Structural Studies of Liquid Crystals by X‐Ray Diffraction

Abstract

This section is concerned with how structural information about liquid crystal phases may be obtained from X-ray diffraction studies. In addition to presenting a brief overview of some of the more quantitative aspects, we will also attempt to give a simple pictorial view of how diffraction patterns may be analyzed qualitatively in terms of the mesophase structures. For the background X-ray diffraction theory relevant to partially-ordered systems, a number of classical texts should be consulted [l-31.Here we will focus on the bulk static structures of the phases formed by low molar mass calamitic (rod-like) mesogens. The principles involved in structural studies of columnar phases, and of polymeric and lyotropic systems, are very similar. The study of liquid crystal surfaces requires reflectivity and/or glancing angle diffraction techniques [4 -61.A number of reviews describe X-ray studies of liquid crystals up to about 1980 [7-101. A monograph by P. S. Pershan [ l l ] reviews X-ray work up to 1987, and reprints many of the most significant papers in the field. A number of more recent texts cover various structural aspects of liquid crystal phases [ 12-141. Other reviews describe the structural classification of liquid crystals [ 151, the nature of the ordering within smectic phases [ 16-181, the structures of ferroelectric and chiral smectic phases [ 191, the structures of frustrated smectics [20 -241, the structures of columnar discotic liquid crystals [25], and X-ray studies of side group liquid crystalline polymers [26].Orientational order in liquid crystals is most effectively studied using 'molecular probe' techniques such as NMR spectroscopy. However, the study of translational order, that is structure, requires diffraction techniques, usually involving X-rays or neutrons. A disadvantage of diffraction is that it does not distinguish between static and dynamic disorder. However, this can be achieved using quasielastic neutron scattering.Diffraction is a phenomenon which comes about when radiation is elastically scattered from atoms in a sample (primarily from electrons for X-ray diffraction). The various scattered wavelets from the different atomic sites combine, undergoing constructive or destructive interference, depending on the relative phases of the differ-

Lanthanide(III) Dodecanoates: Structure, Thermal Behaviour, and Ion-Size Effects on the Mesomorphism

“…For a smectic C phase, an increase in layer spacing would be expected with increasing temperature. [26] The presence of an SmA phase is further supported by the homeotropically aligned regions in the optical texture observed by hot-stage polarized optical microscopy. After the clearing point, only one weak broad band is observed in the X-ray diffractograms.…”

Facile synthesis and mesomorphic properties of 4-hydroxybutyl 4-(4-alkoxybenzoyloxy) benzoate mesogens

“…On the WAXD curve measured at a temperature between T m and T i , there is one diffraction peak at 2θ = 3.5 • , which is a typical diffraction peak observed for the liquid crystalline smectic phase layer structure. The diffuse diffraction peak at about 2θ = 21 • is similar to that of the nematic phase, since the lateral packing of the molecules remains liquid-like [17]. When the sample was heated to a temperature above its T i , only the wide diffraction peak at about 2θ = 21 • was observed indicating that the liquid crystalline phase melted at the higher temperature.…”

Effect of unsaturated acyl chains on structural transformations in triacylglycerols

“…The diffraction pattern of the a 2 -phase could be associated with a liquid crystal structure [21]. Nevertheless, observation of long-spacing diffraction peaks up to the fifth peak suggests a much better structural order of the TAG along the layer Figure 4.…”