Polymorphism in Thermotropic Liquid Crystals

Demus, D.; Diele, Siegmar; Grande, S.; Sackmann, H.

doi:10.1016/b978-0-12-025006-6.50007-3

Cited by 74 publications

(10 citation statements)

References 256 publications

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“…Smectic liquid crystals may be classified as structured or unstructured phases depending on the molecular order within a layer. A comprehensive study of polymorphism for this kind of liquid crystals was given by Demus et al (1983). Idziak et al, (1994) used a Couette shear cell with concentric cylinders which are transparent to x-rays to determine the orientation of a smectic-A liquid crystal as a function of the shear rate.…”

Section: Smectic Liquid Crystalsmentioning

confidence: 99%

Rheological and phase behaviour of amphiphilic lipids

Muñoz

Alfaro

2000

Grasas y Aceites

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RESUMEN Comportamiento reológico y de fase de lípidos anfifílicosEn este capítulo se realiza una revisión de las distintas estructuras coloidales de asociación que pueden formar los lípidos anfifílicos en estado líquido-cristalino y de sus correspondientes propiedades de flujo en cizalla. Se describe la estructura y comportamiento reológico de cristales líquidos termotrópicos, con énfasis en los de tipo esméctico, fases gel, y cristales líquidos liotrópicos: nemáticos, laminares, laminares diluidos, dispersiones de laminares, hexagonales y cúbicos. Se hace hincapié en la importancia de una buena caracterización reológica y reo-óptica de cara a sus aplicaciones prácticas, desarrollo de formulaciones y como una técnica útil para determinar diagramas de fases y detectar cambios de estructura. Se ha usado una metodología histórica para analizar la evolución de la caracterización reológica e identificación de las diferentes estructuras líquido-cristalinas y estructuras afines. La interpretación de comportamientos de flujo viscoso no newtonianos, fenómenos tixotrópicos y antitixotrópicos y de las propiedades viscoelásticas lineales, determinadas con ensayos dinámicos y de fluencia, y no lineales, descritas mediante la diferencia de esfuerzos normales y ensayos de relajación del esfuerzo, se basa en la interrelación microestructuracomportamiento reológico. La naturaleza policristalina de los cristales líquidos resulta ser muy sensible a la cizalla, debido al cambio de tamaño y orientación de los dominios líquido-cristalinos mientras fluyen. PALABRAS-CLAVE: Comportamiento de fase Comportamiento reológico Lípidos anfifílicos. SUMMARY Rheological and phase behaviour of amphiphilic lipidsThis chapter reviews the different association structures which are likely to be formed by amphiphilic lipids in the liquid-crystalline state and their corresponding shear flow properties. The structure and rheological behaviour of thermotropic liquid crystals, emphasizing the properties of smectic mesophases, and those of lyotropic liquid crystals such as: nematic, lamellar, diluted lamellar, lamellar dispersions, hexagonal and cubic mesophases are described. The importance of a comprehensive rheological characterisation, including rheo-optical techniques, is pointed out for their practical applications, development of formulations and as a useful technique to assist in the determination of phase diagrams. A historical approach has been used to discuss the evolving field of the rheology and structure identification of liquid crystals formed by amphiphilic lipids and surfactants. Non-Newtonian viscous shear flow, thixotropic and antithixotropic phenomena, linear viscoelastic properties -described by dynamic and creep compliance experiments-and non-linear viscoelastic properties -described by the difference of normal stresses and stress relaxation tests are interpreted on the basis of a microstructure-rheology relationship. The polycrystalline nature of liquid crystals turns out to be rather sensitive to shear due to the change of both size...

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Section: Smectic Liquid Crystalsmentioning

confidence: 99%

Rheological and phase behaviour of amphiphilic lipids

Muñoz

Alfaro

2000

Grasas y Aceites

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“…In this way, transition enthalpies of 10 mJ/g can be detected (Fig. 1, see also [1,8], demonstrating the high DSC sensitivity, comparable with that of the best adiabatic instruments [9], which require a three orders of magnitude higher expenditure of substance and time and have a one order of magnitude better temperature accuracy.…”

Section: Special Investigationsmentioning

confidence: 82%

“…First "weak" transitions should be treated. Examples of such transitions with small enthalpies are smectic G -smectic F and cholesteric-blue phase, first observed optically, or smectic A -smectic A and the restacking transitions between different smectic B phases, first found by means of X-ray analysis [1,8]. Heavy samples of up to 30 mg of well-purified material have to be measured in a temperature interval of 20 deg on both sides of the expected point.…”

Section: Special Investigationsmentioning

confidence: 99%

“…scopically observed textures, through conclusions from homologous substances, and mainly from miscibility investigations with phases of reference materials, a complicated and time-consuming procedure, also supported by X-ray analysis [1,4]. The precise determination of transition temperatures then follows.…”

Section: Special Investigationsmentioning

confidence: 99%

“…In spite of the advances in theoretical knowledge about the relations between the chemical structures and physical properties of liquid crystals [1], in investigations of new materials of technical importance empirical research methods are still in use, and many substances are developed which have to be characterized as quickly as possible. The main methods, microscopy and microcalorimetry, are well able to yield information on purity, chemical stability, and stable and metastable phases in the solid and liquid states, etc.…”

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Methodical aspects in thermal analysis of liquid crystalline substances

Wiegeleben

Demus

1988

Journal of Thermal Analysis

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On the basis of experience in liquid crystal research, the optimum conditions are presented for the use of thermo-microseopy and microealorimetry (DSC) in preliminary investigations of the general chemical, thermodynamic and kinetic behaviour. In the second step, special measurements are necessary to obtain exact results on purities, phase types, temperatures, enthalpies, and kinetic parameters of transitions in stable and metastable states.

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Liquid Crystals

Sage

2000

Ullmann's Encyclopedia of Industrial Chemistry

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Polymorphism in Thermotropic Liquid Crystals

Cited by 74 publications

References 256 publications

Rheological and phase behaviour of amphiphilic lipids

Rheological and phase behaviour of amphiphilic lipids

Methodical aspects in thermal analysis of liquid crystalline substances

Liquid Crystals

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