01/21/2002
The critical divergence of the smecticorder correlation lengths parallel and perpendicular to the director, ξ∥,⊥, and the susceptibility, σ0, near the nematictosmecticA phase transition has been measured in a 5 T magnetic field. The high magnetic field reduced the mosaicity and improved the effective transverse resolution by almost 2 orders of magnitude and permitted measurements nearly over four decades of reduced temperature. The results provide important new insights into mosaicity correction. The values of the critical exponents of ξ∥,⊥ and σ0 were ν∥ = 0.79±0.02, ν⊥ = 0.68±0.02, γ = 1.45±0.04, respectively. They become smaller for a narrower nematic range.
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04/01/1998
The nematic (N) to lamellar (Lα) phase transition in binary mixtures of cesiumperfluorooctanoate (CsPFO) and water has been studied by highresolution synchrotron xray scattering at 46.6 weight % CsPFO. The longitudinal correlation length ξ∥ and the susceptibility σ associated with the lamellar phase fluctuations in the N phase, measured over three decades of reduced temperature, diverge with critical exponents ν∥=0.86±0.04 and γ=1.37±0.11, respectively. These results show that the N to Lα phase transition is quantitatively similar to the N to smecticA phase transition of thermotropic liquid crystals with a wider nematic range.
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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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02/24/2011
Simultaneous and direct xray measurements of the smectic layer spacing, molecular tilt, and orientational order in the de Vries smectic A (SmA) and C (SmC) phases of two organosiloxane mesogens reveal that (i) the SmC (tilt) order parameter exponent beta = 0.26 +/ 0.01 for 2nd order SmASmC transitionin excellent agreement with the tricritical behavior, (ii) the siloxane and hydrocarbon parts of the molecules are segregated and oriented parallel to the director with very different degree of orientational order, and (iii) thermal evolution of the effective molecular length is different in the two phases.
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12/01/1994
Smallangle neutron scattering was employed to study the effect of shear flow on the nematic (N) and lamellar (L(alpha)) phases in aqueous solutions of cesium perfluorooctanoate. Shear rates as high as similar to 4000 s(1) were used. The N phase was found to align with the director in the direction of the gradient velocity. The L, phase oriented with lamellae parallel to the shear plane. This change in equilibrium orientation is attributed, primarily, to changes in the value of the Ericksen viscosity parameter at. Subtle shearratedependent director reorientations were also observed in the proximity of the NtoL(alpha) phase transition.
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11/15/2002
Previous studies of critical behavior at the nematic to smecticA transition by highresolution xray scattering were performed using low magnetic fields of 0.10.8 T. In those studies, the transverse resolution was limited by the sample mosaicity which complicated data analysis. In order to understand the effect of sample mosaicity on the measured values of critical exponents, the divergence of the smectic order correlation lengths xi(parallel to,perpendicular to) and susceptibility sigma(o) was studied in a magnetic field ranging from 0.25 to 5 T. The use of high (5 T) field reduced the sample mosaicity and improved the effective transverse resolution by almost two orders of magnitude. Three liquid crystals, two mixtures of 6th and 7th homologs of 4,4(')dialkylazoxybenzene (DnAOB) and 4noctylcyanobiphenyl (8CB) were studied. 15 wt% (D6.15AOB) and 40 wt% (D6.4AOB) mixtures of D7AOB in D6AOB have a wide nematic range, while 8CB has a narrow nematic range. Analysis of the data at different fields revealed a different and proper way to apply the mosaicity correction. The Gaussian mosaicity correction was found to be temperature independent but significantly (similar to3.5 times) smaller than the width of the sharpest q(perpendicular to)scan, which has traditionally been used for mosaicity correction in all previous studies. The values of the critical exponents measured over almost four decades of reduced temperature were: nu(parallel to)=0.79+/0.02, nu(perpendicular to)=0.69+/0.02, gamma=1.46+/0.04 for D6.15AOB; nu(parallel to)=0.79+/0.02, nu(perpendicular to)=0.67+/0.02, gamma=1.44+/0.04 for D6.4AOB; and nu(parallel to)=0.70+/0.02, nu(perpendicular to)=0.52+/0.02, gamma=1.24+/0.04 for 8CB. The results for the two mixtures suggest that in wide temperature range nematics, far from the tricritical point, the exponents may be material independent. No significant effects of mosaicity on the values of the coefficient c of the fourthorder term in the structure factor were observed.
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08/15/1991
The directorfield configuration of a nematic liquid crystal confined to cylindrical cavities of polycarbonate Nuclepore membranes ranging from 0.3 to 0.05mum in radius is determined using deuterium nuclear magnetic resonance (H2 NMR). Spectral patterns from cavities of radius 0.3mum reveal the escapedradial configuration with singular point defects, but as the cylinder size is decreased, the elastic energy imposed by the curvature of the confining walls competes with the anchoring energy to tilt the directors away from their preferred perpendicular anchoring direction, preventing the expected transition to the planarradial configuration. A surface fitting parameter is directly determined by simulating H2NMR line shapes, and by studying a series of samples with different radii, the molecularanchoring strength W0 and surface elastic constant K24 are extracted.
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11/15/1997
Reentrant nematic phases and smectic antiphases occur as intermediate states between two onedimensionally ordered frustrated smectic phases, characterized by two competing and, in general, incommensurate smectic density waves. Using highresolution xray diffraction, we have examined the evolution of the two density waves as a function of temperature in all phases of a mixture of polar liquid crystals octyland decyloxyphenyl nitrobenzoyloxy benzoate (47.4 mol% DB8ONO2+52.6% DB10ONO2). This mixture exhibits at temperatures below 127 degrees C the liquidcrystal phase sequence: smecticA(d), reentrant nematic phases Nd and N1, smecticA(1), tilted antiphase (C) over tilde, smecticA(2), and smecticC2 phases. Detailed investigations of several orderdisorder transitions and the evolution of fluctuation effects in this system are reported.
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11/28/2011
A method of achieving homeotropic alignment of liquid crystals (LCs) by ultraviolet light induced phase separation of LC from its mixture with photocurable prepolymer is reported. Vertical polymer columns and microfibrils developed during the phase separation promote homeotropic alignment of the LC director (i.e., alignment perpendicular to the LCsubstrate interface), suitable for devices based on LCs possessing negative dielectric anisotropy. These vertical structures extend between two substrates and permit the fabrication of highly flexible electrooptical devices with high contrast coupled with fast response times. This simple singlestep technique eliminates the need for the traditional polymer alignment layer predeposited on substrates. (C) 2011 American Institute of Physics. [doi:10.1063/1.3663966]
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07/15/2008
Recent experiments report that the longlookedfor thermotropic biaxial nematic phase has been finally detected in some thermotropic liquid crystalline systems. Inspired by these experimental observations, we concentrate on some elementary theoretical issues concerned with the classical sixthorder Landaude Gennes free energy expansion in terms of the symmetric and traceless tensor order parameter Q(alpha beta). In particular, we fully explore the stability of the biaxial nematic phase giving analytical solutions for all distinct classes of the phase diagrams that theory allows. This includes diagrams with triple, critical, and tricritical points and with multiple (reentrant) biaxial and uniaxial phase transitions. A brief comparison with predictions of existing molecular theories is also given.
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