Polymorphism in Liquid Crystals

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  

Molecular Structure and the Properties of Liquid Crystals (Academic Press, London, 1962). 

de Gennes, The Physics of Liquid Crystals (Clarendon Press, Oxford, 1974). 

Chandrasekhar, Liquid Crystals (Cambridge University Press, Cambridge, 1977). 

Kelker and H. Hatz, Handbook of Liquid Crystals (Verlag Chemie, Weinheim, 1980). 

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Sackmann, H., Demus, D., Problems of polymorphism in liquid-crystals. Mol. Cryst. Liquid Cryst. 1973, 21, 239-273. 

Although polymorphism in plastic crystals is less frequent than in liquid crystals, it does exist. Tetrakis(methylmercapto)methane, C(SCH3)4, for example, has four crystal modifications of which the three high temperature forms have a high degree of plasticity 100). Also, it has been observed that plastic crystals are frequently mutually soluble 16b), a consequence of the less restrictive crystal structures. Phase separation of these solutions occurs often on transition to the fully ordered crystal, giving rise to quite complicated phase diagrams102). 

The employment of high pressure is also indispensable to elucidate the phase situation of liquid crystals. Their rich polymorphism is most conveniently studied by high-pressure differential thermal analysis (DTA). " Not only may the phase behavior of the substance studied change significantly under pressure (pressure-induced and pressure-limited phases may occur), but also several peculiarities have been detected in liquid crystal phases, such as reentrant and tricritical phenomena. Also the properties of ferroelectric and polymeric liquid crystals are markedly altered under pressure. 

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

It is a white, deliquescent solid, very powdery, which exhibits polymorphism on heating, several different crystalline forms appear over definite ranges of temperature -ultimately, the P4O10 unit in the crystal disappears and a polymerised glass is obtained, which melts to a clear liquid. 

Many of the investigated mesogenic compounds show solid state polymorphism. In order to obtain useful information about the arrangement of the molecules in the mesophase from the X-ray data of the single crystals, it is important to investigate the crystal structure of those solid phase which transforms into the liquid crystalline phase. For instance, only the crystal structures of the low temperature solid phases of the compounds MBBA [138, 139], MHPOBC [159], and T15 [81] could be determined, but the... 

A polymorph is a solid crystalline phase of a compound resulting from the possibility of at least two different crystal lattice arrangements of that compound in the solid state [42], Polymorphs of a compound are, however, identical in the liquid and vapor states. They usually melt at different temperatures but give melts of identical composition. Two polymorphs of a compound may be as different in structure and properties as crystals of two different compounds [43,44], Apparent solubility, melting point, density, hardness, crystal shape, optical and electrical properties, vapor pressure, etc. may all vary with the polymorphic form. The polymorphs that are produced depend upon factors such as storage temperature, recrystallization solvent, and rate of cooling. Table 2 suggests the importance of polymorphism in the field of pharmaceutics [45],... 

Both thermodynamic and kinetic factors need to be considered. Take, for instance, acetic acid. The liquid contains mostly dimer but the crystal contains the catemer and no (polymorphic) dimer crystal has ever been obtained. Various computations (R. S. Payne, R. J. Roberts, R. C. Rowe, R. Docherty, Generation of crystal structures of acetic acid and its halogenated analogs , J. Comput. Chem, 1998, 19,1-20 W. T. M. Mooij, B. P. van Eijck, S. L. Price, P. Verwer, J. Kroon, Crystal structure predictions for acetic acid , J. Comput. Chem., 1998, 19, 459-474) show the relative stability of the dimer. Perhaps the dimer is not formed in the crystal because it is 0-dimensional and as such, not able to propagate so easily to the bulk crystal as say, the 1-dimensional catemer. 

Liquid crystal display technology, 15 113 Liquid crystalline cellulose, 5 384-386 cellulose esters, 5 418 Liquid crystalline conducting polymers (LCCPs), 7 523-524 Liquid crystalline compounds, 15 118 central linkages found in, 15 103 Liquid crystalline materials, 15 81-120 applications of, 15 113-117 availability and safety of, 15 118 in biological systems, 15 111-113 blue phases of, 15 96 bond orientational order of, 15 85 columnar phase of, 15 96 lyotropic liquid crystals, 15 98-101 orientational distribution function and order parameter of, 15 82-85 polymer liquid crystals, 15 107-111 polymorphism in, 15 101-102 positional distribution function and order parameter of, 15 85 structure-property relations in,... 

Holbrey, J. D., Reichert, W. M., Nieuwenhuyzen, M., Johnston, S., Seddon, K. R., and Rogers, R. D., Crystal polymorphism in l-butyl-3-methylimidazolium halides Supporting ionic liquid formation by inhibition of crystallization, Chem. Commun., 1636-1637, 2003. 

For liquid crystals of small molecules only the equilibrium transitions have received attention (left side of Fig. 3). In addition to the basic transitions indicated in Fig. 2, the polymorphic transitions have to be added which normally are also first order... 

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. 

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