Thermotropic materials

In this review we are mainly concerned with thermotropic materials, i.e. with liquid crystals and LC-glasses which do not contain a solvent. The transitions of the macro-molecular, thermotropic liquid crystals are governed then by temperature, pressure and deformation. In lyotropic liquid crystals and LC-glasses a solvent or dispersing agent is present in addition. The transitions then also become concentration dependent. 

Only fragmentary details about the structure of the main-chain liquid crystals are known (for a review see Ref.86)). Often condis crystals are confused with liquid crystals, and in many cases lyotropic liquid crystals are not separated from thermotropic materials. The problem is complicated since flexible chains, such as for example poly(gamma-benzyl glutamate)47), can become rigid by a coil-to-helix transformation. Similarly, external stress or quenching can lead to incomplete orientation which may be described as a mesophase. 

However, in 1986, Chandrasekhar [70] published a derivative (Figure 35) which he claimed to show a biaxial nematic phase. This report was interesting because the biaxial nematic phase (Nb), demonstrated in lyotropic systems [71], had been long sought after in thermotropic materials. Further, the molecules were described as bridging the gap between rod- and disc-like materials (a reference perhaps better reserved for polycatenar liquid crystals—vide infra)... 

A third type of organization in lyotropie systems is found in mesophases which are strictly analogous to the columnar phases found in thermotropic materials. Also... 

Fig. 5 Orientation of Thermotropic Material model of LCP as it js beeing hot drawn, (reprinted with permission from ref. 26)...

The liquid crystal phases of a thermotropic material are generated by changes in temperature (see Chapter 3). However, lyotropic liquid crystal phases are formed on the dissolution of amphiphilic molecules of a material in a solvent (usually water). Just as there are many different types of structural modifications for thermotropic liquid crystals (see Chapter 3), there are several different types of lyotropic liquid crystal phase structures. Each of these different types has a different extent of molecular ordering within the solvent matrix. The concentration of the material in the solvent dictates the type of lyotropic liquid crystal phase that is exhibited. However, it is also possible to alter the type of lyotropic phase exhibited at each concentration by changing the temperature. 

Many thermotropic materials have been observed to pass through more than one mesophase between the solid and isotropic liquid phases. Such materials are said to be polymorphous . One can predict the order of stability of these mesophases on a scale of increasing temperature simply by utilizing the fact that raising the temperature of any material results in progressive destruction of molecular order. Thus, the more ordered the mesophase, the closer in temperature it lies to the solid phase. From the description of the various types of order given in Sec. 3, we can immediately draw the following conclusions ... 

The liquid crystalline (mesomorphic) state is produced by a combination of dispersive and attractive forces [5]. In this state of matter, intermolecular interactions are sufficiently weakened to force a substance into a liquid state but attractive dipolar interaction hold the molten molecules into a low dimensional lattice which lacks long range positional order. The weakening of the intermolecular interactions can be accomplished by heating the material which creates molecular motions or by the addition of a liquid component which partially solvates the material. Liquid crystalline phases formed by heating and solvation are termed thermotropics and lyotropics respectively and the remainder of our discussion will focus on thermotropic materials. 

The cholesteric nature of the cellulose derivatives and its optical, thermal and mechanical properties can be tuned by the chemical modification of the original molecular structure, giving rise to a high brand of thermotropic materials. These new features impart specific properties that make them of high interest for photonics, electronics, stimuli-responsive devices as well as biomedical applications. 

The thermotropic materials may be processed by injection moulding, extrusion, thermoforming, blow moulding, etc. To modify their properties they are usually filled with glass fibres, or carbon fibres or other inorganic fillers. 

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