Mesophase examples

Packing parameter Micellar shape Surfactant Mesophase example... 

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

The importance of unsaturation is illustrated by the fact that 2,4-nonadienoic acid [21643-39-0] forms a Hquid crystal phase, whereas the aHphatic carboxyHc acids do not. The two double bonds enhance the polarizabiHty of the molecule and bring iatermolecular attractions to a level that is suitable for mesophase formation. The overall linearity of the molecule must not be sacrificed ia poteatial Hquid crystal candidates. For example, whereas /n j - -aIkoxyciaaamic acids (5) are mesomorphic, the cis isomers (6) are not, a reflection of the greater anisotropy of the trans isomer. 

A variety of aromatic and extended-chain polyamides that spontaneously form a mesophase ia coaceatrated solutioas also have beea syathesized (30). Polybeozamide [24991-08-0] with the foUowiag repeat uoit, is an example. 

The number of examples of Uquid crystalline systems is limited. A simple discotic system, hexapentyloxytriphenylene (17) (Fig. 4), has been studied for its hole mobUity (24). These molecules show a crystalline to mesophase transition at 69°C and a mesophase to isotropic phase transition at 122°C (25). 

Martin [25] has also shown that ammonium salts display similar behavior. [Cetyltrimethylammonium]2[ZnCl4], for example, first melts to an Sc-type liquid crystal at 70 °C and then to an S -type mesophase at 160 °C. The broad diffraction features observed in the liquid-crystalline phases are similar to those seen in the original crystal phase and show the retention on melting of some of the order originating from the initial crystal, as shown in Figure 4.1-6. 

Other examples include ditholium salts, shown in Figure 4.1-8 [30]. The scattering data show that a range of mesophase behavior is present, dependent - as with the metal-containing systems - on alkyl chain length. 

Kuznetsov et al. s methodological approach [72-75] provides another example of attempts to evalue the interphase thickness experimentally. Their approach was based on the hypothesis that the mesophase remains glassy while the bulk of the binder has already passed to the highly elastic state. Investigating the concentration... 

This relation indicates that for hard-core composites, where always it is valid that > fiz. rmin is always outside the mesophase annulus, as it was indicated in the examples of Refs. 17 and 23 ... 

One type of material that has transformed electronic displays is neither a solid nor a liquid, but something intermediate between the two. Liquid crystals are substances that flow like viscous liquids, but their molecules lie in a moderately orderly array, like those in a crystal. They are examples of a mesophase, an intermediate state of matter with the fluidity of a liquid and some of the molecular order of a solid. Liquid crystalline materials are finding many applications in the electronics industry because they are responsive to changes in temperature and electric fields. 

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

Lyotropic LCs can also be described by a simple model. Such molecules usually possess the amphiphilic nature characteristic of surfactant, consisting of a polar head and one or several aliphatic chains. A representative example is sodium stearate (soap), which forms mesophases in aqueous solutions (Figure 8.4a). In lyotropic mesophases, not only does temperature play an important role, but also the solvent, the number of components in the solution and their concentration. Depending on these factors, different types of micelles can be formed. Three representative types of micelles are presented in Figure 8.4b-d. 

Given that the presence of three alkoxy chains in the phenyl group produces such a dramatic change in the properties of the material to the point that columnar mesophases are formed at room temperature, the structure of the aryl isocyanide ligand has been further modified to introduce more paraffinic chains, and examples of metallodendrimers containing monodendrons with an isocyanide group in the focal point, and its gold compound 9, have been reported [26]. 

An example of ionic mesomorphic imidazolium cyanoaurate derivative (32) has been prepared displaying a SmA mesophase in the range of 66-112 °C [61]. 

The nematic nanoparticies have been prepared by a two step synthetic process. First, gold nanopartides are covered with an alkylthiol monolayer (hexyl- and dodecylthiol) in a second step, the alkylthiol-nanoparticles are reacted with the functionalized thiol mesogen in dichloromethane at room temperature to obtain the monolayer-protected liquid crystal gold nanopartides. These materials are chemically stable and display a nematic mesophase at room temperature [67, 68]. Other examples include liquid crystal gold nanopartides functionalized by hexaalkoxy-substituted triphenylene [69]. 

The interference pattern depends both on the symmetry of the liquid crystal mesophase and on the arrangement of the molecules between the glass cover slips. Three examples are given in Fig. 8. 

For example, Marder and co-workers [10] (among others) have studied liquid crystal mesophases induced by the presence of quadrupolar interactions between non-mesomorphic phenyl- and perfluorophenyl-containing moieties (e.g. 1). 

Quadrupolar interactions (termed complementary polytopic interactions by the authors) are also responsible for mesophase induction and modification in a series of disc-like materials (e.g. 2 and 3) as described by, for example, Bushby and co-workers [11-13]. 

A different mechanism is that of charge transfer, of which there are many examples based, in particular, on metallomesogens [14]. In these cases, mesophases can be induced by adding the electron-poor TNF (2,4,7-trinitro-9-fluorenone) and Fig. 9 shows Pd mesogen 4, which shows a particular type of SmA phase when TNF is added [15]. 

More exotic systems have also been described and, for example, Lehn and co-workers described the complementary system, 6, which was found to exhibit a columnar mesophase [18], while neither of the components was mesomorphic. 

Polymers formed between a and c, d and e all failed to show any liquid-crystalline behaviour. However, for all a examined (m = 2,4,6 and 8), nematic phases were observed with b-4 (all monotropic) - a further monotropic nematic material was the copolymer of a-6 and b-3. Unidentified crystal smectic mesophases were reported for a further three examples. 

A number of systems which in polymer literature are normally referred to as mesophases are obtained under kinetic control. Examples are the smectic phase of isotactic polypropylene [18,19], mesomorphic syndiotac-tic polypropylene [20-22], mesomorphic PET [23,24], and other instances where intermediate degrees of order result after quenching polymers from the melt to temperatures often close to Tg. In these cases disorder is plausibly more static than in bundles close to T0 and these phases usually crystallize upon heating to an appropriate temperature in the stable crystal phases. 

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