Liquid Crystal MBBA Research Articles

Synergistically controlled in situ growth of silver nanoparticles leading to organization of anisotropic liquid crystalline medium

ABSTRACT An in-depth macroscopic and microscopic investigation is carried out regarding understanding phase ordering and organisation of nematic phase of a room temperature nematic liquid crystal (NLC) MBBA (N-(4-methoxybenzylidene)-4-butylaniline) in the presence of silver nanoparticles (AgNPs) which are synthesised in situ in a single step using MBBA as a template as well as a reducing and stabilising unit. A synergistic effect between anisotropic LC structure and morphologies of AgNPs is observed. The LC-AgNP composite exhibits better thermal stability and phase ordering compared to its neat LC counterpart as indicated by combined study of optical polarization microscopy (OPM) and differential scanning calorimetry (DSC). While OPM study exhibits emergence of smectic mesophase in LC-AgNP composites, DSC manifests significant enhancement in the isotropic temperature and associated enthalpy as well as a change in the kinetic behaviour in LC-AgNP composites compared to neat LC. Also, an alternation in the electronic properties of LC-AgNP composites compared to neat LC is recognised. Such ordering at the macroscopic scale is well supported by our microscopic observations through spectroscopic studies such as Fourier transform infrared (FTIR) and X-ray diffraction (XRD) which provides valuable insights regarding understanding the organisation process more closely.

Dielectric and conductivity properties of liquid crystal MBBA doped with single-walled carbon nanotubes

The effect of single-walled carbon nanotubes (SWCNTs) on the dielectric and conductivity properties of nematic liquid crystal 4-methoxybenzylidene-40-butylaniline (MBBA) has been studied. It was shown that the additive of SWCNTs with concentration of 0.3% leads to an increase in the order parameter of MBBA. Observation under the polarization microscope has shown that the presence of SWCNTs in MBBA increases the clearing temperature from 46.3°C to 47.4°C. This change can be explained by the improvement of the liquid crystalline orientation order. As a result, the transition to a complete disordering of the molecules (transition to an isotropic state) requires more heat energy, correspondingly, more temperature. The dielectric and conductivity measurements have showed that the longitudinal component of the dielectric permittivity decreases while the transverse component increases as well as dielectric anisotropy. The percolation effect promotes to the dominance of hopping electron conductivity over ionic conductivity, leading to an increase in electric conductivity.

Dielectric and conductivity properties of liquid crystal MBBA doped with SWCNTs

The effect of single-walled carbon nanotubes (SWCNTs) on the dielectric and conductivity properties of nematic liquid crystal 4-methoxybenzylidene-4′-butylaniline (MBBA) has been studied. It was shown that the additive of SWCNTs with concentration of 0.3% leads to an increase in the order parameter of MBBA. In this case, the clearing point is raised, the longitudinal component of the dielectric permittivity decreases while the transverse component increases as well as dielectric anisotropy. The percolation effect promotes to the dominance of hopping electron conductivity over ionic conductivity, leading to an increase in electric conductivity.

Экспериментальное исследование реологических свойств жидкого кристалла, модифицированного углеродными нанотрубками

The results of an experimental study of the rheological properties of a nematic liquid crystal MBBA modified with carbon nanotubes depending on the mass concentration of the filler are presented. The measurements were performed on the Physica MCR 501 rheometer (Anton Paar, Austria) by the method of oscillations in the controlled deformation mode. It is shown that the introduction of a modifier into a liquid crystal changes the temperature of the phase transition to the isotropic phase of Ti. The dependence of Ti on the concentration of the filler is constructed. The flow curves and the curves of the complex viscosity of the samples were measured in the concentration range from 0.015to 0.085 wt.%. It is shown that all flow curves can be approximated by one functional dependence: the rheological Tscheuschner equation. The parameters of the approximating curves are calculated. It is shown that the doped samples, in contrast to the dispersion medium, exhibit viscoelastic properties, the greater the greater the concentration of the filler. The dependences of the phase shift angle between tangential stresses and the strain value δ on the concentration of the modifier are investigated. It is shown that as the shear flow velocity increases, the values of δ →π/2, which corresponds to a purely viscous flow. The values of tangential stresses at which the transition to a viscous flow occurs, depending on the concentration of the modifier, are calculated..

Флексоэлектрические домены в бинарной смеси нематиков

Flexoelectric domains formation in mixture of nematic liquid crystals (NLC) MBBA and 5CB with anisotropy of dielectric permeability ε_a ~ 0 under the action of dc electric voltage is investigated. Threshold characteristics of flexoelectric domains formation (threshold voltage and spatial period) in the studied mixture are measured. The difference of flexoelectric coefficients |e1-e3| is determined. It is shown that in the mixture of nematics with ε_a ~ 0 , the threshold voltage is determined only by the ratio k/(e1-e3) (k - average elastic constant), which allows to calculate the difference in flexocoefficients |e1-e3| directly, excluding the properties of substrate surfaces. The definition method of the flexoelectric coefficients, proposed and approved in paper, can be useful for the new synthesized NLC with ε_a <- 0.5.

Экспериментальное исследование магнитных свойств жидкого кристалла MBBA и ферромагнитных суспензий на его основе

This article shows the measured dependences of the Frederiksz threshold field for liquid crystal (LCD) MBBA and ferromagnetic suspension (FN) based on it. Magnetite particles (Fe3O4) were used as magnetic particles, the volume fraction of which was f = 4∙10 -4 . Electron microscopy showed that magnetite particles have a quasispherical shape. The particle size distribution is constructed. The average particle diameter was 10 nm. A technique for preparing samples for doping a nematic matrix with ferromagnetic particles and a liquid crystal cell device are described. Dielectric measurements are performed on a precision RLC WK 6505B analyzer in a constant magnetic field with induction from 0 to 0.6 T. It is shown that the Frederiksz threshold field for the FN is significantly smaller than for the nematic matrix. Temperature dependences of the Frederiksz threshold field are constructed. It is shown that with a temperature rise the threshold field values decrease.

Optical, morphological and dielectric characterization of MBBA liquid crystal-doped hydrogels

Abstract In this work, MBBA liquid crystal-doped hydrogels are investigated through of Scanning Electron Microscopy (SEM), Polarized Optical Microscopy (POM), Optical Transmittance (OT) and Dielectric Spectroscopy (DS) techniques. For the synthesis of the hydrogels and of their composites a solution (20% water and 80% acetone) of the potassium periodate (KIO 4 ) was prepared. The samples were prepared by radicalar photochemical polymerization of acrylamide monomer (AAm) in the presence of N , N ′-methylene-bis-acrylamide (MBAAm) as a cross-linking agent. Potassium periodate (KIO 4 ) was used as a photopolymerization initiator. In such synthesis, acrylamide monomer (AAm-Aldrich, 14, 866–5) was added with cross-linking agent MBAAm (N, N-methylene-bis-acrylamide) and liquid crystals MBBA (N-(4-methoxybenzilidene)-4(butylaniline)). The acetone was used to solubilize the liquid crystal MBBA in the solution. The resulting solution was placed and sealed between two glass plates coated with transparent and conducting films of tin oxide (SnO 2 ) with spacer of 25 μm. All samples were investigated at 15.0 °C, 23.0 °C and 35.0 °C. The electrical behavior of the samples was characterized by using a Solartron 1260A impedance-gain phase analyzer in the frequency range from 10 mHz to 10 MHz with amplitude of 10 mV. The experimental results from DS technique are discussed by using Davidson–Cole and Havriliak–Negami approaches. As one of main results of this work we noticed that the cavities of the hydrogels synthesized with the use of acetone can be able to confine the free ions of the liquid crystal added to the polymeric matrix.

Analysis of the Influence of the Molecular Volume to Predict Experimental Pressure-Temperature Behavior in the Isotropic-Nematic Phase Transition of PAP, 5CB, MBBA and EBBA

In this work, we have analyzed the experimental pressure-temperature behavior at the isotropic-nematic phase transition of the liquid crystals PAP, 5CB, MBBA, and EBBA at 1 atm by using the HERSW Convex Peg model in conjunction with the IPCM model. We have calculated the molecular volume values for the hard and attractive cores from theoretical quantum calculations at the PM3, PM6, B3LYP/6-311++G(d,p)//PM6, and M06/6-311++G(d,p)//PM6 levels of theory. The results suggest that the best theoretical prediction of the experimental pressure-temperature behavior is obtained when the molecular volume is evaluated at the DFT level.

The microwave heating mechanism of N-(4-methoxybenzyliden)-4-butylaniline in liquid crystalline and isotropic phases as determined using in situ microwave irradiation NMR spectroscopy.

Microwave heating effects are widely used in the acceleration of organic, polymerization and enzymatic reactions. These effects are primarily caused by the local heating induced by microwave irradiation. However, the detailed molecular mechanisms associated with microwave heating effects on the chemical reactions are not yet well understood. This study investigated the microwave heating effect of N-(4-methoxybenzylidene)-4-butylaniline (MBBA) in liquid crystalline and isotropic phases using in situ microwave irradiation nuclear magnetic resonance (NMR) spectroscopy, by obtaining (1)H NMR spectra of MBBA under microwave irradiation. When heated simply using the temperature control unit of the NMR instrument, the liquid crystalline MBBA was converted to the isotropic phase exactly at its phase transition temperature (Tc) of 41 °C. The application of microwave irradiation at 130 W for 90 s while maintaining the instrument temperature at 20 °C generated a small amount of isotropic phase within the bulk liquid crystal. The sample temperature of the liquid crystalline state obtained during microwave irradiation was estimated to be 35 °C by assessing the linewidths of the (1)H NMR spectrum. This partial transition to the isotropic phase can be attributed to a non-equilibrium local heating state induced by the microwave irradiation. The application of microwave at 195 W for 5 min to isotropic MBBA while maintaining an instrument temperature of 50 °C raised the sample temperature to 160 °C. In this study, the MBBA temperature during microwave irradiation was estimated by measuring the temperature dependent chemical shifts of individual protons in the sample, and the different protons were found to indicate significantly different temperatures in the molecule. These results suggest that microwave heating polarizes bonds in polar functional groups, and this effect may partly explain the attendant acceleration of organic reactions.

Anomalous heat capacity for nematic MBBA near clearing point

The pretransition properties of the heat capacity on the both sides of the nematic - isotropic phase transition for the liquid crystal MBBA (n-methoxybenzylidene-n -butylaniline) are discussed on the basis of the Landau-de Gennes theory. Agreement between theory and experiment allows to obtain the critical exponents gamma and beta.

The Novel Stable Nitroxide Radicals as Perspective Spin Probes for Study of Orientation Order of Liquid Crystals and Polymers

Stable nitroxide radicals with rigid core were used as spin probes for investigation of orientational order of nematic liquid crystals MBBA and 5CB, uniaxially stretched polyethylene and porous polyethylene. The orientation distribution functions of probe molecules were determined. The order parameters of the true orientation axes were estimated. It was demonstrated that the novel nitroxide radicals show orientational alignment of both liquid crystals and polymers more effectively than ordinary spin probes.

Liquid crystal diffusion in thin films investigated by PFG magnetic resonance and magnetic resonance imaging

NMR is a well-established technique to study orientational order and dynamics of liquid crystals. We present a study of liquid crystal MBBA properties in both bulk and constrained thin film capillaries (thickness ∼200 μm) at different temperatures. Translational self-diffusion measurements have been performed in the nematic and isotropic phases. For constrained samples in the isotropic phase the diffusion coefficient has orientational dependence ( D ∥/ D ⊥ ≈ 7). To characterize the diffusion distribution of liquid crystals within the thin film, a diffusion mapping experiment was performed. The method provides accurate diffusion coefficients ( D ∥ and D ⊥) measurements with a pixel resolution of 4 μm.

Characterisation and intrinsic magnetic resonance properties of nickel nanoparticles

Nickel nanoparticles have been extensively characterised by atomic force microscopy (AFM), scanning electron microscopy (SEM) and confocal micro-Raman spectroscopy. AFM underestimates the particle size compared to SEM measurements. It is shown that Raman spectroscopy can detect the nanometre-thick NiO layer on the particles having frequency shifts of the modes indicative of phonon confinement. The magnetic properties of the particles are studied by ferromagnetic resonance (FMR) of magnetic field aligned particles. The alignment is achieved by suspending the particles in the liquid crystal MBBA and freezing the liquid in a 0.4 T DC magnetic field. The in-field solidification locks the direction of maximum magnetisation of the particles parallel to the direction of the applied DC magnetic field. This removes the effects of dynamical particle fluctuations of the nanoparticles on the magnetic properties allowing a study of the intrinsic magnetic properties of the magnetic nanoparticles. The intensity of the FMR signal decreased with lowering temperature for the particles frozen in the liquid in a 0.4 T DC magnetic field. The effect is suggested to be due to a reduction of the microwave skin depth with lowering temperature.

Dynamic 1H NMR Investigation of the Liquid Crystal MBBA Confined in Porous Silica

The polymorphism and molecular dynamics of the liquid crystal MBBA [N-(p-methoxybenzylidene)-p-n-butylaniline], confined in controlled pores glasses (CPG) porous glasses of 82, 156, and 337 Å pore diameter, were investigated using 1H nuclear magnetic resonance spectroscopy (NMR). The transition temperatures are determined from the analysis of NMR line-shapes and spin-lattice relaxation times (T1). The results show that the depression of the phase-transition temperatures is linear versus inverse pore diameter. The motion parameters of the phases were derived from an isotropic rotational diffusion model (BPP). In the nematic and glassy nematic phases, the results show that the confinement weakly influences the dynamics. In the crystalline phases, the confinement results in a reduction of the motion activation energies and an important phase diagram modification. To interpret the experimental results with regard to the bulk characteristics properties, a Cole–Cole distribution model was introduced. The width distribution parameter was found to depend linearly on the inverse pore diameter in the crystalline phases.

Onset of electroconvection of homeotropically aligned nematic liquid crystals

We present experimental measurements near the onset of electroconvection (EC) of homeotropically aligned nematic liquid crystals Phase 5A and MBBA. A voltage of amplitude square root 2V0 and frequency f was applied. With increasing V0, EC occurred after the bend Freedericksz transition. We found supercritical bifurcations to EC that were either stationary bifurcations or Hopf bifurcations to traveling convection rolls, depending on the sample conductances. Results for the onset voltages Vc, the critical wave numbers kc, the obliqueness angles thetac, and the traveling-wave (Hopf) frequencies at onset omegac over a range of sample conductances and driving frequencies are presented and compared, to the extent possible, with theoretical predictions. For the most part good agreement was found. However, the experiment revealed some unusual results for the orientations of the convection rolls relative to the direction selected by the Freedericksz domain.

Spatiotemporal chaos in electroconvection of a homeotropically aligned nematic liquid crystal

We present patterns of electroconvection (EC) for the homeotropically aligned nematic liquid crystal MBBA. A voltage V = square root of 2V0 sin(2pift) was applied. With increasing V0, the bend Freedericksz transition at VF was followed by the onset of EC at Vc > VF. We found four distinct pattern types. First, a primary supercritical Hopf bifurcation to traveling waves (TW's) of convection rolls occurred. The structure factor S(k) of this state reflected the azimuthal anisotropy of the underlying Freedericksz state. For f < fL approximately 75 Hz there was a superposition of two oblique-roll modes (pattern I). These patterns were chaotic in space and time. For larger f the patterns consisted of chaotic TW normal rolls (pattern II). Here the chaos was attributable to the motion of dislocations and domain walls between left- and right-traveling waves. A secondary bifurcation yielded pattern III; it had no dominant TW frequency but had broadband chaotic dynamics dominated by the motion of dislocations. This pattern type had been referred to by others as a "chevron pattern;" its structure factor still revealed azimuthal anisotropy. Finally, at somewhat larger identical with epsilon = V2/Vc2 -1 a highly disordered pattern IV with defect dynamics was found. This state had been studied before by Kai and co-workers and was referred to by them as "phase turbulence." It had a structure factor that was (within our resolution) invariant under rotation. For patterns I, II, and III, S(k) contained crescent-shaped peaks. The peak shape was qualitatively different from the case of planar EC where the structure factor has an elliptical cross section. We present measurements of the widths 1/xik and 1/xitheta in the radial (k) and the azimuthal (theta) directions. For small epsilon (patterns I and II) we found that xik was consistent with the usual Ginzburg-Landau scaling xik approximately epsilon(-nuk) with nuk approximately 1/2. However, for xitheta we found xitheta approximately epsilon(-nutheta) with nutheta approximately 3/4. Presumably this anomalous scaling of xitheta is associated with the Goldstone mode of homeotropic EC. We also show data for the height S0 of the structure factor that are consistent with S0 approximately epsilonbeta with beta approximately -0.5, implying that S0 diverges at onset. This differs from the case of domain chaos in rotating Rayleigh-Bénard convection where experiment is consistent with beta = 1/2 and thus with a vanishing S0. The difference between the shape of the structure-factor cross section and between the exponents, for the present case, for planar EC, and for domain chaos suggests that there are different universality classes for spatiotemporal chaos.

Optical and morphological characterization of polyacrylamide hydrogel and liquid crystal systems

In this work, the thermotropic liquid crystal MBBA ( N-(4-methoxybenzilidene)-4-butylaniline), entrapped on hydrogels, based on cross-linked polyacrylamide (PAAm), was studied. The liquid crystalline phases of system were characterized by polarized optical microscopy (POM), refractive index, optical transmittance, scanning electron microscopy (SEM) and water loss. It was verified the presence of birefringence on hydrogel + liquid crystal. The dynamic of formation of such birefringence finished 40 days after the hydrogel synthesis. The effective birefringence Δ n, i.e., the difference on refractive index of polyacrylamide hydrogel to refractive index of hydrogel + liquid crystal (Δ n 1) and the difference on refractive index of liquid crystal (MBBA) to refractive index of hydrogel + liquid crystal (Δ n 2) are dependent of content of acrylamide (AAm) and MBBA on hydrogel. The increase on Δ n 1 and Δ n 2 with the polyacrylamide content on hydrogel was attributed to decreasing of the mobility liquid crystal inside the hydrogel. Also, an increase on MBBA concentration in the polymeric matrix provides a reduction in the values of optical transmittance in the system. The morphology observed by SEM shows that hydrogel + liquid crystal is more compact that PAAm hydrogels. The presence of MBBA causes an increase in hydrophobicity. The water loss speed is favored by the increase in the amount of MBBA present in the hydrogels.

Orientational Dynamics in Nematic Liquid Crystal under Decay Poiseuille Flow

We report the results on the orientational dynamics and instabilities in nematic liquid crystal MBBA induced by decaying Poiseuille flow. The experiments were carried out in the rectangular wedge-shape LC cell with thickness varying perpendicular to the flow direction and the confining plates provided homeotropic alignment of a nematic liquid crystal. Increasing the values of initial pressure difference the orientational instability corresponding to the escape of the director out of the shear plane was observed for the first time. This “out-of-plane” transition takes place at some critical pressure difference ΔPc which depends on the local thickness of NLC layer. The results of numerical simulations of governing nematodynamic equations are in a good agreement with experimental data on the threshold of “out-of-plane” instability.

New Aspect of the Voltage/Confinement Ratio Phase Diagram for a Confined Homeotropic Cholesteric

We have investigated the behavior of a cholesteric mixture, consisting of the negative dielectric anisotropy liquid crystal MBBA and a chiral dopant S811(1%b.w.). In the phase diagram a.c. applied voltage versus confinement ratio, in addition to the transition previously observed, before reaching the domain of the transition TIC(Translationnary Invariant Cholesteric)–modulated TIC, we have observed a new stability domain for the topological bubble. These bubbles are now surrounded by the TIC domains and necessarily associated to an hyperbolic − 1 defect of the in plane director component.We have also obtained experimentally topological defect chains, formed by topological dipoles similar to the ones formed in 2 compound materials: liquid crystal/solid spheres or liquid crystal/liquid droplets.

Novel pulsed switched power supply for a fast field cycling NMR spectrometer

In this paper, we outline the operating principles of a pulsed switched power supply for a fast field-cycling nuclear magnetic resonance spectrometer. The power supply uses a variant of a four-quadrant chopper with a duty cycle that defines the average output current. With this topology only two semiconductors are necessary to drive hundreds of amperes with an output power of several kilowatts. The output current ripple has a well-defined shape that can be reduced to acceptable values by a careful design of the semiconductors’ controlling circuits and drivers. A power supply prototype was tested with a home build air-core magnet operating with fields between 0 and 0.21 T. The system is computer controlled using pulse generator and data acquisition PC cards, and specific user-friendly home-developed software. A comparative proton relaxometry study in two well-known liquid crystal compounds 5CB and MBBA was performed to check the reproducibility of the T 1 measurements.