10/11/2011
Several experiments have reported that ferroelectric nanoparticles have drastic effects on nematic liquid crystalsincreasing the isotropicnematic transition temperature by about 5 K, and greatly increasing the sensitivity to applied electric fields. In a recent paper [Lopatina and Selinger, Phys. Rev. Lett. 102, 197802 (2009)], we modeled these effects through a Landau theory, based on coupled orientational order parameters for the liquid crystal and the nanoparticles. This model has one important limitation: Like all Landau theories, it involves an expansion of the free energy in powers of the order parameters, and hence it overestimates the order parameters that occur in the lowtemperature phase. For that reason, we now develop a new MaierSaupetype model, which explicitly shows the lowtemperature saturation of the order parameters. This model reduces to the Landau theory in the limit of high temperature or weak coupling, but shows different behavior in the opposite limit. We compare these calculations with experimental results on ferroelectric nanoparticles in liquid crystals.
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06/01/2007
The distance of closest approach of hard particles is a key parameter of their interaction and plays an important role in the resulting phase behavior. For nonspherical particles, the distance of closest approach depends on orientation, and its calculation is surprisingly difficult. Although overlap criteria have been developed for use in computer simulations [ VieillardBaron J. Chem. Phys. 56 4729 (1972); Perram and Wertheim J. Comput. Phys. 58 409 (1985)], no analytic solutions have been obtained for the distance of closest approach of ellipsoids in three dimensions, or, until now, for ellipses in two dimensions. We have derived an analytic expression for the distance of closest approach of the centers of two arbitrary hard ellipses as a function of their orientation relative to the line joining their centers. We describe our method for solving this problem, illustrate our result, and discuss its usefulness in modeling and simulating systems of anisometric particles such as liquid crystals.
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06/01/2007
The distance of closest approach of hard particles is a key parameter of their interaction and plays an important role in the resulting phase behavior. For nonspherical particles, the distance of closest approach depends on orientation, and its calculation is surprisingly difficult. Although overlap criteria have been developed for use in computer simulations [ VieillardBaron J. Chem. Phys. 56 4729 (1972); Perram and Wertheim J. Comput. Phys. 58 409 (1985)], no analytic solutions have been obtained for the distance of closest approach of ellipsoids in three dimensions, or, until now, for ellipses in two dimensions. We have derived an analytic expression for the distance of closest approach of the centers of two arbitrary hard ellipses as a function of their orientation relative to the line joining their centers. We describe our method for solving this problem, illustrate our result, and discuss its usefulness in modeling and simulating systems of anisometric particles such as liquid crystals.
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03/10/2011
Photoisomerizationinduced phase transition of neat liquidcrystalline azobenzene chromophore (LCAC) and its effect on phase diagrams of its mixtures with reactive mesogenic diacrylate monomer (RM257) have been investigated experimentally and theoretically. Upon irradiation with ultraviolet light, the nematic phase of LCAC transformed to isotropic, while the crystal phase showed corrugated textures on the surface (i.e., ripples). The phasetransition temperatures and corresponding morphologies of the blends have been investigated by means of differential scanning calorimetry and optical microscopy. A theoretical phase diagram of a binary nematic and crystalline system was constructed by selfconsistently solving the combined free energies of FloryHuggins, MaierSaupe, and phasefield theory. The calculation revealed various coexistence regions such as nematic + liquid (N1 + L2), crystal + liquid (Cr1 + L2), crystal + nematic (Cr1 + N2), and crystal + crystal (Cr1 + Cr2) over a broad range of compositions including the singlephase nematic (N1, N2) of the corresponding constituents. The calculated liquidus lines were in good accord with the depressed mesophaseisotropic transition points. The present paper demonstrates the effect of transcis photoisomerization on the mesophase transitions of neat LCAC and the phase diagram of LCACRM257 as well as on the ripple formation (i.e., periodic undulation) on the azobenzene crystals.
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06/18/2012
Deuteron nuclear magnetic resonance is used to study the phase segregation behavior of photoisomerizable liquid crystal diheptylazobenzene (7AB) confined into cylindrical pores of Anopore membranes. It is demonstrated that the concentration of both components in a binary trans7AB and cis7AB mixture can be controlled dynamically using UVillumination stimulated transtocisphotoisomerization and thermally induced cistotrans backisomerization. The so far elusive temperatureconcentration phase diagram of such system is determined by comparative analysis of the behavior in bulk, thinplanar, and Anoporeconfining geometry.
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07/10/1993
Three different incommensurate phases predicted by the phenomenological theory of frustrated smectics were reportedly found in mixtures of DB8OCN and 8OBCAB and DB7OCN and 8OCB. The results of our highresolution Xray diffraction study of these materials show that these phases are, indeed, coexistences of two or more smecticA phases. The correct phase diagrams of the two systems are found to be in excellent agreement with the phenomenological models. At this time, there remain no known materials that exhibit an incommensurate smecticA phase.
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01/01/2006
A homologous series of shapepersistent Vshaped molecules has been designed to form the biaxial nematic phase. Phenyleneethynylene moieties are attached to a bent fluorenone unit to create an apex angle of about 90°, which is determined from the single crystal structure. Two mesogens, one symmetric and another unsymmetric, have been synthesized by attaching a cyano group to one or both of the peripheral phenyl units, respectively. These groups introduce local dipoles essential for the formation of the nematic phases. The tendency to form a crystalline phase is reduced by laterally substituted hexyloxy chains which allow the nematic phase to be supercooled to a glassy state. Two of the three fluorenone derivatives exhibit a transition from the uniaxial nematic to the biaxial nematic phase. This transition has an undetectably small transition enthalpy, but the Xray diffraction, polarizing optical microscopy, and conoscopy reveal the presence of the biaxial order in the low temperature nematic phase.
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07/08/2010
Polarized Raman spectroscopy was used to investigate the development of orientational order and the degree of phase biaxiality in a bentcore mesogenic system. The values of the uniaxial order parameters ⟨P200⟩ and ⟨P400⟩, and biaxial order parameters ⟨P220⟩, ⟨P420⟩, and ⟨P440⟩, and their evolution with temperature were determined. The temperature dependence of almost all order parameters reveals a second order transition from the uniaxial to biaxial nematic phase with ⟨P220⟩ increasing to ∼0.22 before a first order transition to the smecticC phase, upon cooling.
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10/15/2006
The combined effects of the tendency of cholesterol to order lipids in the liquid phase and the coupling between lipids in the two leaves of a bilayer are investigated theoretically utilizing a Landau free energy. We show that as a consequence of these combined effects, lateral phase separation in the outer leaf between cholesterolrich and poor liquids causes a similar, but weaker, phase separation in the inner leaf. Just as the areal density of lipids in the outer leaf increases in the cholesterolrich regions, so the areal density of lipids also increases in the inner leaf. Thus, the areal density in the inner leaf varies spatially, reflecting spatial variations of the areal density in the outer leaf. This provides a mechanism for proteins attached to the inner leaf via a hydrocarbon tether to respond to variations in the composition of the outer leaf. We also note that the effect of coupling between the leaves should be observable in artificial bilayers.
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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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