Polymorphism in thermotropic liquid crystals

A considerable body of experimental data has been accumulated over the years in regard to the relationship between mesomorphism and chemical constitution, especially for rod-like molecules, but we shall not be discussing this aspect of the problem. For a concise review of current trends in the chemistry of mesogenic compounds, with emphasis on the special requirements for technological applications, reference may be made to a recent book edited by Gray.  

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Many thermotropic liquid crystals have been observed to show more than one mesophase between the solid and isotropic liquid phases. They are said to be polymorphous. The arrangment of different mesophases on a scale of increasing temperature is based on the fact that raising the temperature of any material results in a gradual decrease in the degree of molecular order. Thus, the more ordered the mesophase, the closer in temperature it lies to the solid phase. According to the description of molecular order of various mesophases in Section 1.1, some general observations may be made on the polymorphism in thermotropic liquid crystals as follows ... 

D. Demus, S. Diele, S. Grande, H. Sackmatm, Polymorphism in thermotropic liquid crystals, in Advances in Liquid Crystals, vol. 6 (1987)... 

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. 

A thermotropic liquid crystal is capable of polymorphism, the situation in which a material exhibits more than one liquid-crystalline modification between the solid and liquid phases. For instance, when a calamitic liquid crystal melts it can do so via transitions between a number of smectic phases and the nematic phase. For the purposes of phase... 

The existence of the liquid-crystalline state of matter is not only a function of temperature. A large number of organic materials show liquid crystallinity in properly chosen solvents. Systems which exist in the liquid-crystalline state in a definite range of temperature are called thermotropics, while the second group is known as lyotropics. Both groups show a rich polymorphism. While mesophases of rod-like thermotropic liquid crystals can essentially be subdivided into two... 

The study of liquid crystals rapidly becomes complex because both the thermotropic and lyotropic types are polymorphic. The lyotropic type exists in at least six phases according to Brown Johnson. Materials of this type generally exhibit a molecular weight in the range of 250-500. Many of these materials are described as lipids, and frequently as phospholipids. On addition of water to a crystal composed of these materials, the molecular structure initially collapses to form a lamellar structure. Further dilution may result in additional structural changes before an isotropic solution is reached. 

A literature survey reveals that the term thermotropism is mostly used to describe temperature-dependent ordering of LCs—usually referring to phase changes that occur between the temperature below which the LC crystallizes and the temperature above which it becomes an isotropic liquid. Since the study of crystals is a separate (but somewhat related) field, the term can easily be borrowed to describe thermally induced polymorphic transitions in crystals. 

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