04/09/2004
A biaxial nematic phase had been predicted with D2h symmetry, wherein the mesogen’s long and short transverse axes are simultaneously aligned along the two orthogonal, primary and secondary directors, n and m, respectively. The unique lowangle xray diffraction patterns in the nematic phases exhibited by three rigid bentcore mesogens clearly reveal their biaxiality. The results of xray diffraction can be readily reproduced by ab initio calculations that explicitly include the bentcore shape in the form factor and assume shortrange positional correlations.
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11/19/2012
Molecular shape is an important factor in determining the material properties of thermotropic liquid crystals (LCs). We synthesized and investigated several LC compounds formed by asymmetrically bent molecules with a rigid fourring core in the shape of the letter 'L'. We measured the temperature dependencies of dielectric permittivities, birefringence, splay K1 and bend K3 elastic constants, splay viscosity eta(splay) and flow viscosities eta(parallel to) and eta(perpendicular to). The bendsplay anisotropy delta K31  K3  K1 is negative, similar to the case of nematic LCs formed by symmetrically bent molecules of Vshape. The dielectric anisotropy Delta epsilon and birefringence are positive in the entire nematic range. The splay viscosity eta(splay) and the flow viscosities eta(parallel to) and eta(perpendicular to) are smaller than the viscosities measured for the symmetric Vshaped bentcore materials at similar temperatures. The ratio Gamma = eta(splay)/eta(parallel to,perpendicular to) is in the range 54 that is typical for rodlike LCs. The reported Lshaped bentcore nematic LCs combine the useful features of bentcore LCs (such as a negative delta K31, suitable for formulation of broadrange blue phases) with the relatively low viscosities, a property typical for rodlike LCs and beneficial for electrooptic switching applications.
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01/01/2006
We have developed a fully nonlocal model to describe the dynamic behavior of nematic liquidcrystal elastomers. The free energy, incorporating both elastic and nematic contributions, is a function of the material displacement vector and the orientational order parameter tensor. The free energy cost of spatial variations of these order parameters is taken into account through nonlocal interactions rather than through the use of gradient expansions. We also give an expression for the Rayleigh dissipation function. The equations of motion for displacement and orientational order are obtained from the free energy and the dissipation function by the use of a Lagrangian approach. We examine the free energy and the equations of motion in the limit of longwavelength and smallamplitude variations of the displacement and the orientational order parameter. We compare our results with those in the literature. If the scalar order parameter is held fixed, we recover the usual viscoelastic theory for nematic liquid crystals.
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10/15/1994
Freestanding monodomain thick films of three liquid crystalline compounds were examined by xray scattering. These compounds exhibit a unique enantiotropic crystalBhexaticBsmecticA phase sequence with no evidence of herringbone order. The hexatic order parameter Cg changes in the hexatic phase with critical exponent beta=0.15+/0.03. Temperature dependence of the inplane positional order correlation length suggests a strongly firstorder hexaticB to crystalB phase transition with indiscernible enthalpy change.
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04/15/1995
Two compounds of the 1(4’alkoxyphenylamino3[5’’(2’’menthylpirydil)]propen1one3 (PIRn) series that exhibit the hexaticB (HexB) to smecticA (SmA) and the crystalB (CryB) to HexB phase transitions were studied by high resolution ac calorimetry. The HexB–SmA phase transition was found to be of first order and continuous for PIR7 and PIR9, respectively. The tricritical point was located at the concentration for which TCryB–HexB/THexB–SmA =0.980±0.004, i.e., a hexatic range of 7 K. At both sides of the tricritical point, nonclassical and nonthreedimensionalXY specific heat exponents α were observed. The CryB–HexB phase transition was of first order and was associated with a small or immeasurable Cp anomaly.
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01/24/2013
A twodimensional (2D) solid lacks longrange positional order and is diffusive by means of the cooperative motion of particles. We find from molecular dynamics simulations of hard discs that 2D colloids in solid and hexatic phases show seemingly Fickian but strongly heterogeneous dynamics. Beyond translational relaxation time, the meansquare displacement is linear with time, t, implying that discs would undergo Brownian diffusion and the selfpart of the van Hove correlation function [G(s)(r, t)] might be Gaussian. But dynamics is still heterogeneous and G(s)(r, t) is exponential at large r and oscillatory with multiple peaks at intermediate length. We attribute the existence of several such peaks to the observation that there are several clusters of discs with discretized mobility. The cluster of marginally mobile discs grows with time and begins to percolate around translational relaxation time while clusters of fast discs emerge in the middle of the marginally mobile cluster.
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07/20/2012
Using a range of optical techniques, we have probed the nature of orientational order in a thermotropic bentcore liquid crystal, which features a shapepersistent molecular architecture designed to promote a biaxial nematic phase. In the upper range of the nematic phase (enantiotropic regime), dynamic light scattering reveals strong fluctuations attributable to the biaxial order parameter, in addition to the usual uniaxial director modes. Assuming a Landautype expansion of the orientational free energy, we estimate the correlation length associated with these fluctuations to be similar to 100 nm. At lower temperatures, and mainly in the monotropic regime of the nematic, we observe by optical conoscopy an apparently biaxial texture, which develops when the sample temperature is changed but then relaxes back to a uniaxial state over time scales much longer than observed in the light scattering measurements. A combination of fluorescence confocal polarizing microscopy and coherent antiStokes Raman scattering confirms that the conoscopic texture arises from a flowinduced reorientation of the molecules, associated with a large thermal expansion coefficient of the material, rather than from the spontaneous development of a macroscopic secondary optical axis. We discuss a model to account for the observed behavior at both high and low temperatures based on the temperaturedependent formation of nanoscale, biaxially ordered complexes among the bentcore molecules within a macroscopically uniaxial phase.
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10/15/1995
Phase transitions in finitethickness nematogenic materials bounded by two planar surfaces and characterized by identical surface interactions linearly proportional to the order parameter have been studied theoretically by solving the coupled nonlinear EulerLagrange equations. The surface interaction was assumed to favor molecular orientation in the surface plane with no rubbed or preferred direction. The related problem of a semiinfinite film having a single surface has been studied previously at temperatures above the bulk nematicisotropic phase transition point TNI. For that geometry and physically relevant elastic constants, it was shown that, in addition to the bulk transition, there is a second transition at higher temperatures between biaxial and uniaxial ordering of the surface layer when the strength of the surface coupling is not too weak. It is shown here that this double phase transition reduces to a single one for sufficiently thin layers.
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09/15/2002
We report specificheat measurements for a series of liquid crystals imbedded in a porous cylindrical geometry. Above the nematictoisotropic transition and dependent on nematic width (or chain length), the specific heat shows a small peak. In analogy to known ellipsometry results, the peak is believed to be the signature of a nematic prewetting transition.
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12/27/2010
We propose lyotropic chromonic liquid crystals (LCLCs) as a distinct class of materials for organic electronics. In water, the chromonic molecules stack on top of each other into elongated aggregates that form orientationally ordered phases. The aligned aggregated structure is preserved when the material is deposited onto a substrate and dried. The dried LCLC films show a strongly anisotropic electric conductivity of semiconductor type. The fieldeffect carrier mobility measured along the molecular aggregates in unoptimized films of LCLC V20 is 0.03 cm(2) V1 s(1). Easy processibility, low cost, and high mobility demonstrate the potential of LCLCs for microelectronic applications. (C) 2010 American Institute of Physics.
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