Low Molecular Mass Liquid Crystals

The basic structure of low molecular mass liquid crystals or monomers of liquid crystalline polymers is schematically shown below... 

The above argument is suitable for polymer liquid crystals as well. In fact, the static properties of polymer liquid crystals are basically the same as those for the low molecular mass liquid crystals. But their dynamics are quite different because of their polymeric structure. 

Gray (1961) has had a good account for the structure and properties of low molecular mass liquid crystals. In this section we discuss only a few structural elements having the most impact on the properties of liquid crystals. The structural elements include those imposing on molecules, the side-steps , the kinks, the flexible spacers, the bulky substituents, and others. The discussion starts with poly(l,4-phenylene)s as the parent or reference molecules. 

The liquid crystals, in fact, meet the above requirements large molecular length, linear molecular configuration and with polar terminals. Unfortunately, low molecular mass liquid crystals have difficulty in maintaining the non-center symmetry after electrical polarization. Moreover, it is difficult to process the low molecular mass liquid crystal materials. Apparently, polymers are good candidates in these respects. 

Thermotropic Polymeric Liquid Crystals (PLCs) which are formed by regularly alternating mesogenic elements and flexible spacer groups in the main chain are currently the focus of intensive investigation The standard meso-gens, such as biphenyl, stilbene, azo or azoxybenzene derivatives, which form the core of low molecular mass liquid crystals (LMLCs), are used in synthesis of PLCs as well. 

In this paper, one of the examples of the use of such oriented films as film matrix for information recording is examined. We used the principle of thermo-recording, which is well known for low molecular mass liquid crystals >. On the transparent film of the homeotropi-cally oriented LC polymers (Fig. la), regions of local overheating are created with the aid of a laser beam. 

The laser-recorded information can be kept for a long time if the sample is cooled below the glass transition temperature. From this viewpoint, LC polymers differ beneficially from low molecular mass liquid crystals. 

In the last section of the book, lyotropic systems are treated. These concern derivatives of cellulose in various solvents as well as solutions of synthetic PLCs in low molecular mass liquid crystal solvents and polypeptide solutions in water. The last article illustrates the tremendous variety of polymeric bio-mesogens encountered in living matter. 

The molecules displaying LC phases are highly anisotropic. They might be seen as rigid rods or ellipsoids of revolution with I d. The basic structure of low molecular mass liquid crystals or monomers of LC polymers is given in Figure 5. 

To informatively discuss the structure and properties of liquid crystalline polymers, it is necessary to briefly describe and define liquid crystallinity, building on the knowledge developed for low molecular mass liquid crystals. 

Before examining the properties of polymers which exhibit liquid crystalline characteristics it is appropriate to mention briefly that it is possible to disperse low molecular mass liquid crystals in a polymer matrix and hence achieve an immobilization of the liquid in a solid, yet still achieve many of the desirable optical characteristics of the pure media. These dispersed phases are called polymer dispersed liquid crystals and will not be discussed here. These materials, however, are being shown to have many applications and clearly may be technologically important in the future. ... 

The third section of the book addresses recent research efforts in making polymer-dispersed liquid crystals consisting of nematic or cholesteric low- molecular-mass liquid crystals in flexible-chain polymers or LCPs. These S3rstems are prepared by polymerization of reactive monomers in the presence of Uquid ciystads stabilized by flexible-chain polymer or LCP. Special attention has been paid to the use of these materials in display and electro-optical devices. 

Constitution and Structure of Low-Molecular-Mass Liquid Crystals 260... 

In many aspects, this situation is similar to that of low-molecular-mass liquid crystals, which also comhine seemingly two uncombinative words - liquids and crystals - and, therefore, they are, like LC polymers also very often called hybrid materials or centatus of nature. ... 

Table 1 Some low-molecular-mass liquid crystals forming different LC phases ...

The study of LC polymers is of interest in its own right, inspired by the need to clarify the nature and specific features of LC state of macromolecular compounds.On the other hand, the interest toward this field is accoimted for by the possibility of creating polymeric systems, combining the unique properties of low-molecular-mass liquid crystals and... 

Poly(propyleneimine), poly(amidoamine), siloxane, carbo-silane, and other derivatives are most often used as initial polymer matrices. Spacers are usually aliphatic chains of various lengths, and mesogenic fragments cover the whole arsenal of classical low-molecular-mass liquid crystals. 

Conventional low-molecular-mass liquid crystals (LC) are anisotropic fluids composed of relatively stiff rod-like molecules. The nematic phase is characterized by long-range orientational order in a preferred direction, given by the director n. 

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