An Introduction to Liquid Crystallinity

Center for Materials Characterization and Department of Chemistry, University of North Texas, Denton, Texas 76203-5308, USA 

To begin with, we divide liquid-crystalline materials into monomer liquid crystals (MLCs) and polymer liquid crystals (PLCs). This convenient terminology is due to Samulski, who also specified that a compound is classified as an MLC irrespective of the fact whether it can or cannot polymerize. Not long ago we had an explosion of new applications of MLCs. We might well be entering now into a similar period with PLCs. 

To acquire a certain perspective, I shall provide first a brief history of MLCs and PLCs. Then I shall discuss mesophases liquid crystals constitute only one of three kinds of mesophases. Further, I shall compare heterogeneous (that is, ordinary) composites, molecular composites and PLCs. Then we shall go to the heart of this chapter the nature of liquid crystallinity and its manifestations. On this basis we shall be able to survey existing and potential structures of PLCs, using a classification developed earlier. We shall see connections between a place in this classification and properties. Thus, this chapter provides an overview of the field. Chapters 1-3 form the first part of the book, including structures, characterization and dynamics. Subsequent chapters deal with specific properties, synthesis procedures, morphologies, processing and applications. 

Researchers synthesizing liquid crystals, whether MLCs or PLCs, are proud of new materials they develop, and justly so. However, as in many areas of endeavor, humans were not exactly the first. In 1989 Li and Yu found that the middle gland of silk fibroin is liquid-crystalline— nematic to be more accurate. Hence silkworms were producing LC materials many thousands of years before human beings started to imitate them. 

The first scientific description of liquid crystals was provided by the Austrian botanist Friedrich Reinitzer. This was in 1888, hence in 1988 we had celebrations of 100 years of discovery of liquid crystals. We also had celebrations in 1989, because in 1889 the German scientist Otto Lehmann coined the name liquid crystals . Lehmann confirmed the experimental results of Reinitzer, which was important, since some people did not believe Reinitzer. However, Reinitzer was not exactly grateful he maintained that the name liquid crystals is wrong and constitutes a contradiction. 

---

Brostow W (1992) An introduction to liquid crystallinity (Liquid crystalline polymers from structures to applications). Elsevier Applied Science, New York Brostow W, D Souza NA, Kubat J, Maksimov R (1999) Creep and stress relaxation in a longitudinal polymer liquid crystal prediction of the temperature shift factor. J Chem Phys 110 9706-9712... 

Gangadhara et al. have linked the cyanobiphenyl mesogen via a dicarbox-imide-group to an oxanorbornene ring system Vl-n, n=2-8 (see Fig. 4). Polymerization was carried out with Schrock type initiator 4. The dicarboximide linkage probably hindered the formation of LC phases even the introduction of relatively long spacers between the polymer backbone and the mesogen did not lead to liquid crystalline monomers or polymers [41]. 

The principle of the conservation of angular momentum is often used to argue for the symmetry of the extra stress that is, Xxy = Tyx, and so forth. In that case there are six independent components, not nine. The angular momentum argument requires an explicit but often unstated assumption that there is no structure in the fluid that is capable of generating local torques, which seems generally to be the case for polymers (except perhaps for liquid crystalline polymers), and, except for a brief introduction to liquid crystals in Chapter 13, we will assume stress symmetry throughout. 

For the liquid crystalline behavior, the position of the ester group was crucial. Introduction of an ethyl spacer in 9 led to the complete loss of the mesophase and... 

Gray has provided an excellent book on the liquid crystalline state as known in 196222. It includes many detailed definitions. Although he notes the wide occurrence of liquid crystals in biological tissue, he limited his discussion of such crystals to the introduction. Materials in the liquid crystalline state can be divided into two major groups, those that are thermotropic and lyotropic. 

SCLCPs combine liquid crystalline properties and polymeric behavior in one material. If the mesogenic unit is fixed directly to the polymer main chain, the motion of the liquid crystalline side chain is coupled with the motion of the polymer backbone, preventing the formation of a LC mesophase. Therefore, Finkelmann and Ringsdorf proposed that the introduction of a flexible spacer between the main chain and the mesogenic unit would decouple their motions, allowing the mesogenic moiety to build up an orientational order [29,30]. 

The two algorithms already developed and used to reproduce ESR line-shapes of paramagnetic species in free diffusion are applied in this subsection to the case of spin probes dissolved in liquid crystalline mesophases. The main point of diffoence with the previously examined cases is due to the introduction of an orienting potential v ose nature is directly reflected in the structure of the Fokker-Planck opoator, whidi in the difiusional assumption is given by Eq. (2.6). The explicit form of the potential we use in this... 

Solubilization can be defined as the preparation of a thermodynamically stable isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the introduction of an additional amphiphilic component or components. The amphiphilic components (surfactants) must be introduced at a concentration at or above their critical micelle concentrations. Simple micellar systems (and reverse micellar) as well as liquid crystalline phases and vesicles referred to above are all capable of solubilization. In liquid crystalline phases and vesicles, a ternary system is formed on incorporation of the solubilizate and thus these anisotropic systems are not strictly in accordance with the definition given above. 

---