Role of Molecular Structure on X-ray Diffraction in Uniaxial and Biaxial Phases of Thermotropic Liquid Crystals † (original) (raw)
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Physical Review A, 1991
We have used x-ray specular reflection to study the structure of the airliquid-crystal interface of the lyotropic liquid crystal formed from binary mixtures of cesium perfluoro-octanoate (CsPFO) and water. In the isotropic phase the surface is coated with a monolayer of CsPFO separated by layers of water from one or more smectic bilayers of CsPFO. As for the case of thermotropic liquid crystals, the isotropic-to-nematic phase transition has no effect on the surface structure, and as the temperature is lowered towards the nematictosmectic-A transition temperature, the number of surface-induced smectic layers increases dramatically (e.g. , approximately 100 layers were observed). Theoretical modeling of the reflectivity excludes the possibility that the surface bilayers are arrays of micelles. X-ray scattering from critical smectic short-range order in the bulk, studied by tuning the spectrometer away from the spec'ular condition, indicates that the scattering is fundamentally different from short-range smectic order in thermotropic systems.
X-ray studies of the phases and phase transitions of liquid crystals
Acta Crystallographica Section A Foundations of Crystallography, 2005
A short review is given of recent X-ray diffraction studies of the phases and phase transitions of thermotropic liquid crystals. The areas covered are twistgrain-boundary phases, antiferroelectric phases studied with resonant X-ray diffraction and smectic phases within gel structures. In all areas, X-ray diffraction has played a key role. Nonetheless, open questions remain: the nature of the smectic C variant of the twist-grain-boundary phase, the origin of antiferroelectric phases, and whether novel Bragg glass states exist for smectic A gel samples.
Recent Developments in Biaxial Liquid Crystals:. AN NMR Perspective
International Journal of Modern Physics B, 2010
A survey of recent studies of biaxial liquid crystals (LCs), whose nematic and/or smectic-A phases do not possess optical uniaxiality (viz., more than one optical axis exists), is given in this review. In particular, we emphasize on how Nuclear Magnetic Resonance (NMR) spectroscopy can help to advance the understanding of phase biaxiality in general, and to examine recent debates on the existence of biaxial nematic phase reported in low molecular mass bent-core or V-shaped mesogens. A general discussion of orientational order parameters is detailed, particularly in smectic-C (SmC) and biaxial nematic phases. How these orientational order parameters can be determined by various techniques such as NMR, IR absorbance and Raman scattering studies, will be mentioned. Recent X-ray observations of SmC clusters in the nematic phase of V-shaped mesogens are highlighted and contrasted with probable theory. Moreover, deuterium and carbon-13 NMR techniques are briefly reviewed, and their possible utilization to identify phase biaxiality in these biaxial LC systems is explored.
Liquid Crystals, 2012
Polarising optical microscopy (POM) shows evidence for a reversible uniaxial nematic (N U ) to biaxial nematic (N B ) phase transition in a quiescent melt of bent-core mesogens (BCM). A second superimposed Schlieren texture, attributed to a second director field, grows into the N U phase on lowering the temperature and disappears again on heating, indicating a fully reversible N B to N U phase transition. This POM result together with recent X-ray data provides a new signature for the N B phase in this class of BCMs, and offers a new method for assessment of new biaxial nematic phases.
Physical review. E, 2016
The temperature dependence of the orientational order parameters 〈P_{2}(cosβ)〉 and 〈P_{4}(cosβ)〉 in the nematic (N) and twist-bend nematic (N_{tb}) phases of the liquid crystal dimer CB7CB have been measured using x-ray and polarized Raman scattering. The 〈P_{2}(cosβ)〉 obtained from both techniques are the same, while 〈P_{4}(cosβ)〉, determined by Raman scattering is, as expected, systematically larger than its x-ray value. Both order parameters increase in the N phase with decreasing temperature, drop across the N-N_{tb} transition, and continue to decrease. In the N_{tb} phase, the x-ray value of 〈P_{4}(cosβ)〉 eventually becomes negative, providing a direct and independent confirmation of a conical molecular orientational distribution. The heliconical tilt angle α, determined from orientational distribution functions in the N_{tb} phase, increases to ∼24^{∘} at ∼15 K below the transition. In the N_{tb} phase, α(T)∝(T^{*}-T)^{λ}, with λ=0.19±0.03. The transition supercools by 1.7 K,...
Liquid Crystals, 2010
The observation and quantification of the biaxiality in the nematic phase of thermotropic liquid crystals is one of the topical problems in the science of liquid crystals. The biaxiality in the refractive index and/or relative permittivity, both constituting tensors of the second rank, are expressed in terms of the four scalar order parameters S, D, P and C and the relevant molecular quantities. In this paper we review the theory of determining these order parameters using polarised infrared spectroscopy and present the method that we have developed in obtaining results for the order parameters of tetrapodes (with symmetrical and asymmetrical mesogens) and tripodes with symmetrical mesogens. In the tetrapodes, four mesogens are linked through the siloxane chains to the central Si atom. In the tripode, three mesogens are linked to a benzene ring via the oxygen atom through the siloxane spacers. In the first case a platelet-like structure is formed where the major director corresponds to the highest refractive index. A disc-like structure is formed for a tetrapode with symmetrical mesogens and for a tripode. The order parameters are calculated in the framework of the quasi-flat platelets. For the discotic-like structure, the major director corresponds to the lowest of the three refractive indices. For the three cases, the compounds are shown to exhibit a biaxial nematic phase. Biaxiality in the refractive indices is expressed in terms of the biaxial order parameters. A comparison of the results on the IN U and N U-N B transitions for tetrapodes compare favourably with the predictions from the mean field models of de Gennes, Virga and his co-workers.
Quasi-binary picture of thermotropic liquid crystals and its application to cubic mesophases
Chemical Physics Letters, 2002
A quasi-binary (QB) picture of thermotropic liquid crystals is proposed on the basis of thermodynamic observations. The experimental conformational entropy of long alkyl chains attached to a (semi)rigid core of mesogenic molecules indicates that the chain is highly disordered in liquid crystalline states. These disordered chains serve as Ôintramolecular solventÕ or Ôself-solventÕ judging from a close resemblance between phase diagrams of neat (against chain length) and binary (against composition) systems. The application of the QB picture to the classic examples of thermotropic cubic mesophases (in ANBC series) shows that the essential structural motif is triply periodic minimal surface.