Liquid-crystalline molecules Subject

Molecularlv Doped Thermotropic Liquid Crystalline Polymer. The idea of the nonlinear optical medium which is the subject of this paper results from a synthesis of the ideas of the discussion above and a few concepts from nonlinear optical molecular and crystal physics. As discusssed several places in this volume, it is known that certain classes of molecules exhibit tremendously enhanced second-order... 

In recent years, the behaviour of liquid crystalline polymers including elastomers has been a subject of considerable interest 104,105). It is known that small molecule liquid crystals turn into a macroscopic ordered state by external electric or magnetic fields. A similar behaviour seems to occur for liquid-crystalline polymer networks under mechanical stress or strain. 

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

A sandwich-like liquid crystalline stmcture consisting of two large organic molecules with a liquid crystal of hydronium between them is the key to the electronic mechanisms of the nervous system. This stmcture is known as an Activa. Its characteristics and mode of operation is the subject of Chapter 8. 

In addition, these chromophores are very delicate molecules from an energy state perspective and should not be subject to detergents and other reactants such as sodium based complex salts. Although, it is possible (and actually easy) to return the chromophores of vision to the liquid crystalline state after extraction and purification, it is not easy to prevent conversion of the chromophores into their chromogens or similar retinoids. These decomposition products do not exhibit the resonance phenomena (and resulting spectral characteristics) no matter what state of matter they are in. 

Microscopic foam films are most successfully employed in the study of surface forces. Since such films are small it is possible to follow their formation at very low concentrations of the amphiphile molecules in the bulk solution. On the other hand, the small size permits studying the fluctuation phenomena in thin liquid films which play an important role in the binding energy of amphiphile molecules in the bilayer. In a bilayer film connected with the bulk phase, there appear fluctuation holes formed from vacancies (missing molecules) which depend on the difference in the chemical potential of the molecules in the film and the bulk phase. The bilayer black foam film subjected to different temperatures can be either in liquid-crystalline or gel state, each one being characterised by a respective binding energy. 

The cholesterol activity on lipid bilayers has been subjected to extensive studies. As described above, cholesterol belongs to the class of neutral (non-polar) lipids and can be characterized as an amphiphilic molecule having a hydroxyl group at the C-3 position and a non-polar region. Frequently, cholesterol is incorporated into lipid membranes and studies concerning the influence on the phase transition between gel and liquid crystalline phases of the lipid bilayers hav been published [26, 27]. 

Molecular conformation is an important subject of concern in liquid crystalline polymers. It is also the basic subject of polymer science. The mean square end-to-end distance or the radius of gyration is often used to characterize the size of a polymer molecule. According to the definition, the end-to-end distance is the sum of each rod s vectors in sequence... 

Surfactant molecules commonly self-assemble in water (or in oil). Even single-surfactant systems can display a quite remarkably rich variety of structures when parameters such as water content or temperature are varied. In dilute solution they form an isotropic solution phase consisting of micellar aggregates. At more concentrated surfactant-solvent systems, several isotropic and anisotropic liquid crystalline phases will be formed [2]. The phase behavior becomes even more intricate if an oil (such as an alkane or fluorinated hydrocarbon) is added to a water-surfactant binary system and the more so if other components (such as another surfactant or an alcohol) are also included [3], In such systems, emulsions, microemulsions, and lyotropic mesophases with different geometries may be formed. Indeed, the ability to form such association colloids is the feature that singles out surfactants within the broader group of amphiphiles [4]. No wonder surfactants phase behavior and microstructures have been the subject of intense and profound investigation over the course of recent decades. 

In the phrase liquid-crystalline, the crystalline adjective refers to the faa that these materials are sufSdentiy ordered to diffract an X-ray beam in a way analogous to that of normal crystalline materials. On the other hand, the liquid part specifies that there is frequently sufSdent disorder for the material to flow like a liquid. liquid crystals can be divided into thermotropic, that exhibit a phase transition with change of temperature, and lyotropic, that exhibit phase transition as a function of both temperature and concentration of the LC molecules in a solvent. Both low molecular wdght materials and polymers " can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides. It can also occur with flexible chains, however, and it is these flexible chains in the elastomeric state that are the focus of the present discussion. LC networks of flexible chains have the following three properties (1) they can be extensively deformed (as described for elastomers throughout this book), (2) the deformation produces alignment of the chains, and (3) alignment of the chains is central to the formation of LC phases. Elastomers of this type have been the subject of numerous studies, as described in several detailed reviews. ... 

Liquid crystals were first identified in 1888 ° in a molecule extracted from carrots. The first observed phase was the cholesteric or chiral nematic phase, whose shape is the subject of this chapter. Liquid crystals are made of molecules of particular shapes which, under certain conditions, align to form the phase shown in Figure 17.7. The first synthetic liquid crystals were manufactured in 1889. ° Liquid crystalline phases have been observed in many biopolymers. °... 

Finally, it is worth mentioning that polymers such as PTS-12 (].) and 3-BCMCJ (2) can be melted without decomposition the melts show liquid crystalline behaviour which disappears upon further increase in temperature, A sharp transition to the isotropic state has not been obseirved. It is recommended to do further research on this subject which could be relevant to the general area of polymeric thermotropic liquid crystals as well as to the further discussion of the temperature dependence of chain stiffness in PDA molecules. 

A relatively new class of polymers, the liquid-crystalline polymers, exhibits orientational order, i.e. alignment of molecules along a common director in the molten state. Liquid-crystalline polymers are used, after solidification, as strong and stiff engineering plastics and fibres. Functional liquid-crystalline polymers with unique electrical and optical properties are currently under development. The fundamental physical and rheological aspects of liquid-crystalline polymers are the third subject of this chapter (section 6.5). 

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