Molar Mass Nematic Liquid Crystals Research Articles

Field-induced director dynamics in confined nematic liquid crystals imposed by a strong orthogonal electric field

The field-induced director dynamics for a low molar mass nematic liquid crystal (LC) has been investigated theoretically based on the hydrodynamic theory including the director motion with appropriate boundary and initial conditions. Analysis of the numerical results for the turn-on process provides evidence for the appearance of the spatially periodic patterns in 4-n-pentyl-4'-cyanobiphenyl LC film, only in response to the suddenly applied strong electric field orthogonal to the magnetic field. It has been shown that at the values of the voltage of 200 V across the 194.7 μm LC film and the magnetic field of 7.05 T directed at the angle α=1.57(~89.99°) between two fields, there is a threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provides the nonuniform rotation mode rather than the uniform one, whereas the lower values both of the amplitude and the angle α [<1.565(~88.81°)] dominate the uniform mode. During the turn-off process, the reorientation of the director to its equilibrium orientation is characterized by the complex destruction of the initially periodic structure to a monodomain state.

Field-induced non-uniform director reorientation for a low molar mass nematic imposed by a strong orthogonal electric field

The director dynamics for a low molar mass nematic liquid crystal (LC) has been investigated theoretically based on the hydrodynamic theory including the director motion. Analysis of the numerical results for the turn-on process provides an evidence for the appearance of the spatially periodic patterns in 4-n-pentyl-4′-cyanobiphenyl LC film, only in response to the suddenly applied strong electric field orthogonal to the magnetic field. It has been shown that at the values of the voltage in 200V across the 194.7μm LC film and the magnetic field of 7.05T directed at the angle α=1.57(~89.99°) between two fields there is the threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provide the nonuniform rotation mode rather than the uniform one, whereas the lower values both of the amplitude and the angle α (<1.565(~88.81°)) dominate the uniform mode. During the turn-off process, the reorientation of the director to its equilibrium orientation is characterized by the complex destroying of the initially periodic structure to a monodomain state.

Ferroelectric Response and Induced Biaxiality in the Nematic Phase of Bent‐Core Mesogens

AbstractThe still undiscovered fluid ferroelectric nematic phase is expected to exhibit a much faster and easier response to an external electric field compared to conventional ferroelectric smectic liquid crystals; therefore, the discovery of such a phase could open new avenues in electro‐optic device technology. Here, experimental evidence of a ferroelectric response to a switching electric field in a low molar mass nematic liquid crystal is reported and connected with field‐induced biaxiality. The fluid is made of bent‐core polar molecules and is nematic over a range of 120 °C. Combining repolarization current measurements, electro‐optical characterizations, X‐ray diffraction and computer simulations, ferroelectric switching is demonstrated and it is concluded that the response is due to field‐induced reorganization of polar cybotactic groups within the nematic phase. This work represents significant progress toward the realization of ferroelectric fluids that can be aligned at command with a simple electric field.

Finite Element Approximations of the Ericksen–Leslie Model for Nematic Liquid Crystal Flow

The Ericksen-Leslie model describes dynamics of low molar-mass nematic liquid crystals, where the spatiotemporal distribution of defects defining texture is represented by the director unit vector field ${\bf d}: \Omega_T \rightarrow \mathbb{S}^2$. It consists of the Navier-Stokes equations with an extra viscous stress tensor, and a convective harmonic map heat flow equation to govern the dynamics of the director field. Two fully discrete finite element methods, for a regularized system using the Ginzburg-Landau regularization and for the limiting Ericksen-Leslie model, are proposed, and well-posedness and related discrete energy laws are established. For the regularized model, unconditional convergence of finite element solutions towards weak solutions of the continuum model as well as convergence towards measure-valued solutions of the limiting Ericksen-Leslie model are verified when the mesh parameters and the regularization parameter successively tend to zero. Computational experiments are also presented to show the importance of balancing numerical and regularization parameters, to compare regularized and direct approaches, and to show numerically the finite-time formation, annihilation, and evolution of point defects.

Interfacial nematodynamics of heterogeneous curved isotropic-nematic moving fronts

The early stages of liquid crystal phase ordering upon thermal quenches of isotropic phases into unstable and metastable temperature ranges is studied using two-dimensional (2D) computational solutions of the governing Landau-de Gennes (L-dG) equations for low molar mass nematic liquid crystals and analysis based on the corresponding interfacial nematodynamic model. The early phase ordering stage, for both unstable and metastable quenches of the isotropic phase, is shown to lead to highly textured nematic spherulites through a mechanism of interfacial defect nucleation. The underlying mechanisms of interface-driven texturing are elucidated using complementary 2D computational parametric studies of the bulk L-dG equation and analysis of the IN model. It is shown that for highly curved nanodomains and realistic elastic anisotropy, sharp interfacial transitions between uniaxial and biaxial states arise and are resolved by interfacial defect nucleation, which upon subsequent migration into the spherulite's interior leads to strong texturing. This paper shows that texture formation in the early stages of phase ordering is interface driven, and due to low interface tension, elastic anisotropy, and large curvature. Interfacial defect shedding in highly curved, low tension, anisotropic interfaces is a significant defect nucleation mechanism that needs to be taken into account when considering texturing processes.

Orientational Order and Dynamics of Nematic Multipodes Based on Carbosilazane Cores Using Optical and Dielectric Spectroscopy

The birefringence, refractive indices, and dielectric measurements have been carried out for the double- and triple-branched multipodes based on the carbosilazane cores. The orientational order parameters have been obtained using both the optical and the dielectric measurements. The dielectric response is identified to arise from the dipoles of the carboxyl and the alkoxy groups in the ortho position of the benzene ring. Results are analyzed in the framework of the mean-field theory. At least three molecular modes are resolved in the dielectric relaxation spectra in the nematic phase. The lower frequency process is retarded, and the other two processes are accelerated significantly with respect to the relaxation rate in the isotropic phase. The acceleration effect is even much stronger than the retardation effect. The acceleration factors are also found to be much higher than those for the low molar mass nematic liquid crystals. These results show that the anisotropy of the macromolecule plays a significant role in determining the nematic potential which governs the relaxation dynamics. The results on the relaxation dynamics show that the mesogens are decoupled from the core of the macromolecules.

Short wavelength light reflecting films from side‐chain liquid crystal homopolymers with chiral spacers

AbstractA series of acrylate monomers with alkoxy tails of varying lengths are synthesised and polymerised. The butoxy analogue had a stable enantiotropic cholesteric liquid crystalline phase which formed a grandjean texture when prepared as a thin film between glass slides. The polymer was mixed with a low molar mass nematic liquid crystal in various proportions and the pitch of the chiral nematic phases were determined using a cano‐wedge cell technique. The polymer prepared from (S)‐2‐(4‐butoxyphenyl‐4′‐benzoyloxy)‐1‐methyl ethyl acrylate had a pitch length of 113 nm which indicates that the polymer film could be employed in optical devices requiring selective reflection of light with short wavelengths in the region of 170 nm. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Morphology Development and Dynamics of Photopolymerization-Induced Phase Separation in Mixtures of a Nematic Liquid Crystal and Photocuratives

Dynamics of photopolymerization-induced phase separation (PIPS) and morphology development in mixtures of low molar mass nematic liquid crystal (LC) 4-n-heptyl-4‘-cyanobiphenyl (designated K21) and photocuratives (designated NOA65) have been investigated by means of optical microscopy, light scattering, and differential scanning calorimetry. The equilibrium phase diagram of the LC/monomer (uncured NOA65) system was first established theoretically on the basis of the Flory−Huggins/Maier−Saupe theory. The calculated phase diagram displayed isotropic, liquid + liquid, liquid + nematic, and pure nematic coexistence regions in conformity with experiment. The effect of photopolymerization on the phase diagram was examined through comparison of the calculated phase diagrams of LC/linear polymer with LC/cross-linked polymer mixtures. Photopolymerization of the LC/monomer mixtures resulted in phase separation due to instability caused by an increase in molecular weight and subsequent network formation. Photopolymerization initiated in the isotropic phase gave uniformly distributed polygonal-shaped LC droplets in contrast to photopolymerization initiated in the two-phase region. Of particular interest is that the size of the phase-separated LC droplet increases with LC concentration. The network morphology is reminiscent of an interconnected thread seemingly running through the interstices of the polygonal droplets. Temporal evolution of structure factors was analyzed in the context of dynamical scaling law.

Photorefractive Bragg gratings in nematic liquid crystals aligned by a magnetic field

Photorefractive Bragg gratings are observed in low-molar-mass nematic liquid crystals doped with electron donor and acceptor molecules. This is accomplished by alignment of the nematic liquid crystals in a 0.3 T magnetic field, which produces thicker homeotropic aligned samples than traditional surfactant techniques. Grating fringe spacings as low as 3.7 μm are achieved with 176-μm-thick samples, producing grating Q values of 33. Up to this point, low molar mass nematic liquid crystals have exhibited photorefractive gratings with Q⩽1. Asymmetric two-beam coupling and photoconductivity experiments are performed to verify the photorefractive origin of the gratings.

Phase Behavior of Mixtures of Low Molar Mass Nematic Liquid Crystal and in Situ Photo-Cross-Linked Polymer Network

Phase equilibria and mesophase ordering behavior of mixtures of low molar mass nematic liquid crystal (LC) and photo-cross-linked polymeric network have been investigated theoretically and experimentally. The nematic LC used was a single component, viz., 4-n-heptyl-4‘-cyanobiphenyl (K21). The polymer network was formed by photo-cross-linking multifunctional thiolene-based optical adhesive (NOA65). On the basis of a simple addition of the free energy densities of isotropic mixing and nematic ordering along with the elastic free energy density of the network, phase diagrams have been established by solving the total free energy equation self-consistently. The effects of average repeat units between cross-linked points, network functionality, and concentration dependence of network models on the phase diagrams of LC/polymer network were examined. The calculated phase diagrams displayed isotropic network + nematic solvent (I + N), isotropic network + isotropic solvent (I + I), and isotropic swollen network (I...

Phase transitions in mixtures of a side-on-side chain liquid crystalline polymer and low molar mass nematic liquid crystals

Miscibility phase diagrams of mixtures of side-on side chain liquid crystalline polymers (s-SCLCP) and low molar mass liquid crystals (E48 and E44) have been established by means of polarized optical microscopy and light scattering. E48 and E44 are cyanobiphenyl-based eutectic nematic liquid crystal (LC) mixtures with nematic-isotropic transition temperatures of 93 and 105 C, respectively. The phase diagram of the s-SCLCP/E48 system reveals the coexistence of an isotropic nematic region and a single nematic phase in order of descending temperature. The single nematic phase suggests that the pair is miscible in the nematic region. On the other hand, the s-SCLCP/E44 mixture shows liquid liquid and nematic nematic coexistence phases, suggestive of the immiscibility character of the pair. These nematic phase diagrams of the s-SCLCP/E48 and s-SCLCP/E44 have been analysed in the context of the combined Flory-Huggins (FH) free energy for isotropic mixing and the Maier-Saupe (MS) free energy for nematic ordering of the mesogens. This combined FH/MS theory is capable of predicting the observed nematic phase diagrams consisting of liquid liquid, liquid nematic, nematic nematic, and the pure nematic regions. The change of colour accompanying the appearance and disappearance of the inversion walls may be attributed to the temperature dependence of birefringence.

Dector Field Paterns of Nematic Polymer Solutions at Asynchronous Rotation in a Magnetic Field

A variety of different patterns is observed when a magnetic field rotating with an angular frequency ω is applied on a nematic cell with homeotropic boundary conditions. To study the role of backflow in the formation of these dissipative structures, we compared the behaviour of dilute solutions of a mesogenic side group polymethacrylate in a low molar mass nematic liquid crystal (5CB) with that of the pure solvent. The dissolved polymer molecules lead to a strong reduction of the existence range of the so-called VRL-TI (viscosity reduction lattice transverse instability) and complex states. For mass fractions w > 2 % of the polymer these states were found to be suppressed completely due to the efficient reduction of backflow. The growth rate of the viscosity reduction lattices, which can still be observed, is in addition dependent on the polymer concentration. Numerical calculations were performed to explain the behaviour.

Physical properties of mixtures of low molar mass nematic liquid crystals with photochromic fulgide guest dyes

Abstract This paper considers how a low concentration (> 2 per cent w/w) of a photochrome dissolved in a liquid crystal affects the physical properties of the host mesogen. A novel class of photochromes, the fulgide based dyes, have been used to give a new approach in this area of study. Unlike many other light active dyes, the fulgides exhibit only a relatively small shape change. Because of this, the influence of the changes in the physical properties of the guest (for example, the dipole moment) are readily studied. Both thiophene and furan fulgide doped liquid crystal systems were studied in the irradiated and non-irradiated states and as a function of concentration. The changes in phase transition temperature, refractive index and order parameter for these various states as compared to the pure host liquid crystal have been measured to investigate the way in which the dye alters the mesomorphic properties of the host, on a molecular level.

Orientational transition in radial flow of a nematic liquid

Stationary in-plane and out-of-plane numerical solutions to the Leslie-Ericksen equations are obtained for the creeping, radial, pressure-driven out-flow of a low molar mass nematic liquid crystal between concentric, narrowly spaced, parallel disks with planar director surface orientation. The kinematics are described by the Ericksen number, E. At low E, elastic and shear torques keep the director in-plane for all radial distances. At sufficiently high E, elongation is able to orient the director out of the shear plane. The transition is a supercritical bifurcation: the stable stationary in-plane mode bifurcates into two stationary out-of-plane modes. The critical Ericksen number depends on the radial distance from the entrance hole, since elongation weakens more rapidly than the restoring elastic and shear torques as the edge of the cell is approached. The dissipatively equivalent out-of-plane modes are characterized by a net azimuthal secondary flow.