Lyotropic polymorphism

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... 

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... 

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,... 

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. 

Amphiphilic lipopeptides with a hydrophobic paraffinic chain containing from 12 to 18 carbon atoms and a hydrophilic peptidic chain exhibit lyotropic meso-phases and good emulsifying properties. The X-ray diffraction study of the mesophases and of dry lipopeptides showed the existence of three types of mesomorphic structures lamellar, cylindrical hexagonal and body-centred cubic. Two types of polymorphism were also identified one as a function of the length of the peptidic chain and the other as a function of the water content of the mesophases. The emulsifying properties of the lipopeptides in numerous pairs of immiscible liquids such as water/ hydrocarbons and water/base products of the cosmetic industry showed that small amounts of lipopeptides easily give three types of emulsions simple emulsions, miniemulsions and microemulsions. 

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. 

If then we abandon the standard three-dimensional Euclidean perspective and adopt this non-Euclidean two-dimensional view, it can be seen that stable polymorphs are characterised by a global geometric constraint surface density 2"1, and a local constraint Gaussian curvature, . We shall see in Chapter 4 that this description is identical to one that accounts for the mesophase behaviour of lyotropic liquid crystals in amphiphile-water mixtures. 

Garti N, Libster D, Aserin A. Lipid polymorphism in lyotropic liquid crystals for triggered release of... 

Polymorphs Modification I (PBA-LiCl3-DMAc) Modification II (PBA-LiCl2 cocrystal) Modification III (PBA) Lyotropic nematic (26)... 

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. 

As there is a lack of detailed phase diagrams exhibiting lyotropic phase chirality it may be sufficient to discuss briefly the polymorphism of two frequently used achiral systems. The first example is presented by the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) in a binary mix-... 

Lamellar and hexagonal polymorphs are the most commonly reported mesophases in lyotropic liquid crystals, usually identified from their scattering spectra. 

Most mesogenic salts derived from aliphatic acids, (R-COO) M (R = alkyl (normal and branched) or alkenyl M=Li, Na, K, Rb, Cs, NH4, Tl, Pb or other metal) form layered structures (lamellar phases, neat phases) that are similar to smectic A phases. However, mesogenic salts form double layers and are not miscible with smectic A phases [276]. Some of the materials show very complicated polymorphism with a large number of mesophases [276-279]. In general, the transition temperatures of the salts are quite high compared with those of nonpolar liquid crystals. Most of the salts can also form lyotropic liquid crystals. 

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... 

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