

Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

ABSTRACTWe report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay (), twist (), and bend () elastic constants of the uniaxial nematic (N) phase formed by flexible dimers of DTC5C9 and compare their behaviour to that of a corresponding monomer MCT5. DTC5C9 forms a twist-bend nematic (Ntb) at temperatures below the N phase. Anisotropic properties of MCT5 are typical of the rod-like mesogens. In particular, birefringence increases as the temperature is reduced, following the classic behaviour, described by Haller. The elastic constants also follow the standard behaviour, with their ratios being practically temperature independent. In contrast, DTC5C9 shows a dramatic departure from the standard case. Birefringence changes non-monotonously with temperature, decreasing on approaching the N–Ntb phase transition. decreases strongly to 0.4 pN near the N–Ntb transition, although remains finite. The ratios of the elastic constants in DTC5C9 show a strong temperature dependence that can be associated with the bend-induced changes in the orientational distribution function. The measured elastic properties are consistent with the tendency of the dimeric molecules to adopt bent configurations that give rise to the Ntb phase.

Large reduction in the magnitude and thermal variation of Frank elastic constants in a gold nanorod/nematic composite

We report measurements of splay and bend Frank elastic constants in a composite comprising a nematic liquid crystal doped with a small concentration of sterically stabilized gold nanorods. The composite exhibits not only a large reduction in the magnitude of the threshold voltage for switching (Vth, 20%), as well as of the splay (K11, 40%) and bend (K33, 40%) elastic constants, but also presents an unprecedented feature: a substantial diminution in the temperature dependence of these parameters, almost to the point of becoming thermally invariant. This observation is significant because the electro-optic switching of liquid-crystal devices is largely controlled by the K11 and K33 elastic constants. Electrical conductivity measurements also show interesting behavior upon the inclusion of nanorods. Whereas the intrinsic Arrhenius behavior governing the temperature dependence is enhanced, the frequency dependence shows qualitative features of Jonscher’s universal model, albeit with a higher exponent. Further, photoisomerization of an azobenzene guest component provides an additional influence on the elastic constants. The results are discussed in terms of (a) the effect of the order parameter dependence seen from the viewpoint of an extended mean-field model, and (b) local order. The advantage of incorporating nanorods with photofunctionality is also pointed out.

Effect of iron phthalocyanine (FePc) concentration on electrical and dielectric properties of the nematic liquid crystal composites

Electrical and dielectric properties of iron phthalocyanine (FePc) dispersed E63 nematic LC were investigated using admittance measurements in the frequency range of 100Hz - 20MHz and current measurements in the bias range of 0–20V. In order to observe the effect of FePc concentration, E63 was dispersed with 1, 3 and 5% wt/wt FePc. Basic dielectric parameters such as dielectric anisotropy (Δε), crossover frequency (fc), real and imaginary parts of dielectric permittivity (ε' and ε") and relaxation time (τ) of the samples were obtained from the experimental data, along with threshold voltage (Vth), splay elastic constant (K11) and conductivity parameters (σac and σdc). Dispersal of FePc and its concentration caused Vth, K11, σdc and fc of the LC composites increase whereas τ is lowered. LC composites were found to exhibit relaxation in low and high frequency regions such that the former was masked due to high conductivity in the samples and hence electric modulus formalism was applied to determine low frequency relaxation. Prominent effect of FePc dispersal was found in σdc values in a way that σdc of the LC composite of E63 with 5% wt/wt FePc is about 53 times larger than that of neat E63.

Influence of virtual surfaces on Frank elastic constants in a polymer-stabilized bent-core nematic liquid crystal.

Effect of a polymer network on the threshold voltage of the Fréedericksz transition, Frank elastic constants, switching speed, and the rotational viscosity are investigated in a polymer-stabilized bent-core nematic liquid crystal with different polymer concentrations. These polymer networks form virtual surfaces with a finite anchoring energy. The studies bring out several differences in comparison to similar studies with a calamitic liquid crystal as the nematic host. For example, on varying the polymer content the threshold voltage decreases initially, but exhibits a drastic increase above a critical concentration. A similar feature-reaching a minimum before rising-is seen for the bend elastic constant, which gets enhanced by an order of magnitude for a polymer content of 2.5 wt %. In contrast, the splay elastic constant has a monotonic variation although the overall enhancement is comparable to that of the bend elastic constant. The behavior changing at a critical concentration is also seen for the switching time and the associated rotational viscosity. The presence of the polymer also induces a shape change in the thermal dependence of the bend elastic constant. We explain the features observed here on the basis of images obtained from the optical and atomic force microscopy.

Measurement of splay elastic constant and rotational viscosity of an induced nematic binary system

ABSTRACTWe investigated the temperature dependence of the splay elastic constant (K11) and rotational viscosity (γ1) of a binary liquid crystal system comprising 5-trans-n-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (4DBT) and 4-cyano-4′-n-undecyloxy-biphenyl (11OCB), exhibiting induced nematic phase. Both the splay elastic constant and rotational viscosity increased following lowering of the temperature. In the vicinity of smectic A-nematic (SmA-N) transition, both the relaxation time and rotational viscosity exhibited a strong pretransitional behaviour. The critical exponent (ν) presented here led to valuable qualitative information about the pre-transitional behaviour of the rotational viscosity data near the SmA-N phase transition. The extracted ν values ranging between 0.336 and 0.352 are in fair agreement with those predicted in the de Gennes model as compared to that by the mean-field model.

Diminished Splay Stiffening in Weak Gels of Calamitic-Bent-Core Nematic Composites.

Composites of calamitic and bent-core nematic molecules exhibiting a nematic to nematic-gel transformation have been investigated using thermal, electrical, X-ray, and mechanical probes. The studies focusing on the Frank elastic behavior show a surprising result that the thermal behavior of the threshold voltage and the dependent splay elastic constant (K11) are remarkably different in temperature regions identified as weak and strong gels. In the former gel, the parameters exhibit values significantly smaller than the higher-temperature fluid nematic, effectively canceling out the underlying thermal variation due to the order parameter. This is especially attractive from the viewpoint of display devices. The X-ray diffraction data suggest that the fibers have a plastic nature in the weak gel and 3D-crystalline ordering in the strong gel. We argue that the different elastic behavior in the two gel phases is caused by the nature of the fibers; they are stiff in both gels but the interfiber interaction is weaker in the weak gel allowing the splay elastic constant to be lowered. The X-ray and rheological data lend support to the characterization of the fibers.

Unusual temperature dependence of elastic constants of an ambient-temperature discotic nematic liquid crystal.

We report the first experimental studies on the temperature dependence of viscoelastic properties of a room temperature discotic nematic liquid crystal. The splay elastic constant is greater than the bend elastic constant and both show unusual temperature and order parameter dependence. The rotational viscosity is remarkably larger than conventional calamitic liquid crystals. We provide a simple physical explanation based on the columnar short-range order to account for the the unusual temperature dependence of the elastic constants.

Temperature dependences of the electrooptical properties of rodlike nematic liquid crystals doped with hockey-stick-shaped liquid crystals

We investigated the temperature dependences of the dielectric anisotropy, birefringence, order parameter, splay elastic constant, and rotational viscosity of rodlike nematic liquid crystals (RLCs) doped with hockey-stick-shaped liquid crystals (HLCs). Although the order parameter of the HLC-RLC mixtures was similar to that of the pure RLC, the dielectric anisotropy and the birefringence of the mixtures were decreased or increased depending on the structure of the HLC molecule. In addition, the activation energies of the mixtures were different, which implies that the intramolecular structure of the HLC molecule had more influence on the electrooptical properties of the HLC-RLC binary mixtures than the inter-molecular interaction between the HLC and the RLC molecules.

Thermodynamical, optical, electrical and electro-optical studies of a room temperature nematic liquid crystal 4-pentyl-4′-cyanobiphenyl dispersed with barium titanate nanoparticles

Dispersion of ferroelectric Barium Titanate Nano Particles (BTNPs) in a nematic liquid crystal (NLC) display material namely 4-pentyl-4′-cyanobipheny (5CB) has been studied by differential scanning calorimeter, UV–Vis spectroscopy, polarizing light microscopy, dielectric spectroscopy and electro optical studies. BTNPs have been dispersed in three different concentrations in pure 5CB viz. 0.05, 0.5 and 5.0wt.%. It has been observed that with the increase of BTNP concentrations in 5CB, the nematic to isotropic transition temperatures, parallel components of dielectric permittivity and hence dielectric anisotropy decreases significantly unlike in the earlier studies. Threshold voltage for the Freedericksz transition, switching voltage and splay elastic constant have also decreased due to the dispersion of BTNPs. Relaxation frequency and activation energy of an observed relaxation mode corresponding to molecular rotation about the short axis are almost the same for pure and its nanocomposites. The ionic conductivity has increased by two orders of magnitudes for 0.05wt.% of nanocomposites. The optical study suggests that due to dispersion of BTNPs band gap has decreased.

Twist, tilt, and orientational order at the nematic to twist-bend nematic phase transition of 1″,9″-bis(4-cyanobiphenyl-4′-yl) nonane: A dielectric,H2NMR, and calorimetric study

The nature of the nematic-nematic phase transition in the liquid crystal dimer 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) has been investigated using techniques of calorimetry, dynamic dielectric response measurements, and (2)H NMR spectroscopy. The experimental results for CB9CB show that, like the shorter homologue CB7CB, the studied material exhibits a normal nematic phase, which on cooling undergoes a transition to the twist-bend nematic phase (N(TB)), a uniaxial nematic phase, promoted by the average bent molecular shape, in which the director tilts and precesses describing a conical helix. Modulated differential scanning calorimetry has been used to analyze the nature of the N(TB)-N phase transition, which is found to be weakly first order, but close to tricritical. Additionally broadband dielectric spectroscopy and (2)H magnetic resonance studies have revealed information on the structural characteristics of the recently discovered twist-bend nematic phase. Analysis of the dynamic dielectric response in both nematic phases has provided an estimate of the conical angle of the heliconical structure for the N(TB) phase. Capacitance measurements of the electric-field realignment of the director in initially planar aligned cells have yielded values for the splay and bend elastic constants in the high temperature nematic phase. The bend elastic constant is small and decreases with decreasing temperature as the twist-bend phase is approached. This behavior is expected theoretically and has been observed in materials that form the twist-bend nematic phase. (2)H NMR measurements characterize the chiral helical twist identified in the twist-bend nematic phase and also allow the determination of the temperature dependence of the conical angle and the orientational order parameter with respect to the director.