Description of the Liquid - Crystalline Structures

The basic columnar structure is as illustrated in fig. 1.1.8(a) it is somewhat similar to the hexagonal phase of soai -water and other lyotropic systems (fig. 1.2.2). However, a number of variants of this structure have been found. Fig. 6.1.2 presents the different two-dimensional lattices of columns that have been identified here the ellipses denote discs or, more precisely, cores that are tilted with respect to the column axis. Table 6.1.1 gives the space groups of the columnar structures formed by some derivatives of triphenylene. (These are planar space groups that constitute the subset of the 230 space groups when symmetry elements relating to translations along one of the axes, in this case the column axis, are absent.) 

High resolution X-ray studies have been reported on the columnar phases of a few compounds. The measurements were made on very well oriented monodomain samples obtained by preparing freely suspended liquid crystal strands, typically about 200 /mi in diameter and 1.5-2 mm 

Deuterium NMR spectroscopy of the discotic phase of hexa-n-hexyloxy triphenylene has led to similar conclusions. Spectra of two selectively deuterated isotopic species, one in which all aromatic positions are substituted and the other in which only the a-carbon side chains are substituted, bring out the difference between the order parameters of the cores and the tails. Fig. 6.1.3 gives the quadrupole splittings of the aromatic and the a-aliphatic deuterons versus temperature in the meso-phase region. It is seen that the rigid core is highly ordered, the orientational order parameter s ranging from 0.95 to 0.90, whereas the a-aliphatic chains are in a disordered state. 

These studies emphasize the fact that any realistic theory of the statistical mechanics of discotic phases cannot treat the molecules as rigid discs, but has to take into account the conformational degrees of freedom of the hydrocarbon chains. 

Columnar mesophases are also formed when the flat core of the molecule is replaced by a conical one as in the cyclotricatechylene hexaesters (fig. 6.1.4(a)). With macrocyclic molecules, which are hollow at the centre (fig. 6.1.4(A)), the columns are in the form of tubes these mesophases have been described as tubular .  

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The ability to form a lamellar liquid crystalline film depends on the spontaneous curvature of the surfactant aggregates, or the CPP, which is a convenient and intuitive description of the surfactant molecular structure. Kabalnov and Wennerstrbm [16] have shown that, for the formation of a water bridge between two water droplets, a large free energy is required for a surfactant with a high CPP, while the free energy required for a surfactant with a low CPP is lower. Hence, the stability of a surfactant double layer increases with an increase of the CPP of the surfactant. 

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

The problem of the structure of liquid water has attracted much attention, but as yet no completely satisfactory solution to it has been found. We shall postpone the discussion of this problem until after the description of the structure of certain crystalline hydrates of simple Bubstances. 

Thus, polymers with mesogenic groups in side chains form structural mesophases of the same types as low-molecular liquid crystals. This makes it possible to apply traditional mesophase classification for the description of the structure of LC polymers. At the same time, the structure of some of comb-like polymers (see Table 5) considered as crystalline, may probably be treated as one of highly-ordered smectic mesophases (SH or Sj), whose study is only started74). 

The following table lists the liquid crystalline materials that are useful as gas chromatographic stationary phases in both packed and open tubular column applications. In each case, the name, structure, and transition temperatures are provided (where available), along with a description of the separations that have been done using these materials. The table has been divided into two sections. The first section contains information on phases that have either smectic or nematic phases or both, while the second section contains mesogens that have a cholesteric phase. It should be noted that each material may be used for separations other than those listed, but the listing contains the applications reported in the literature. 

The light microscope can be used to identify liquid crystalline phases, characterize molecular order, and quantify the distribution of defects. As such, the technique is seldom exploited to maximum advantage, particularly if studies are restricted to observations between crossed polars in transmitted light. Also, there is much scope for misinterpreting the observed contrast. TTiis chapter offers a systematic description of the options (and pitfalls) that are relevant to a research microscopist studying the structure of liquid crystalline polymers. 

As a starting point for considering the effect of impurity and solvent on crystallization, the growth and interaction process is examined in the framework of the fundamental solid-state, interfacial, and liquid phase (solute-solvent-impurity) chemistry. The solid-state chemistry is specific to a given crystalline material and the nature of the bonds (e.g., ionic, covalent, van der Waals, etc.) that hold the crystal structure together. A complete description of the solid-state aspects of crystal growth is beyond the scope of this... 

A liquid crystal dimer is composed of molecules containing two conventional mesogenic groups linked via a flexible spacer. These materials show quite different behaviour to conventional low molar mass liquid crystals and in particular their transitional behaviour exhibits a dramatic dependence on the length and parity of the flexible spacer. In this review a comprehensive overview of the relationships between molecular structure and liquid crystallinity in dimers is provided. This includes a description of the novel modulated and intercalated smectic phases exhibited by dimers. 

In this Chapter the basic approaches used to describe nematic liquid crystalline (NLC) systems in slab geometries under the effect of confinement are introduced. We review both, the microscopic and macroscopic approaches, however, the emphasis is on the latter. We also show the correspondence between the approaches on different levels. Special attention is devoted to effects of the confinement on the LC order and consequently to the interactions arising from that. More precise descriptions of the techniques and also more detailed results have been already published elsewhere [9-12,15-18]. In the following Section we first shortly review the microscopic origin of order and define the appropriate order parameter. Then we review the basic microscopic and macroscopic theoretical approaches to describe LC systems. In the third Section we describe in short the effect of confinement in two different types of NLC systems. The fourth Section is devoted to macroscopic interactions between confining walls, especially the ones characteristic for ordered systems. We conclude the Chapter with the discussion on the observability of structural and fluctuation forces in NLC systems. 

The unique morphologies of liquid crystalline polymers cause them to undergo numerous thermal transitions which can be observed in a Differential Scanning Calorimeter (DSC) scan. Figure 5 shows a typical scan with descriptions of the transitions and interpretations of the structural arrangements in the several phases. 

The most intensive period of monolayer research took place during the decade before the Second World War. Thus the basic interpretations of Il-A isotherms were done before the characteristic crystalline and liquid-crystalline structural features were known. Unfortunately much of the later monolayer work has also been limited to the early concepts only, without the knowledge on three-dimensional lipid phases being fully utilized. Reports of molecular arrangement in monolayers are therefore often misleading and the nomenclature used is inadequate. A structural description of monolayer phases will first be given based on relations with crystalline and liquid-crystalline phases. A hypothetical pressure-area (II-A) isotherm shown in Fig. 8.23 will be used and the nomenclature according to Harkins (1952) is applied. 

The issue is the description of the molecular changes accompanying the phase transitions in systems which show liquid crystalline phases. While much is known on the morphological changes, little is known on what happens to the molecular structure through the various phase transitions. 

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