Thermotropic systems

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

The development is reviewed of liquid-crystalline polymers whose mesophase formation derives from the nature of the chemical units in the main chain. The emphasis lies primarily on highly aromatic condensation polymers and their applications. The general properties of nematic phases formed by such polymers are surveyed and some chemical structures capable of producing nematic phases are classified in relation to their ability to form lyotropic and thermotropic systems. The synthesis, properties, physical structure and applications of two of the most important lyotropic systems and of a range of potentially important thermotropic polymers are discussed with particular reference to the production and use of fibres, films and anisotropic mouldings. 

Academic and industrial interest in liquid-crystalline polymers of the main-chain type has been stimulated by certain special properties shared by lyotropic and thermotropic systems that exhibit a nematic phase. Although these special properties affect both the processing into fibres and other shaped articles and the physical behaviour of the products, the product behaviour is at least partly attributable to the novel processing behaviour. 

Tihe term lyotropic mesomorphism is used to describe the formation of thermodynamically stable liquid crystalline systems through the penetration of a solvent between the molecules of a crystal lattice. In contrast to the thermotropic mesomorphism shown by many pure substances, lyotropic mesomorphism always requires the participation of a solvent. Lyotropically mesomorphous systems, however, are usually as sensitive to changes in temperature as thermotropic systems. So far, lyotropic mesomorphism has been observed almost exclusively in lipid systems containing water. Lipids that show lyotropic mesomorphism frequently... 

As indicated above, much interest exists in dynamic behavior of thin aligned layers of nematic liquid crystals. It is not surprising to find, therefore, that measurement of the anisotropy of transport properties has been the objective of many studies of thermotropic systems. The literature on anisotropic thermal conductivity in nematic liquid crystals has been reviewed recently by Rajan and Picot (12). Among the studies of anisotropic diffusion are those of Yun and Fredrickson (13), Bline... 

Transient behaviour of lyotropic MCLCPs is similar to that of thermotropic MCLCPs as discussed in Sect. 15.7 (Fig. 15.47). An example is given in Fig. 16.35 for PpPTA in sulphuric acid (Doppert and Picken, 1987). It shows damped oscillating behaviour, which is in contradistinction to conventional polymers, but which is also found as a result of simple rheological models, like the Jeffreys model (see Chap. 15). This behaviour turns out to be a general feature of lyotropic MCLCPs. The oscillatory behaviour is easier to measure for lyotropic than for thermotropic systems, where it is less pronounced. 

The crystallinity of the mesomorphic copolymers was extremely low as judged from the X-ray diffraction pattern, in which only one broad reflection was observed. This may be an essential point with regard to the chain mobility. Since the long side chains are considered to play a role as solvent molecules, the thermotropic system may correspond to a highly concentrated solution. a-Helical main chains embedded in the matrix of side chains take the cholesteric arrangement because of the chirality of the main chain. 

Liquid crystals can be classified into lyotropic and thermotropic systems. The rheology of thermotropic liquid crystals is less documented, but in general, nematic liquid crystals were found to show Newtonian flow, whereas smectic and cholesteric liquid crystals demonstrated more or less pronounced plug flow. Plug flow is typical for non-Newtonian, shear thinning liquids. ... 

Liquid crystal phases, or mesophases, are characterized by a partial order, intermediate between the full orientational and translational disorder of the isotropic liquid phase and the full orientational and translational order of the crystalline phase. Thermotropic liquid-crystal phases are obtained for a given compound (or possibly a mixture) as a function of temperature, while the so-called lyotropic liquid-crystal phases are obtained as a function of the concentration of a given solute in a solvent Typical examples of the latter systems are the various types of aggregates formed by amphiphilic molecules either in water or in organic solvents. In this chapter we will be interested only in thermotropic systems. An interesting review on lyotropic ionic liquid crystals can be found in Ref. [2],... 

P. J. Flory and G. Ronca, Mol. Cryst. Liq. Cryst., 54(3-4), 311 (1979). Theory of Sptems of Rodlike Particles. II. Thermotropic Systems with Orientation-Dependent Interactions. M. Vacatello and P. J. Flory, Polym. Commun., 25(9), 258 (1984). Helical Conformations of Isotactic Poly(methyl Methacrylate). Energies Computed with Bond Angle Relaxation. 

In contrast to the Onsager and Flory theories, the Maier-Saupe theory no longer takes into account molecular steric effects as the basic interaction but instead proposes that the van der Waals interactions between molecules are the basis for forming a liquid crystal phase. The van der Waals interaction depends on molecular orientations. The Maier-Saupe theory adopts a rather simple mathematical treatment and can easily take into account the relationship of system properties to temperature. This theory has been successfully applied to a thermotropic system of small molecular mass liquid crystal. 

Flory PJ, Ronca G (1979) Theory of systems of rodlike particles. II. Thermotropic systems with orientation-dependent interactions. Mol. Cryst. Liq. Cryst. 54 311... 

Flory, P. J. Ronca, G. "Theory of Rod-like Particles - Part II Thermotropic Systems with Orientation-Dependent Interactions", Mol. Cryst. Llq. Cryst., 1979, Vol. 54, pp. 311-330. 

Mesophase can be thermotropic or lyotropic in nature. For thermotropic systems, the size of the molecules, or the molecular weight... 

Molecules which combine the features of the rod and the disc may be expected to form new types of mesophases. An example is the biaxial nematic phase reported in thermotropic systems (see 6.6). Malthete et a/. have prepared an interesting series of mesogens shaped like stick insects called phasmids (fig. 6.1.5(n)). Some of them form columnar mesophases the structure proposed for the hexagonal phase is shown schematically in fig. 6.1.5( >). 

Toulouse describes as topological rigidity . These and other ideas have yet to be investigated experimentally. The availability of the phase in simple thermotropic systems is likely to make it conveniently possible to test some of these predictions. 

In some polymer liquid crystals, several mesophases can be identified. In main-chain liquid crystal polymers there is usually a transition from the crystal to a mesophase, whereas in more amorphous systems when a glass transition is present, the mesophase may appear after this transition has occurred. In multiple transition thermotropic systems, the increase in temperature leads to changes from the most-ordered to the least-ordered states, i.e., crystal (k) smectic (S) nematic (N) isotropic (i). 

In true thermotropic systems the liquid-crystalline phase exists over a range of temperatures between the melting point, or glass-transition tern-... 

Many works deal with the variation of the helicoidal pitch in cholesteric phases as a function of temperature and composition. The best way to elucidate the origin of the twist seems to be to compare the pitch variations in lyotropic and in thermotropic systems. The first accurate work in this field is due to Robinson (1958-66) who studied PBLG (polybenzyl-L-glutamate) a synthetic polypeptide in organic solvents as dioxane, ethylic alcohol, chloroform etc. and Cano (1967) who made measurements of the pitches of nematic paraazoxyphenetol with different amounts of cholesterol benzoate. 

For example, if the time for the process to occur is faster than the longest relaxation time, then the fluid behaves more like an elastic solid. For the liquid crystalline systems there seems to be two relaxation times which are important. One is the time for relaxation of orientation and the other is the time for relaxation of stress. Whereas these phenomena are connected for flexible chain polymers, they seem to separate for liquid crystalline polymers. In other words, there are several stress free states for LCP. Some of the behavior observed may be partly due to the copolymer nature of thermotropic systems. However, the lyotropic systems based on the polyamide structure also exhibit similar behavior. 

Liquid crystalline systems can be either lyotropic or thermotropic. In the case of lyotropic systems, liquid crystalline characteristics are found in solution above a critical concentration, and are concentration as well as temperature dependent. On the other hand, thermotropic systems show liquid crystalline behavior in the melt and are temperature dependent with respect to mesophase behavior. 

Athermal Systems," Mol. Cryst. Liq. Cryst., 54, 289-310 (1979), "2. Thermotropic Systems with Orientation-dependent Interactions," Mol. Cryst. 

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