Liquid crystalline phases anisotropic structures

It is not possible to predict from the related crystal structure alone whether the compound will melt to a liquid crystalline phase or not, because the anisotropic molecules (calamitic and discotic ones) form in favourable anisotropic packing. As a rule long shaped rod-like molecules quite often possess a layered arrangement in the solid state regardless of whether the compound is mesogenic or not. 

The detection of liquid crystal is based primarily on anisotropic optical properties. This means that a sample of this phase looks radiant when viewed against a light source placed between crossed polarizers. An isotropic solution is black under such conditions (Fig. 12). Optical microscopy may also detect the liquid crystal in an emulsion. The liquid crystal is conspicuous from its radiance in polarized light (Fig. 13). The structure of the liquid crystalline phase is also most easily identified by optical microscopy. Lamellar liquid crystals have a pattern of oil streaks and Maltese crosses (Fig. 14a), whereas ones with hexagonal arrays of cylinders give a different optical pattern (Fig. 14b). 

Not only can molecules constituting the oriented phase be thus studied. Solutes present in this mesophase will also be oriented through operation of anisotropic intermolecular forces. An example is that of deuterium-labeled D gramicidin. When dissolved in a nematic mesophase, it displays a series of deuterium doublets. Their residual splittings 6v are almost temperature-independent. This points to a rigid structure for the peptide, an helix that reorients about the director of the liquid crystalline phase (18). 

CB is 4-cyano-4 -pentylbiphenyl. Liquid-crystal phases can also form when amphiphilic molecules associate to form anisotropic structures which, in turn, spontaneously order into liquid-crystalline phases. 

A liquid crystal is a general term used to describe a variety of anisotropic structures formed by amphiphilic molecules, typically but not exclusively at high concentrations. Hexagonal, lamellar, and cubic phases are all examples of liquid crystalline phases. These phases have been examined as drug delivery systems because of their stability, broad solubilization potential, ability to delay the release of encapsulated drug, and, in the case of lamellar phases, their ability to form closed, spherical bilayer structures known as vesicles, which can entrap both hydrophobic and hydrophilic drug. This section will review SANS studies performed on all liquid crystalline phases, except vesicles, which will be considered separately. Vesicles will be considered separately because, with a few exceptions, generally mixed systems, vesicles (unlike the other liquid crystalline phases mentioned) do not form spontaneously upon dispersal of the surfactant in water and because there have been many more SANS studies performed on these systems. 

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. 

It is well-established that solutes incorporated in liquid crystalline phases are oriented in a manner and to an extent that depends primarily on structural similarities between the solute and the mesogen (2a-b,3-8). The rotational and diffusive mobility of both the solute and the mesogen is rendered strongly anisotropic, an effect that also correlates (for the former) with solute/mesogen structural similarities. 

In the cases of the hydrogen-bonded materials described in the previous sections, single homogeneous liquid-crystalline phases without phase separation are displayed by the formation of intermolecular hydrogen bonds. Here, liquid-crystalline physical gels, anisotropic functional materials with heterogeneous self-organized structures (Type B in Fig. 2), are discussed. 

Comilescu G et al (1998) Validation of protein structure from anisotropic carbonyl chemical shifts in a dilute liquid crystalline phase. J Am Chem Soc 120(27) 6836-6837... 

Optical microscopy is particularly useful for investigating various anisotropic features of polymeric systems. Polymers are intrinsically anisotropic objects and, when arranged in anisotropic structures such as crystalline and liquid crystalline phases, they display macroscopic optical anisotropy. These are investigated using a polarizing microscope, i.e., under crossed polarizers. 

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 past few years a considerable number of papers were published which were concerned with liquid crystalline structures in polymeric systems. Different routes were employed to obtain polymers with liquid crystalline structures or even thermodynamically stable liquid crystalline phases 1,, In general monomers containing mesogenic groups - groups which are known to have a tendency towards the formation of liquid crystalline structures, or rigid groups were used. Cases are known where the monomers exhibit liquid crystalline phases Z) In that case the polymerization can be performed in anisotropic melts frozen-in liquid crystalline structures and textures can be obtained in many instances i . In other cases the monomers do not display liquid crystalline phases. The formation of liquid crystalline polymer structures may nevertheless be possible due to the restriction of the motions of the individual repeat units -3),... 

The papers presented in this symposium give some indication of the wide variety of polymers which are now known to form liquid crystalline phases Polymeric liquid crystals are usually classified according to the mesophase structure e g., nematic, cholesteric, smectic A, etc ). However, these classes are quite broad For example, the cholesteric lyotropic phases formed by synthetic polypeptides in suitable solvents differ markedly from the cholesteric thermotropic phases formed from silicone polymers with cho-lesteryl ester side chains. In particular, the driving forces behind the formation of the mesophases are quite different for these two examples, being essentially due to chain stiffness in the first case and to anisotropic dispersion force interactions in the second case It may therefore be useful to classify polymeric liquid crystals according to the polymer chain structure ... 

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