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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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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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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04/15/2002
Xray diffraction from Anodisc membrane filters is measured and analyzed. The patterns are consistent with pores of constant diameter and a Gaussian distribution of porepore spacing. The mean distance between pores, measured using x rays is 0.37 mum compared to 0.32 mum calculated from the nominal density of pores/cm(2). The results may be modeled both in terms of the convolution of a structure factor of the pores with the resolution function, and by modeling the source as a collection of incoherent sources with an ideal monochromator crystal. The incoherent source analysis provides an explanation for the common observation that the resolution function in many xray spectrometers is better fit to a sum of Lorentzians, rather than a single Lorentzian.
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05/05/1998
The synthesis of a new series of liquid crystalline polyesters with bilaterally linked mesogens is described. The investigation of their mesomorphic behaviors by differential scanning calorimetry, Xray diffraction, and polarization microscopy is presented. These new polymers exhibit only the nematic phase. Xray diffraction studies reveal the existence of biaxial fluctuation in the nematic phase in two homologues of the series.
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03/01/2004
A biaxial nematic phase with D2h symmetry has the mesogen's long and short transverse axes simultaneously aligned along the two orthogonal directors, n and m, respectively. Lowangle xray diffraction measurements in the nematic phase exhibited by three rigid bentcore mesogens clearly reveal its biaxiality. The diffraction results can be readily reproduced by ab initio calculations of molecular form factor assuming shortrange positional correlations in the nematic phase with the apex of the core aligned along m.
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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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08/01/2003
The biaxial nematic liquid crystalline phase was predicted several decades ago. Several vigorous attempts to find it in various systems resulted in misidentifications. The results of Xray diffraction and optical texture studies of the phases exhibited by rigid bentcore molecules derived from 2,5bis(phydroxyphenyl)l,3,4oxadiazole reveal that the biaxial nematic phase is formed by three compounds of this type. Xray diffraction studies reveal that the nematic phase of these compounds has the achiral symmetry D2h, in which the overall long axes of the molecules are oriented parallel to each other to define the major axis of the biaxial phase. The apex of the bentcores defines the minor axis of this phase along which the planes containing the bentcores of neighboring molecules are oriented parallel to each other.
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01/01/2003
The nematic to smecticA (NA) phase transition would appear to be the simplest example of a freezing transition in nature. However, it has proven to be one of the most challenging problems of liquid crystal science and equilibrium statistical mechanics. One quantitative method to study this transition, that yields high quality data of almost transparent interpretation, has been Xray measurements of the divergence of smectic order correlations as the SmA phase is approached from the nematic side. However, even data from these experiments have proved difficult to interpret because of the imperfect magnetic alignment or “mosaic spread” of the nematic phase. This mosaic spread limits the experimental transverse resolution and hence affects the data very close to the transition. Previous studies employed magnetic fields of ∼0.1–0.8 T. The use of a high (5 T) field, in the work we recently completed, improved the effective transverse resolution by almost two orders of magnitude allowing us to obtain results essentially free of mosaic spread. The generally used technique for removing this effect has now been tested. The high field results shed new light on this transition and reveal a new and proper way to apply the mosaicity correction.
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