Liquid crystals basic concepts

To help readers who are not familiar with liquid crystals several terms are defined below. Those who are interested in learning the basic concepts of these materials in more detail are referred to the references [1,5 c, 8],... 

The purpose of this chapter is to explain theoretically the formation of liquid crystal phases in polymer systems and to provide the basic concepts for designing and synthesizing liquid crystalline polymers. Liquid crystalline polymers combine features of both polymers and liquid crystals, thus we discuss the materials from two sides liquid crystallinity and polymer properties. Theoretical descriptions have encountered many difficulties in the past. One is that the present theoretical understanding of neither polymers or liquid crystals is complete. 

In this Chapter we want to introduce some basic concepts and present an overview of the mechanisms, that are in our opinion responsible for the alignment of liquid crystals on solid and soft-matter interfaces. In addition we will present and discuss recent developments and results in surface sensitive... 

The chapters of this book expand on the ideas mentioned here, as well as others that have arisen from the efforts of many talented scientists studying this rather unusual phenomenon over the past 40 years. To get the most out of this book, the reader should be familiar with the basics of liquid crystal phenomenology, including molecular structure, phases, textures, and typical sample geometries, as well as theoretical concepts, including curvature elasticity theory and some elements of the fluid dynamics of liquid crystals. 

Chapter 4 will introduce the reader to the basic concepts of the X-ray analysis of crystals and its applications to particular liquid crystal phases. It should be noted that in the present literature this problem is not adequately dealt with anywhere, and this chapter attempts to rectify this deficiency. Chapter 5 covers phase transitions, one of the key problems of the liquid crystal physics, and which has been widely discussed in other texts at very different levels. In this chapter I give only a detailed explanation of the basic crmcepts of the phase transitions between most important mesophases. 

The decisive (and a posteriori trivial) idea came to me from the concepts of liquid crystallinity and from the examination of the most recurrent and general structural features of the low molecular weight compounds known to exhibit liquid crystal behaviour. The basic assumptions were the following ... 

Just as for biological beings, the liquid crystalline phase structure and simultaneously the functionality of liquid crystalline elastomers are strictly limited to a defined temperature regime. Similar to low molar mass liquid crystals and LC polymers this temperature regime is determined by the chemical constitution of the polymer networks. For the synthesis and investigation of liquid crystalline elastomers the basic concepts of liquid crystals, LC polymers, and polymer networks have to be brought together. 

Figure 1. Concept of basic structures in liquid crystal synthesis (a) basic structure developed by D. Vorlander and its modification (b) possibilities of deriving liquid crystal structures by exchanging a single subunit in MBBA (c) structural modifications of 5CB (4-cyano-4 -pentylbiphenyl).

Chapters 1 and 2 introduce the main phases and basic properties of liquid crystals and other anisotropic fluids, such as soaps, foams, mono-layers, fluid membranes and fibers. These chapters do not include difficult mafliematical formulas and are probably suitable for imdergraduates or for other professionals, such as K-12 teachers. Chapter 3 describes the nature of phase transitions based on the phenomenological Landau-de Gennes theories, and on the self-consistent mean-field theories that use concepts in statistical physics. 

BASIC CONCEPTS OF THE ELECTRO- AND MAGNETOOPTICAL EFFECTS IN LOW-MOLECULAR-WEIGHT LIQUID CRYSTALS... 

As a summary, we have reviewed the main experimental results and concepts concerning the physical properties of the liquid crystals of bent-core molecules. We demonstrated that a number of seminal findings and basically new concepts emerged in the field in the last decade. However scientists regularly explore unusual , surprising and not understood phenomena, and there are more unexplained observations than well understood ones. We are completely sure that the next decade of the physics of bent-core liquid crystals will bring new physics, and will be as rich in beautiful observations as was the first decade. 

In the next few sectiorts we introduce some basic concepts and definitions, such as order parameter, short- and long-range order, phase transition, and so on, which form the basis for describing the ordered and disordered phases of liquid crystals. 

An elementary example of flow in nematic liquid crystals is introduced in Section 5.2. This example motivates the basic ideas of flow stability and instabiUty in nematics and also introduces the concepts of flow-aligning and non-flow-ahgning nematic liquid crystals. Section 5.3 presents an example of flow where it is possible to have a transverse component of flow, sometimes called a secondary component of flow. This secondary component of flow is in addition to the component of flow that is present in the direction of an applied pressure gradient, and is in a direction perpendicular to that of the pressure gradient. 

Falling-film crystallization utilizes progressive freezing principles to purify melts and solutions. The technique established to practice the process is inherently cyclic. Figure 20-15 depicts the basic working concept. First a crystalline layer is formed by subcooling a liquid film on a vertical surface inside a tube. This coating is then... 

By necessity, the treatment of solid state kinetics has to be selective in view of the myriad processes which can occur in the solid state. This multitude is mainly due to three facts 1) correlation lengths in crystals are often much larger than in fluids and may comprise the whole crystal, 2) a structure element is characterized by three parameters instead of only by two in a liquid (chemical species, electrical charge, type of crystallographic site), and 3) a crystal can be elastically stressed. The stress state is normally inhomogeneous. If the yield strength is exceeded, then plastic deformation and the formation of dislocations will change the structural state of a crystal. What we aim at in this book is a strict treatment of concepts and basic situations in a quantitative way, so far as it is possible. In contrast, the often extremely complex kinetic situations in solid state chemistry and materials science will be analyzed in a rather qualitative manner, but with clearcut thermodynamic and kinetic concepts. 

In Chapter 3 we described the structure of interfaces and in the previous section we described their thermodynamic properties. In the following, we will discuss the kinetics of interfaces. However, kinetic effects due to interface energies (eg., Ostwald ripening) are treated in Chapter 12 on phase transformations, whereas Chapter 14 is devoted to the influence of elasticity on the kinetics. As such, we will concentrate here on the basic kinetics of interface reactions. Stationary, immobile phase boundaries in solids (e.g., A/B, A/AX, AX/AY, etc.) may be compared to two-phase heterogeneous systems of which one phase is a liquid. Their kinetics have been extensively studied in electrochemistry and we shall make use of the concepts developed in that subject. For electrodes in dynamic equilibrium, we know that charged atomic particles are continuously crossing the boundary in both directions. This transfer is thermally activated. At the stationary equilibrium boundary, the opposite fluxes of both electrons and ions are necessarily equal. Figure 10-7 shows this situation schematically for two different crystals bounded by the (b) interface. This was already presented in Section 4.5 and we continue that preliminary discussion now in more detail. 

In comparing the three states of matter, our approach will be as follows. First, we will compare the three (3) basic states of matter, namely gases, liquids and solids. We will then contrast these states energetically and atomistically. Next, we will discuss structures of solids and the factors involved in determination of crystal structure. Finally, we will introduce the concept of the defect solid and how such defects affect the macroscopic properties of the solid state. 

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