Anchoring properties of a nematic liquid crystal on anisotropic hydroxypropylcellulose films

Thin solid films of hydroxypropylcellulose (∼15-30 µm) prepared from liquid crystalline and isotropic aqueous solutions are used as liquid crystal alignment layers. Using the standard nematic liquid crystal 5CB we measured the interface properties of these solid films as a function of the polymer concentration in the aqueous precursor solution, expressed in terms of zenithal and azimuthal anchoring orientations and extrapolation lengths. The hydroxypropylcellulose thin films are found to induce a planar orientation of 5CB independently of the polymer concentration, with the alignment along the polymer backbone. The zenithal anchoring strength is found to be strong and essentially independent of the temperature far from the nematic-isotropic transition, with an extrapolation length ξ_θ≈50 nm. The zenithal anchoring becomes weaker near the nematic-isotropic transition, as expected. The azimuthal anchoring strength is found to be intermediately weak and strongly dependent on the polymer concentration, with an extrapolation length varying from ξ_θ≈250 nm to ξ_ϕ≈500 nm. These films are particularly interesting since their surface topography and morphology may be tuned by varying a few parameters in the film preparation process, such as the polymer concentration in the aqueous solution.     

DNA hybridization-induced reorientation of liquid crystal anchoring at the nematic liquid crystal/aqueous interface

Interactions between DNA and an adsorbed cationic surfactant at the nematic liquid crystal (LC)/aqueous interface were investigated using polarized and fluorescence microscopy. The adsorption of octadecyltrimethylammonium bromide (OTAB) surfactant to the LC/aqueous interface resulted in homeotropic (untilted) LC alignment. Subsequent adsorption of single-stranded DNA (ssDNA) to the surfactant-laden interface modified the interfacial structure, resulting in a reorientation of the LC from homeotropic alignment to an intermediate tilt angle. Exposure of the ssDNA/OTAB interfacial complex to its ssDNA complement induced a second change in the interfacial structure characterized by the nucleation, growth, and coalescence of lateral regions that induced homeotropic LC alignment. Fluorescence microscopy showed explicitly that the complement was colocalized in the same regions as the homeotropic domains. Exposure to noncomplementary ssDNA caused no such response, suggesting that the homeotropic regions were due to DNA hybridization. This hybridization occurred in the vicinity of the interface despite the fact that the conditions in bulk solution were such that hybridization did not occur (high stringency), suggesting that the presence of the cationic surfactant neutralized electrostatic repulsion and allowed for hydrogen bonding between DNA complements. This system has potential for label-less and portable DNA detection. Indeed, LC response to ssDNA target was detected with a lower limit of approximately 50 fmol of complement and was sufficiently selective to differentiate a one-base-pair mismatch in a 16-mer target.     

Determination of solvatochromic solvent polarity parameters for the characterisation of some nematic liquid crystal in anisotropic and isotropic phases

Characterisation of liquid crystals (LCs) as solvents is needed, to obtain the polarity and solvatochromic polarity parameters of these media. Polarity parameters demonstrate the effects of LC media on the photo-physical behaviour of solute molecules in an anisotropic medium. The practical limitations in determining solvent polarity scale parameters for LCs can overcome the overlapping absorption band of LCs and solvent-sensitive standard compounds or their insolubility in LCs. In this work, we report Kamlet–Abboud–Taft polarity functions of some nematic LCs in different temperatures and phases, isotropic and anisotropic, with the solvatochromic method, using the Reichardt's dye and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-phenolate standard probe. In addition, a new azo and coumarin dye were used as probes to obtain some solvatochromic polarity parameters. Finally, a new polarity parameter, the LC anisotropic matrix, is introduced.     

Non-isothermal kinetics of melting and nematic to isotropic phase transitions of 5CB liquid crystal

This work reports a non-isothermal kinetics of the melting and the nematic to isotropic (N–I) phase transitions of the pentylcyanobiphenyl (5CB) liquid crystal compared with octylcyanobiphenyl (8CB) liquid crystal using calorimetric technique. Temperature scans and heating rate scans were performed for 5CB and 8CB from 280 to 333 K at various rates using differential scanning calorimetry from 0.5 to 20 K min⁻¹. Double activation was observed for 5CB for two heating rate regimes whereas 8CB indicated single activation only. The 5CB has smaller enthalpy and entropy of the transitions and needs larger activation than 8CB. This kinetic change can be explained in terms of the length scale and mobility of the liquid crystal molecules.     

Diffusion of brownian particles in the isotropic phase of a nematic liquid crystal

Abstract

We report a light scattering study of the translational diffusion of a suspension of silica spheres in the liquid crystal, 4-n-pentyl-4'-cyanobiphenyl. We observe a small but significant increase of the effective hydrodynamic radius of the colloidal particles as the transition to the nematic phase is approached. This effect can be understood in terms of orientational pre-wetting of the silica spheres.     

Diffusion of brownian particles in the isotropic phase of a nematic liquid crystal

We report a light scattering study of the translational diffusion of a suspension of silica spheres in the liquid crystal, 4-n-pentyl-4'-cyanobiphenyl. We observe a small but significant increase of the effective hydrodynamic radius of the colloidal particles as the transition to the nematic phase is approached. This effect can be understood in terms of orientational pre-wetting of the silica spheres.     

Dynamics of dissipative structures and the transition to turbulence in a nematic liquid crystal

It has been observed for the first time that the domain oscillations of a nematic liquid crystal subject to a transverse DC electric field are self-organized in space as concentric and spiral phase waves. The formation of the phase wave sources has been described. The transition of the hexagonal convective structure to turbulence has been investigated by the temporal Fourier analysis method. It has been found that a route to turbulence passes through a continuous widening of peaks in the power spectrum of light intensity transmitted through the cell.     

Interfacial tension of a nematic liquid crystal/water interface with homeotropic surface alignment

Pendant drop experimental results are presented for the temperature dependence of the interfacial tension between water and the immiscible nematic liquid crystal 4'-pentyl-4-biphenylcarbonitrile (5CB) in the presence of the adsorbed surfactant cetyltrimethylammonium bromide (CTAB). Adsorption of the surfactant lowers the interfacial tension value and is also known from earlier work to induce a transition in liquid crystal surface alignment from planar to homeotropic [Brake et al. Langmuir 2003, 19, 6436.]. Discrepancies exist in the literature regarding the density of 5CB, and the density difference between 5CB and water in any case is very small. However, from the ability to form pendant 5CB drops, one may infer that the density of 5CB exceeds that of water over the entire temperature range studied (28-41 degrees C), in disagreement with the predictions of one earlier report on 5CB. The interfacial tension is shown to exhibit a relative maximum near the bulk 5CB nematic-isotropic transition temperature T(NI), regardless of which published data set of 5CB density values is used to analyze the measurements, with a possible discontinuity in tension occurring at T(NI). The anomalous shape of the interfacial tension curve, depending on the choice of the 5CB density data set, may be quite similar to that recently reported for the interface between 5CB and a hydrophobic, isotropic molten polymer (Rai et al. Langmuir 2003, 19, 7370).     

Ordering of a monotropic nematic liquid crystal in dielectric medium at phase transition temperature: A computational analysis

Ordering in a monotropic nematic liquid crystal, 4’-n-butyl-4-cyanobiphenyl, was examined by the methods of quantum mechanics and intermolecular forces. The atomic charges and dipole moment at each atomic center were evaluated by the CNDO/2 method. The configurational energy was computed using the Rayleigh-Schrödinger perturbation method. The total interaction energies obtained by these computations were used to calculate the probability of each configuration in a dielectric medium (noninteracting and nonmessogenic solvent, benzene) at the phase transition temperature using the Maxwell-Boltzmann formula. In the dielectric medium, the energies/probabilities are redistributed, and there is a considerable rise in the probability of interactions, although the order of preference remains the same. An attempt was made to develop a new and interesting model of nematogen in a dielectric medium. A theoretical support is offered to the experimental observations.     

Computer simulation studies of anisotropic systems XXIX. Quadrupolar Gay-Berne discs and chemically induced liquid crystal phases

Liquid crystal phases can be induced chemically by mixing compounds whose specific interactions are such that the transition temperature for the induced phase is higher than the melting points of the two compounds. A particularly dramatic example of such behaviour is the creation of a columnar nematic and a hexagonal columnar phase on mixing discotic multiynes with 2,4,7-trinitrofluorenone. Although the intense colour of the mixture indicates a strong charge-transfer band, it is uncertain as to whether the charge-transfer interaction between unlike molecules is enough to stabilize the induced liquid crystal phases. An alternative explanation for the formation of such phases involves an electrostatic quadrupolar interaction between the components,whose quadrupole moments differ in sign. This interaction weakens the face-to-face attraction for like particles while strengthening it for unlike particles. We have explored this possible explanation for chemically induced liquid crystal phases in discotic systems by modelling the basic interaction between discs with a Gay-Berne potential, to which is added a point quadrupolar interaction. We have determined the phase behaviour of the pure systems and their binary mixtures with constant pressure Monte Carlo simulations. It would seem that the quadrupolar interaction can account for many of the features of chemically induced liquid crystals.     

An estimation of the interfacial tension between the nematic and isotropic states of a liquid crystal

A method is described for the determination of the interfacial tension between two liquids. It is based on the evaluation of the oscillations performed     

Study of the isotropic to smectic-A phase transition in liquid crystal and acetone binary mixtures

The first-order transition from the isotropic (I) to smectic-A (Sm?A) phase in the liquid crystal 4-cyano-4(')-decylbiphenyl (10CB) doped with the polar solvent acetone (ace) has been studied as a function of solvent concentration by high-resolution ac-calorimetry. Heating and cooling scans were performed for miscible 10CB+ace samples having acetone mole fractions from x(ace)=0.05 (1 wt?%) to 0.36 (10%) over a wide temperature range from 310 to 327 K. Two distinct first-order phase transition features are observed in the mixture whereas there is only one transition (I-Sm?A) in the pure 10CB for that particular temperature range. Both calorimetric features reproduce on repeated heating and cooling scans and evolve with increasing x(ace) with the high-temperature feature relatively stable in temperature but reduced in size while the low-temperature feature shifts dramatically to lower temperature and exhibits increased dispersion. The coexistence region increases for the low-temperature feature but remains fairly constant for the high-temperature feature as a function of x(ace). Polarizing optical microscopy supports the identification of a smectic phase below the high-temperature heat capacity signature indicating that the low-temperature feature represents an injected smectic-smectic phase transition. These effects may be the consequence of screening the intermolecular potential of the liquid crystals by the solvent that stabilizes a weak smectic phase intermediate of the isotropic and pure smectic-A.     

Studies on the electro-optical properties of chiral nematic liquid crystal/aerosil particle composites

Chiral nematic liquid crystal (N*-LC)/aerosil particle (AP) composites were prepared. After the composite was sandwiched between ITO glass substrates, the inner surfaces of which had been treated for homogeneous orientation of LC molecules, the LC molecules tended to be aligned with planar texture. Upon the application of an electric filed, a focal conic texture with memory effect was induced and the composite exhibited light scattering. An electric field-induced homeotropic state was obtained after the application of a high electric field. The effects of the content of the APs and the pitch length of the N*-LC on the transmittances of the initial state and the focal conic texture, the driven voltage and the memory effect were investigated.     

Optical response of a nematic liquid crystal cell at the splay - bend transition: a model and dynamic simulation

We study the dynamical optical response of a nematic liquid crystal cell that undergoes the splay-bend transition after applying a voltage across the cell. We formulate a simplified model that takes into account both the flexoelectric coupling and the surface rotational viscosity. The dynamic equations of the model are solved numerically to calculate the temporal evolution of the director profile and of the transmittance. We evaluate the response time as a function of a number of parameters, such as dielectric and elastic anisotropies, asymmetry of the surface pretilt angles, anchoring energy, surface rotational viscosity and flexoelectricity.     

Alternative dynamics for the symmetric splay to bend transition in a nematic liquid crystal layer

A pathway for a coherent topological transition from the splay to bend director configuration in liquid crystal layers has been previously described via order reconstruction in a two-dimensional sheet in the plane of the liquid crystal layer. In this paper we provide the details of the modelling of an alternative pathway that is based on the nucleation, motion and annihilation of an array of disclination lines. Electrical current flow onto the cell is calculated based on the two pathways and compared with data.     

Tuning the physical properties of a nematic liquid crystal elastomer actuator

In this report we demonstrate the ability to tune the physical properties of a liquid crystal elastomer (LCE) by varying the amount and type of crosslinking within the elastomer network. LCE films composed of a single mesogenic compound were capable of uniaxial contraction when thermally actuated through the nematic to isotropic phase of the material. We probed the physical properties of the LCE films while varying the amount and concentration of two crosslinking agents and measured actuation strains of 10-35%, elastic moduli of 3-14 MPa, and transition temperatures ranging between 75 and 60°C. The viscous losses of the elastomers and the estimated work capable of being produced by the films were also evaluated. The ability to tune the physical properties of the LCE films allows for a wide range of applications including robotics, microelectromechanical systems (MEMS), shape-changing membranes, and/or microfluidics.     

Tuning the physical properties of a nematic liquid crystal elastomer actuator

In this report we demonstrate the ability to tune the physical properties of a liquid crystal elastomer (LCE) by varying the amount and type of crosslinking within the elastomer network. LCE films composed of a single mesogenic compound were capable of uniaxial contraction when thermally actuated through the nematic to isotropic phase of the material. We probed the physical properties of the LCE films while varying the amount and concentration of two crosslinking agents and measured actuation strains of 10–35%, elastic moduli of 3–14 MPa, and transition temperatures ranging between 75 and 60°C. The viscous losses of the elastomers and the estimated work capable of being produced by the films were also evaluated. The ability to tune the physical properties of the LCE films allows for a wide range of applications including robotics, microelectromechanical systems (MEMS), shape‐changing membranes, and/or microfluidics.     

Nanostructure and dielectric properties of a twist-bend nematic liquid crystal mixture

We present freeze-fracture transmission electron microscopy (FF-TEM), dielectric spectroscopy and electro-optic measurements on a dimeric liquid crystal mixture, which previously was proposed to form the twist-bend nematic (N_tb) phase. Our FF-TEM studies provide a direct image of a 10.5 nm periodic structure, consistent with the expected nanoscale, heliconical twist-bend modulation of the molecular orientation. Dielectric measurements in the 100 Hz to 10 MHz range reveal three nearly Debye-type dispersion processes in the nematic and the twist-bend phase. Low frequency 8 V/µm electric fields applied on planar cells cause the optical-scale stripe texture (another characteristic feature of the N_tb phase) to disappear. Higher (>16 V/µm) fields gradually realign the heliconical axis along the electric field; it relaxes back after the field removal.