Subject thermotropic

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. 

Molecularlv Doped Thermotropic Liquid Crystalline Polymer. The idea of the nonlinear optical medium which is the subject of this paper results from a synthesis of the ideas of the discussion above and a few concepts from nonlinear optical molecular and crystal physics. As discusssed several places in this volume, it is known that certain classes of molecules exhibit tremendously enhanced second-order... 

The study of the cholesteric mesophases obtained by doping thermotropic nematics with chiral, nonracemic compounds, has lead to relevant information about the stereochemistry of the dopants. Chiral interactions change the structure of the phase and therefore molecular chirality can be mapped onto an achiral (nematic) phase to yield a superstructural phase chirality. In 1984 Sol-ladie and Zimmermann published the first review summarizing the state of the art at that time.52 Later on, several review articles updated this subject.53-55... 

Research on liquid crystalline polymers(LCP) is a fashionable subject with the goal of developing speciality polymers of superior mechanical and thermal properties. Besides these properties, other interesting properties of LCP have not been fully utilized. We are trying to use thermotropic LCP for photon-mode image recording material. 

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]. 

Further challenges need to be addressed for the functional development of liquid-crystalline cellulosics, coupled with the elucidation of fundamental aspects of their chiral nature. In addition, the potential thermotropicity of cellulose microcrystallites with the surfaces modified adequately and manipulation of the cholesteric sense (handedness) in the helically arranged molecular assembly are embryonic but fascinating subjects in this research field. 

Most of the subject polymers that have been studied to date have contained flexible spacers composed of polymethylene chains. The monomers or precursors for these spacers are readily available as either diols or diacids or as dihalides, and this availability has led to the preparation of several homologous series of thermotropic polymers with different mesogenic units. Indeed, the first reported main chain, thermotropic LC polymer contained a polymethylene flexible spacer... 

The rheology of low molecular weight thermotropic compounds has been a subject of considerable theoretical and experimental analysis In general, liquid crystals are easily oriented by surfaces, electromagnetic fields and mechanical stress or shear, and the degree of orientation, in turn, affects their melt viscosity. The rheological behavior of a liquid crystal is known to be greatly dependent on the nature and also on the texture of its mesophase. 

The arrangement of chiral molecules in thermotropic liquid crystals is more complex, since entire volumes of space - rather than the bounded twisted ribbons discussed above - must be ed subject the constraint of a preferred twist between neighbouring molecules. The simplest examples of such mesophases are the cholesteric liquid crystals, discovered last century, (c/. section 5.1.8). This class of thermotropic liquid crystals derives its generic name from chiral cholesterol derivatives (shown below), which were found a century ago to exhibit peculiar optical changes as they were heated. 

During the course of our study, we had the opportunity to evaluate the only available all aromatic, thermotropic polyester, LCP2000 from Celanese. Presumably it was a copolymer containing hydroxybenzoic (HBA) and hydroxy naphtholic acid (HNA) moieties. The sample was subjected to fiber spinnings and plastic moldings as were the thermotropic polyester-carbonates, using the same equipment and under comparable conditions. 

Within the extensive literature on this subject, there are many examples of the synthesis of thermotropic polyesters, polyesteramides, polycarbonates, polyethers, polyurethanes and polyester-imides. Until recently, the main omissions had been thermotropic polyamides and polyimides however, many examples of polyamides that show lyotropic behaviour have been known for a long time. 

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

Cellulose and some derivatives form liquid crystals (LC) and represent excellent materials for basic studies of this subject. A variety of different structures are formed, thermotropic and lyotropic LC phases, which exhibit some unusual behavior. Since chirality expresses itself on the configuration level of molecules as well as on the conformation level of helical structures of chain molecules, both elements will influence the twisting of the self-assembled supermolecular helicoidal structure formed in a mesophase. These supermolecular structures of chiral materials exhibit special optical properties as iridescent colors, and... 

Phase behavior and morphology in conventional polymers are heavily dependent on the thermal history of the sample, as is obvious to anyone even remotely familiar with macromolecules. Polymer liquid crystals (PLCs) are clearly subject to similar constraints by virtue of their macromolecular identity. In addition, a number of thermal properties are specific to PLCs as a result of the interaction between macromolecular behavior and the molecular ordering characteristic of LC mesophases. This chapter focuses on just such features of thermal history, as revealed by the interplay of kinetic and thermodynamic factors observed in thermotropic polymers. 

This chapter deals exclusively with thermotropics (pure polymer systems). This review deals with longitudinal one-comb and network PLCs and blends of longitudinal PLCs and conventional flexible chain polymers (Figure 10.2). An excellent introduction to the subject including a presentation of definitions and classifications is given by Brostow [10]. It is important to emphasize that a given PLC at a given... 

The way, or sequence, in which thermotropic transitions occur is defined in the following ways. The liquid crystal to isotropic liquid transition is called the clearing or isotropization point, and this transition, like those between liquid crystal phases, is essentially reversible and occurs with little hysteresis in temperature. The melting point of a material is usually a constant, but the recrystallization process can be subject to supercooling. Mesophases formed on the first heating cycle of a material are thermodynamically stable, and are called enantiotropic phases, whereas phases that are formed below the melt point on cooling cycles, and are revealed... 

The zero shear viscosity scales with Nf" to contrast Af dependence for isotropic polymers [20] So far, we have examined the dynamics of rod-Uke macromolecules in isotropic semi-dilute solution. For anisotropic LCP solutions in which the rods are oriented in a certain direction, the diffusion constant increases, and the viscosity decreases, but their scaling behavior with the molecular weight is expected to be unchanged [2,17], Little experimental work has been reported on this subject. The dynamics of thermotropic liquid crystalline polymer melts may be considered as a special case of the concentrated solution with no solvent. Many experimental results [16-18] showed the strong molecular weight dependence of the melt viscosity as predicted by the Doi-Edwards theory. However, the complex rheological behaviors of TLCPs have not been well theorized. 

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