Mesophases ordered structures

Most solid materials produce isotropic liquids directly upon melting. However, in some cases one or more intermediate phases are formed (called mesophases), where the material retains some ordered structure but already shows the mobility characteristic of a liquid. These materials are liquid crystal (LCs)(or mesogens) of the thermotropic type, and can display several transitions between phases at different temperatures crystal-crystal transition (between solid phases), melting point (solid to first mesophase transition), mesophase-mesophase transition (when several mesophases exist), and clearing point (last mesophase to isotropic liquid transition) [1]. Often the transitions are observed both upon heating and on cooling (enantiotropic transitions), but sometimes they appear only upon cooling (monotropic transitions). 

In recent studies, Friberg and co-workers (J, 2) showed that the 21 carbon dicarboxylic acid 5(6)-carboxyl-4-hexyl-2-cyclohexene-1-yl octanoic acid (C21-DA, see Figure 1) exhibited hydrotropic or solubilizing properties in the multicomponent system(s) sodium octanoate (decanoate)/n-octanol/C2i-DA aqueous disodium salt solutions. Hydrotropic action was observed in dilute solutions even at concentrations below the critical micelle concentration (CMC) of the alkanoate. Such action was also observed in concentrates containing pure nonionic and anionic surfactants and C21-DA salt. The function of the hydrotrope was to retard formation of a more ordered structure or mesophase (liquid crystalline phase). 

The property of mesophase that makes it suitable for carbon fiber and premium coke manufacture is that it forms ordered structures under stress which persist following carbonization. However, most carbon fiber production in the 1990s is based on polyacrylonitrile (PAN). 

Other methods of film formation discussed in this book depend on allowing a melt or a solution of the material to be deposited to spread on the substrate and subsequently to solidify. An ordered structure can sometimes be imposed on such a film by the application of an electric or magnetic field if the film is in a mesophase (otherwise known as a liquid crystal) before solidification. However, any such method presupposes that the melt or solution used will spread evenly over the substrate. It is thus important to understand a little about the conditions which allow a liquid to spread on a solid surface. This topic depends on the nature of intermolecular forces, a subject which is of general relevance to the formation of organic films and which is discussed in the following section. 

Beginning with a discussion of the necessary basic physics and chemistry, the book proceeds to a description of the main topics of current research in this field. The Langmuir-Blodgett technique, self-assembly, and methods of film deposition exploiting the ordered structure of mesophases are discussed. Separate chapters are devoted to the properties and computer modelling of both liquid crystals and films at the air/water interface. Order in biomemebranes is also discussed. 

Mesophases of supermolecular structure do not need a rigid mesogen in the constituent molecules. For many of these materials the cause of the liquid crystalline structure is an amphiphilic structure of the molecules. Different parts of the molecules are incompatible relative to each other and are kept in proximity only because of being linked by covalent chemical bonds. Some typical examples are certain block copolymers50 , soap micelles 51 and lipids52. The overall morphology of these substances is distinctly that of a mesophase, the constituent molecules may have, however, only little or no orientational order. The mesophase order is that of a molecular superstructure. 

To improve the effectiveness of the chromatographic separation, a comparison study has been carried out on cyclodextrin and liquid crystal stationary phases Both materials function as "ordered" media with cyclodextrins the inclusion complex formation predominates, whereas the liquid crystals enable interaction of compounds with the ordered structure of the mesophase ... 

Liquid crystals represent a transition between solid crystalline substances and isotropic liquids. On heating, mesophases are formed that have ordered structures which can be nematic, smectic or cholesteric. On further heating, the orientation is disturbed and the phases are converted into an isotropic liquid. The long structure of liquid crystals causes isomers with more drawn-out shapes to be readily dissolved in the ordered liquid crystal substrate ( mesophase ) thus yielding stronger sor-bat-sorbent interactions,... 

That is, the ordered structure of the cholesteric mesophase affects the formation of the traTO-adduct advantageously. Furthermore, the trans/cis product ratio depends significantly on the initial acenaphthylene concentration. In isotropic solutions, the dimerization of singlet-excited acenaphthylene molecules is known to yield exclusively the czv-adduct, whereas a mixture of cis- and traTO-adducts results from triplet-excited solute molecules. The lowering of cu-adduct production in the mesophase has been attributed to the enhanced efficiency of the triplet reaction in comparison with the singlet reaction, as shown by quantum yield measurements [732]. The increase in triplet reaction efficiencies has been ascribed to the increase in the fraction of acenaphthylene-acenaphthylene collisions which have coplanar or parallel-plane orientations with respect to the surrounding solvent molecules, and not to the increase in the total number of collisions per unit time [732]. See references [713, 732, 733] for a more detailed discussion of this photodimerization reaction. 

Hence, the tremendously high equilibrium rigidity and the ordered structure of para aromatic polyamides favouring the formation of mesophases in the concentrated polymer solutions and permitting their use for the manufacture of ultrahigh-modulus fibers is ensured by both the frans-structure of the amide groups and the para position of the phenyl rings. 

The complex with [Li+]/[PO] = 0.10 exhibits a crystalline melting transition followed by a smectic A mesophase. By increasing the salt concentration as in the case of complexes with [Li+]/[PO] = 0.15 0.30, the smectic A phase is suppressed instead, they exhibit a hexagonal columnar mesophase as evidenced by X-ray scattering. The induction of ordered structure in the melt state of the rod—coil molecule by complexation is most probably due to enhanced microphase separation between hydrophobic blocks and polypropylene oxide) block caused by transformation from a dipolar medium to an ionic medium in polypropylene oxide) coil. 

Zeolites were already employed as templates in the synthesis of microporous carbon with ordered structures.[247] The discovery of ordered mesoporous silica materials opened new opportunities in the synthesis of periodic carbon structures using the templating approach. By employing mesoporous silica structures as hard templates, ordered mesoporous carbon replicas have been synthesized from a nanocasting strategy. The synthesis is quite tedious and involves two main steps (i) Preparation and calcination of the silica mesophase, and (ii) filling the silica pore system by a carbon precursor, followed by the carbonization and selective removal of the silica framework. 

Efforts to make high-performance fibers and films from cellulosic mesophases have been made. For example, cellulose fibers produced from cellulosic mesophases show properties superior to those of commercially available fibers. Although these fibers are superior to commercial products, their physical properties are lower than theoretically predicted. This is in part because of that the ordered structures... 

Orientational order plays an important role in solid polymers. It is often induced by industrial processing, for example in fibers and injection- or compression-modulated parts. In polymers with liquid-crystalline properties of the melt or solution, the anisotropies generated by the flow pattern are particularly pronounced. In order to improve the mechanical properties of polymer fibers or films, the degree of orientation is intentionally enhanced by drawing. At the same time, anisotropy of mechanical properties can result in low tolerance to unfavourably directed loads. In many liquid-crystalline polymers, in the mesophase near the transition to the isotropic phase, electric or magnetic fields can induce macroscopic orientational order [1]. Natural polymers such as silk protein fibers, which are biosynthesized and spun under biological condition, also have good mechanical properties because of their ordered structure [2]. 

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