Liquid crystalline polymorphism

Liquid crystalline polymorphism is a frequent occurrence and two or more different LC phases may be exhibited following melting of the crystalline component, and prior to isotropization. It is clear that various combinations of phases may coexist and overlap, especially if phase transitions occur over a broad range of temperatures, as is often found in PLC systems. This last point is visualized in Figure 6.2, which illustrates the well known increase of phase transition temperatures... 

Ciuchi P, Di Nicola G, Pranz H et al (1994) Self-recognition and self-assembly of folic acid salts columnar liquid crystalline polymorphism and the column growth process. J Am Chem Soc 116 7064-7071... 

Alexandridis P, Olsson U and Lindman B 1997 Structural polymorphism of amphiphilic copolymers Six lyotropic liquid crystalline and two solution phases in a poly(oxybutylene)-poly(oxyethylene)-water-xylene system Langmuir 23-34... 

Phosphorus (like C and S) exists in many allotropic modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several amorphous or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P4 tetrahedral molecules, P-P 225 pm. The same molecular form exists in the gas phase (P-P 221pm), but at high temperatures (above 800°C) and low pressures P4 is in equilibrium with the diatomic form P=P (189.5 pm). At atmospheric pressure, dissociation of P4 into 2P2 reaches 50% at 1800°C and dissociation of P2 into 2P reaches 50% at 2800°. 

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. 

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... 

Ice I is one of at least nine polymorphic forms of ice. Ices II to VII are crystalline modifications of various types, formed at high pressures ice VIII is a low-temperature modification of ice VII. Many of these polymorphs exist metastably at liquid nitrogen temperature and atmospheric pressure, and hence it has been possible to study their structures without undue difficulty. In addition to these crystalline polymorphs, so-called vitreous ice has been found within the low-temperature field of ice I. It is not a polymorph, however, since it is a glass, i.e. a highly supercooled liquid. It is formed when water vapour condenses on surfaces cooled to below — 160°C. 

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,... 

With these three different examples it has been demonstrated that the systematics observed for the polymorphism of m-l.c. s is also valid for the side chain polymers, provided that a flexible spacer connects the rigid mesogenic moieties to the polymer main chain. Deviations from this behavior are observed, when the mesogenic moieties are directly linked to the backbone. Under these conditions, normally no liquid crystalline behavior is to be expected, according to the model considerations mentioned in Chap. 2.1. Some examples, however, proved l.c. properties for such systems, which are characterized by two striking properties Very high glass transition temperatures and only smectic structures even in case of short substituents... 

Sewell and co workers [145-148] have performed molecular dynamics simulations using the HMX model developed by Smith and Bharadwaj [142] to predict thermophysical and mechanical properties of HMX for use in mesoscale simulations of HMX-containing plastic-bonded explosives. Since much of the information needed for the mesoscale models cannot readily be obtained through experimental measurement, Menikoff and Sewell [145] demonstrate how information on HMX generated through molecular dynamics simulation supplement the available experimental information to provide the necessary data for the mesoscale models. The information generated from molecular dynamics simulations of HMX using the Smith and Bharadwaj model [142] includes shear viscosity, self-diffusion [146] and thermal conductivity [147] of liquid HMX. Sewell et al. have also assessed the validity of the HMX flexible model proposed by Smith and Bharadwaj in molecular dynamics studies of HMX crystalline polymorphs. 

Force field validation. In addition to ensuring that the force field reproduces results of QC calculations we have compared predictions of MD simulations using this force field with the available experimental data. Gas phase MD simulations using the quantum-chemistry based force field accurately reproduced the gas phase structure of DMNA as determined from electron diffraction studies. Liquid phase MD simulations of DMNA predicted the densities and solubility parameter as well as the activation energy and correlation times associated with molecular reorientation that are in good agreement with experimental data [34], As we will show in Section 4, comparison to structural and thermal data for the three pure crystalline polymorphs of HMX support the overall validity of our formulation and parameterization. 

Figure 22.6. (A) Temperature variation profile during, and (B) three-dimensional plots of the results obtained from, a time-resolved synchrotron X-ray diffraction experiment on SOS (1,3-distearoyl-2-oleoyl-.sn-glycerol) without annealing, a and (3 polymorphic forms LC1 liquid crystalline structure 1. (Reproduced with permission from Sato et al., 1999.)...

While only one paper describes the appearance of an anisotropic aqueous solution for a polymer of type D (19), polymers of type A to C have been investigated in more detail recently with respect to formation of liquid crystalline phases (20-23). In the following two sections, the association behavior in dilute isotropic solution and the liquid crystalline phase region is discussed on the basis of some experimental results. The dilute isotropic solutions are of interest with respect to the question whether polymers of type A and B form micelles similar to the corresponding monomeric amphiphiles. The liquid crystalline phase regime gives information whether the linkage of the amphiphiles via a polymer backbone influences the stability of the anisotropic phases and whether the same polymorphism occurs as is known for monomeric amphiphiles. 

Structure and Polymorphism of Lipopeptides. Amphiphilic lipopep-tldes Cjj(AA)p exhibit mesophases in aqueous solution for water concentrations smaller than about 60 %, The structure of the mesophases and of the dry lipopeptides obtained by evaporation of the mesophase water at a slow rate was determined by X-ray diffraction. Lipopeptides X-ray diagrams obtained are similar to those exhibited by classical amphiphiles (11). They have allowed us to establish the existence of three types of liquid-crystalline structures Isunel-lar, hexagonal and cubic. 

Membrane lipids are invariably polymorphic that is, they can exist in a variety of kinds of organized structures, especially when hydrated. The particular polymorphic form that predominates depends not only on the stmcture of the lipid molecule itself and on its degree of hydration, but also on such variables as temperature, pressure, ionic strength and pH (see References 11 and 12 and article Lipids, Phase Transitions of). However, under physiologically relevant conditions, most (but not all) membrane lipids exist in the lamellar or bilayer phase, usually in the lamellar liquid-crystalline phase but sometimes in the lamellar gel phase. It is not surprising, therefore, that the lamellar gel-to-liquid-crystalline or chain-melting phase transition has been the most intensively studied lipid phase transition... 

Lipids constitute a diverse and important group of biomolecules. Most lipids can behave as lyotropic liquid crystals. In the presence of water, they self-assemble in a variety of phases with different stmcture and geometry. The lipid polymorphic and mesomorphic behavior, i.e., their ability to form various ordered, crystalline, gel, or liquid-crystalline phases as a function of water content, temperature, and composition, is one of the most intriguing features of lipid-water systems. The mutual transformations between these phases and their physiologic implications are the subject of this article. 

The results and discussion presented here focus on two major points which are (i) the number of phases coexisting in the CB such as it is found in tempered chocolate and (ii) the liquid crystalline nature of the variety obtained from rapid quenching of CB. A more detailed reexamination of CB polymorphism is given in Reference 4. 

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