Rigid lyotropic liquid crystallinity

The use of ordered supramolecular assemblies, such as micelles, monolayers, vesicles, inverted micelles, and lyotropic liquid crystalline systems, allows for the controlled nucleation of inorganic materials on molecular templates with well-defined structure and surface chemistry. Poly(propyleneimine) dendrimers modified with long aliphatic chains are a new class of amphiphiles which display a variety of aggregation states due to their conformational flexibility [38]. In the presence of octadecylamine, poly(propyleneimine) dendrimers modified with long alkyl chains self-assemble to form remarkably rigid and well-defined aggregates. When the aggregate dispersion was injected into a supersaturated... 

It is now generally accepted that folding is universal for spontaneous, free crystallisation of flexible polymer chains. It was first of all found in crystallisation from very dilute solutions, but it is beyond doubt now, that also spherulites, the normal mode of crystallisation from the melt, are aggregates of platelike crystallites with folded chains, pervaded with amorphous material. "Extended chain crystallisation" only occurs under very special conditions in the case of flexible chains for rigid polymer chains it is the natural mode ("rigid rod-crystallisation" from the melt in case of thermotropic polymers, and from solution in case of the lyotropic liquid-crystalline polymers both of them show nematic ordering in the liquid state). 

Among melt-spun fibers, those based on thermotropic liquid-crystalline melts have the highest strength and rigidity reported to date, and appear comparable to polyamides spun from lyotropic liquids-crystalline solutions. This was a very active field of research in the 1970s and later, and many comonomers have been reported. Obviously, these compositions must contain three components at a minimum, but many have four or five com-... 

The rigid rod chains in para-aramids tend to form so-called liquid crystals when they are dissolved in polar solvents or heated to certain temperatures. The polymers showing liquid crystalline behavior in melts are called the thermotropic liquid crystalline polymers, and those showing similar properties in solution are called the lyotropic liquid crystalline polymers. These liquid crystals exist in three distinct phases according to their specific structures (Fig. 12.25). 

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

As their name implies, liquid crystals are materials whose structures and properties are intermediate between those of isotropic liquids and crystalline solids (2). They can be of two primary types. Thermotropic liquid crystalline phases are formed at temperatures intermediate between those at which the crystalline and isotropic liquid phases of a mesogenic compound exist. Substances which exhibit thermotropic phases are generally rod- or disc-like in shape, and contain flexible substituents attached to a relatively rigid molecular core. Lyotropic liquid crystalline phases are formed by amphiphilic molecules (e.g. surfactants) in the presence of small amounts of water or other polar solvent. In general, the constituent molecules in a liquid crystal possess orientational order reminiscent of that found in the crystalline phase, yet retain some degree of the fluidity associated with the isotropic liquid phase. 

The generally rigid, extended nature of metal polyynes is also evident in their tendency to form lyotropic liquid crystalline phases " " (see also Chapter 12.05 on the topic of organometallic mesogens). Homometallic polymers, as well as mixed metal systems, that contain Pt, Ni, and Pd, have all been shown to exhibit liquid... 

Rod-like macromolecules or semi-flexible chains such as cellulosics with a certain rigidity may form thermotropic and, in highly concentrated solution with suitable interactions, lyotropic liquid-crystalline phases. 

CNTs can be viewed as highly anisometric rigid rod-like particles. Like other anisotropic and ID molecule, CNTs can form lyotropic liquid crystalline phase... 

Macromolecular LCs were first observed in form of, lyotropic systems (7) and discussed in terms of segmental rigidity (8). Lyotropic liquid crystalline phases derive their anisotropy from interaction with a solvent. They are not further considered in this paper. When thermotropic, polymeric LCs that have an LC-to-isotropic phase transition at the isotropization temperature, T, were first mentioned in the scientific literature (P), it became of interest to find the similarities and differences to small-molecule LCs. 

Fig. 3.1 Building blocks of thermotropic and lyotropic liquid crystalline phases. The upper part of the figure shows two examples of typical thermotropic mesogens. Calamitic mesogens, such as terephthal-bis-(p-butylaniline) (TBBA) [2], can be represented by prolate ellipsoids or rigid rods, while discotic mesogens, such as benzene-hexa-n-octanoate (BH8) [4], are usually described by oblate ellipsoids or discs. The lower part of the figure shows the typical surfactant molecule sodium dodecyl sulfate (SDS), which forms lyotropic phases with water [5], Such a surfactant molecule is basically composed of a polar head group and a flexible hydrophobic tail. These amphiphilic molecules aggregate into different types of micelles, which are the actual mesogens of lyotropic liquid crystals. The shape of the micelles depends mainly on the solvent concentration...

The most rigid chains are those of ladder polyorganosilsesquioxanes. Together with esters of cellulose, these polymers are an example of the lyotropic liquid crystalline system which is formed by semirigid macromolecules. ITie structure of their solid mesophasese has not been studied sufficiently enough, therefore, at present it is not clear whether it can be considered as a disordered crystal or as a liquid crystalline phase. Further study of the structure of the mesophases in element-organic polymers and low molecular substances would undoubtedly be very useful for the development of the concept of disordered crystals. 

Competing tendencies in polymer solutions restrict the types of polymers for which lyotropic liquid crystalline phases can form. On the one hand, a rigid rodlike polymer will produce an ordered phase at a lower concentration than will a flexible polymer. On the other hand, a rigid polymer is generally less soluble than a flexible one, making it difficult to achieve even the reduced concentrations necessary to produce the ordered phase. 

The crystalline phases consist of stacked bilayers of alcohol molecules, which are either in a tilted (y) or perpendicular orientation (a and P) with respect to the plane of the bilayers. A similar stacking of bilayers is found in lamellar liquid crystals but in this case the structure is swollen in a one-dimensional fashion by water and/or oil. Depending on whether the surfactant and cosurfactant chains are in a conformationally disordered molten or an extended rigid state, the lyotropic liquid crystalline phase is called L or phase, respectively. The phase is also known as gel phase. This nomenclature is also used for lipids. Concerning the sur-factant/fatty alcohol/water systans considered here, it must be pointed out that the swelling of a crystals with water results in an Lp phase. 

It would thus be fcwmally possible to assign the described system, which contains a liquid-crystalline phase, to lyotropic liquid crystals. Actually, this system differs fiom true lyotropic liquid-crystalline systems with respect to the mechanism of the onset of the ordoed state since only a decrease in the temperature of the C-LC transition is involved in the rigid-chain polymer-solvent system due to addition of the solvent, and in this sense, the system does not differ from thomotropic liquid-crystalline systems formed in the pure polym with an increase in the temperature. In addition, let us examine the behavior of a system with a fixed concentration of solvent cwresponding to a composition of the system V2 (ho% and below, subscripts 1 and 2 in the volume and weight compositimis of the system refer to the solvent and polymer, respectively). An increase in the temperature to Tj results in the complete transition of the system into the liquid-crystalline state, which corresponds to the usual thermotropic transition, and the solvent does not play any specific role here except for decreasing the melting point of the crystalline phase. With a further increase in the tempaature, the same ttansitions LC -> LC +1 (at and LC +1 -> I (at as in ordinary thermotropic liquid-crystalline systems take place. 

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