Polymorphism thermotropics

Thermotropic liquid-crystalline properties of different metal alkanesulfonates are studied by microscopy and X-ray diffraction [59]. Sodium soaps show smectic polymorphism of smectic A and smectic B phases. Ammonium soaps only show smectic A phases but polymorphism in the crystalline state. Calcium soaps show columnar mesophases. In Figs. 32 and 33 some textures and x-ray diffraction patterns are depicted. 

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. 

The crystallization of 3D-ordered crystalline phases from thermotropic mesophases, envisaged as stable pre-crystalline partially ordered intermediates, is an additional interesting issue which should be considered with care experimentally, theoretically, and with appropriate simulation approaches. Depending upon the nature of the mesophase it can be seen as a crystal-crystal transition or, for conformationally disordered, columnar mesophases, it approaches a true crystallization process. It is quite clear that the preexisting order will play a major role for example if the mesophase is chain-extended, bundle equilibria and chain-folding should not play any role. Indeed available experimental evidence supports this idea. Mechanistic and kinetic features should in general differ widely from the standard chain-folded crystallization processes yielding thin lamellar structures. In a number of cases (polyphosphazenes, polysiloxanes, see below) the crystalline polymorphs obtained from the chain-extended precursor differ from those obtained from solution. 

A remarkable property of lipid bilayers is their structural phase transitions (thermotropic polymorphism). For example, fully hydrated pure diacyl-phosphatidyl cholines exibit one fluid phase. La and three crystalline phases Pp/, Lp/. and Lc (12). Because of the high degree of disorder caused by defects, the Pp/ and Lp/ phases usually are called gel phases. The Pp/ phase is sometimes called a ripple phase, because the surface of the bilayer is rippled (13) and presents a wave-like appearance in electron micrographs (Fig. 2). Depending on the nature of the lipid and the presence of additional components (cholesterol etc.), the Pp/ phase may be present or absent in the phase diagram, and a tilted gel Lp/ could be replaced by the Lp phase, which has similar physical properties but no tilt of the hydrocarbon chains. 

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. 

Polymorphism is also common for thermotropic longitudinal PLCs. Two types of crystal lattice exhibit in these PLCs orthorhombic and... 

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... 

This chapter presents a review of different liquid crystal phases. The main attention is paid to the thermotropic liquid crystals, which manifest rich polymorphism upon variation of temperature. Moreover, the thermotropic phases are subdivided into rod-like or calamitic and discotic ones the latter are discussed only briefly. At first, we discuss achiral media with lyotropic phases included and then consider the role of chirality. 

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. 

To obtain the liquid crystalline state in a polymer network, several strategies are conceivable. They are all based on well known principles evaluated during the last few decades for linear liquid crystalline polymers. The monomer units of the network have to consist of mesogenic moieties, which are either rigid rods or discs in the case of thermotropic polymorphism or amphiphiles in the case of lyotropic polymorphism. The mesogenic units can be attached either as side chains to the monomer units yielding side chain elastomers (Fig. la, b) or directly linked... 

It is interesting to note that two polymorphs have been observed for the aramids [514, 515] with the pseudo-orthorhombic structure exhibiting two chains per unit cell, in agreement with Ward [512, 516] and Windle [517] who suggest the same two chain structure for the thermotropes. 

In conclusion, the number and kinds of thermal phase transitions of singlechain surfactant molecules vary from a simple phase transition, the solid crystalline to isotropic liquid, to a complex polymorphism and mesomorphism. More than one thermotropic state may exist in the same system, each being the stable phase within a particular range of temperature (and pressure). Phase transitions are usually reversible through all the intermediate forms to a structure that is thermodynamically stable at room temperature, or they are partially reversible through one or more, but not all, of these transitions, and the room temperature product is an undercooled form of a phase that is stable at some intermediate temperature [26]. 

In particular, the planar bilayer phase manifests itself in several thermotropic subphases Lp, L p, and )" that differ as to the orientational order and mobility of the lipid molecules. The common characteristic of this polymorphism is that the molecules arrange themselves into structures with a hydrophobic core by orienting their hydrophilic portions towards the water environment. 

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 ... 

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