Liquid crystalline structure

Both low molecular weight materials [145] and polymers [146,147] 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. One reason such liquid-crystalline elastomers are of particular interest is the fact that (i) they can be extensively deformed (as described for elastomers throughout this chapter), (ii) the deformation produces alignment of the chains, and (iii) alignment of the chains is central to the formation of liquid-crystalline phases. Because of fascinating properties related to their novel structures, liquid-crystalline elastomers have been the subject of numerous studies, as described in several detailed reviews [148-150]. The purpose here will be to mention some typical elastomers exhibiting liquid crystallinity, to describe some of their properties, and to provide interpretations of some of these properties in molecular terms. 

Many different structures Liquid crystalline phases Lamellar Hexagonal Reversed hexagonal... 

Liquid crystals exhibit a partially ordered state (anisotropic) which falls in-between the completely ordered solid state and completely disordered liquid state. It is sometimes referred to as the fourth state of matter . In recent years, interest in liquid crystalline thermosets (especially liquid crystalline epoxy) has increased tremendously [33-44]. If the liquid crystal epoxy is cured in the mesophase, the liquid crystalline superstructure is fixed permanently in the polymer network, even at higher temperature. Liquid crystal epoxies are prepared using a liquid crystal monomer [33-38] or by chemical modification of epoxy resin [43] which incorporates liquid crystal unit in the epoxy structure. Liquid crystalline epoxy resins with different types of mesogen such as benzaldehyde azine [33], binaphthyl ether [34, 35], phenyl ester [36, 37] and azomethine ethers [38, 39] have been reported. Depending on the chemical nature of the mesogen, the related epoxies display a wide range of thermomechanical properties. The resins can be cured chemically with an acid or amine [40, 41] or by photochemical curing in the presence of a photo-initiator [3]. Broer and co-workers [42] demonstrated the fabrication of uniaxially oriented nematic networks from a diepoxy monomer in the presence of a photo-initiator. 

The above method of stabilization (referred to as electrostatic stabilization) can be produced by the use of ionic surfactants (of the anionic, cationic, or zwitterionic type). However, for a number of reasons this method of stabilization is not ideal for personal care formulations. First, the stabilization is influenced by the presence of electrolytes in the system, which reduces repulsion and may cause instability. In addition, many ionic surfactants cause skin irritation as a result of their penetration and interaction with the stratum corneum (4). The latter is the main barrier to water loss and it consists of lipids that are organized in a bilayer structure (liquid crystalline), which at high water content is transparent and soft (5). Surfactants that interact with the lipid bilayer and reduce its liquid-like nature (by disrupting the liquid crystalline structure) may cause crystallization of the lipids, and this has a drastic effect on the appearance and smoothness of the skin ( dry skin feeling). 

Lamellar structure liquid crystalline polymer local geometry... 

At the beginning of the Gap the water content of the system is not enough to allow a definite structural liquid-crystalline configuration, but the structured entities, within the system, may be indeed oriented and spatially ordered by means of an impressed electric field. Therefore a Kerr-like effect can be observed, due to the optical anisotropy induced by the field. 

Sek D, Wolinska A and Janeczek H (1986) Structure-liquid crystalline properties relationship of polyesteramides, J Polym Mater 3 225-233. 

Because of their highly ordered structure, liquid crystalline mesophases give very sharp X-ray diffraction signals from which interlamellar distance d) can be calculated using the Bragg equation. The dimensions of the polar and apolar layers were calculated by Kunieda and coworkers [27,28]. The molar fractions of the surfactant (d),) and of its lipophilic chain (4>l) in such a system are given by... 

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