Subject liquid crystal

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

Hopefully with this brief introduction the reader will be able to appreciate fully the chapters which follow and which have been written by experts in the field. The complexity and beauty of the liquid crystalline phase has attracted many able scientists and the applications of liquid crystals in the electronics industry have provided a secure funding base for the subject. This is therefore still a field which is expanding rapidly and many research avenues remain to be explored by newcomers. Perhaps after reading these volumes of Structure and Bonding you will be tempted to join this exciting endeavour. 

The liquid crystal state represents the fourth state of matter and exists between the solid and liquid states, which form its boundaries. The liquid crystal state is reached from the solid state either by the action of temperature (thermotropic liquid crystals) or of solvent (lyotropic liquid crystals) and it is the former that will be the subject of this chapter. 

The 31P n.m.r. of phospholipids has been the subject of a number of papers.62-89 These have been primarily aimed at investigating the conformation and motion of phospholipids in bilayers, but information has also been obtained on gel-to-liquid crystal transformations of phospholipids.65-67 A P31 1H nuclear Overhauser effect indicates that there is little tendency for mixed phosphatidylcholine/phosphatidyl-ethanolamine vesicles to segregate in separate domains.68 69 A phosphonium analogue (23) of choline chloride has been prepared and converted chemically into... 

Liquid-crystal (LC) polymers are the subject of worldwide research and development. (1-5). Commercial films and plastics such as Kevlar (du Pont), Xydar (Dartco) and Ekonol (Sumitomo) utilize the remarkable ability of LC behavior to enhance physical properties. 

We are currently initiating three research projects that include (1) the synthesis of reflective liquid crystal/polymer composite films, (2) a study of microphase separation in hyperbranched block copolymers, and (3) the design and synthesis of polar organic thin films, which is the subject of this proposal. (47 words aim for 41 words)... 

Fig. 32. Schematic representation of the flexo-electric effect, (a) The structure of an undeformed nematic liquid crystal with pear- and banana-shaped molecules (b) the same liquid crystal subjected to splay and bend deformations, respectively.

Neither polymers nor liquid crystals will be studied in this book, but at some stages will be recalled for the sake of comparison. Many books and review articles devoted to these subjects can be found in the literature and interested readers are referred to them (e.g., Nalwa, 1997 Vols. 2-4 Pope Swenberg, 1999 Heeger, 2001). 

Since 1970 the subject of amorphous semiconductors, in particular silicon, has progressed from obscurity to product commercialization such as flat-panel liquid crystal displays, linear sensor arrays for facsimile machines, inexpensive solar panels, electrophotography, etc. Many other applications are at the developmental stage such as nuclear particle detectors, medical imaging, spatial light modulators for optical computing, and switches in neural networks (1,2). 

When a liquid crystal is a metal-containing material, it is called a metallomesogen. It is commonly accepted that the first report on metallomesogens was by Vorlander in 1923, who reported a series of mercury mesomorphic complexes (Figure 7.4) [2]. The subject was not very popular until the end of the 1970s, then the number of reports increased regularly until the 1990s and then stabilized. 

In recent years, the behaviour of liquid crystalline polymers including elastomers has been a subject of considerable interest 104,105). It is known that small molecule liquid crystals turn into a macroscopic ordered state by external electric or magnetic fields. A similar behaviour seems to occur for liquid-crystalline polymer networks under mechanical stress or strain. 

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

We start with dislocations and describe both glissile (conservative) and climb (nonconservative) motion in Chapter 11. The motion of vapor/crystal interfaces and liquid/crystal interfaces is taken up in Chapter 12. Finally, the complex subject of the motion of crystal/crystal interfaces is treated in Chapter 13, including both glissile and nonconservative motion. 

Other methods of film formation discussed in this book depend on allowing a melt or a solution of the material to be deposited to spread on the substrate and subsequently to solidify. An ordered structure can sometimes be imposed on such a film by the application of an electric or magnetic field if the film is in a mesophase (otherwise known as a liquid crystal) before solidification. However, any such method presupposes that the melt or solution used will spread evenly over the substrate. It is thus important to understand a little about the conditions which allow a liquid to spread on a solid surface. This topic depends on the nature of intermolecular forces, a subject which is of general relevance to the formation of organic films and which is discussed in the following section. 

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

These studies throw light on the initial aggregation phenomena, which results in micelles and may also (17) constitute the necessary basis for understanding the subsequent agglomeration to liquid crystals. Very little is known about the thermodynamic conditions for the latter associations. Instead we must rely on empirical data to illuminate the basic mechanisms which determine the association behavior of those substances in concentrated systems. Valuable information for understanding the drastic influence of weak intermolecular forces on the association structures (Figure 1) is obtained from the pronounced temperature dependence of the solubilization which was observed early by Shinoda (29). He and his collaborators (30, 31, 32 33) have since developed this subject. 

An exciting potential application of knowledge about lyotropic liquid crystals is in biological systems. Another chapter in this volume discusses biological systems as its chief subject. Here only a few remarks are made about dynamic phenomena. 

Although dynamic behavior of lyotropic liquid crystals is important in various applications, relatively little work has been done on the subject. 

The self-organization of both thermotropic and lyotropic liquid crystals make these ordered fluids remarkable media for the dispersion and organization (alignment) of CNTs. This subject has been the focus of a recent excellent review by Scalia [231], theoretical work on anchoring at the liquid crystal/CNT interface by Popa-Nita and Kralj [458], and a number of earlier experimental reports on liquid crystal/CNT composites demonstrating that liquid crystal orientational order can be transferred to dispersed CNTs, which is commonly illustrated using polarized Raman spectroscopy [459 -62]. 

It is obvious that this volume cannot be fully comprehensive, but at least it should provide a rough overview, covering some of the important subjects in the field of liquid crystal design and self-assembly. Nevertheless, I hope the present volume will be highly informative and inspiring for chemists and physicists who are interested in developing new materials based on the unique combination of order and mobility provided by the LC state. 

In THE PAST DECADE, IMPROVEMENTS IN infrared spectroscopic instrumentation have contributed to significant advances in the traditional analytical applications of the technique. Progress in the application of Fourier transform infrared spectroscopy to physiochemical studies of colloidal assemblies and interfaces has been more uneven, however. While much Fourier transform infrared spectroscopic work has been generated about the structure of lipid bilayers and vesicles, considerably less is available on the subjects of micelles, liquid crystals, or other structures adopted by synthetic surfactants in water. In the area of interfacial chemistry, much of the infrared spectroscopic work, both on the adsorption of polymers or proteins and on the adsorption of surfactants forming so called "self-assembled" mono- and multilayers, has transpired only in the last five years or so. 

Small-angle light scattering has also been extensively applied to PLCs subject to flow [173]. As in the case of scattering dichroism, SALS patterns arise principally from fluctuations in orientation, and these arc strongest in the vicinity of disclinations, or defects in the director field. The experimental geometries used for SALS in liquid crystals normally use polarizers placed before and after the sample. The arrangements include VV scatter-... 

The final chapter on applications of optical rheometric methods brings together examples of their use to solve a wide variety of physical problems. A partial list includes the use of birefringence to measure spatially resolved stress fields in non-Newtonian flows, the isolation of component dynamics in polymer/polymer blends using spectroscopic methods, the measurement of the structure factor in systems subject to field-induced phase separation, the measurement of structure in dense colloidal dispersions, and the dynamics of liquid crystals under flow. 

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