Disc-shaped amphiphiles

Research on liquid crystals with a disc-like shape started in 1977 [130] when hexa(hep-tanoyl)benzene (38, first synthesized forty years earlier [131]) was proved to form a columnar mesophase (m.p.=/tj, 81.2°C, cl.p.=/ci 87.0°C) [132]. Disc-shaped amphiphilic molecules with saturated cores, such as the hexaesters 39 or even the hexa-ethers 40 of the naturally occurring scyllo-inositol [9, 109, 133-137] form columnar liquid crystals much more easily with me-sophases more stable and very much wider in range than known for the mentioned benzene derivatives. 

The stability of the mesophase decreases drastically when the carbonyl functions of 39 are replaced by CH2-groups this formal 

an intramolecular contrast of hydrophilic and lipophilic parts is necessary for the occurrence of liquid crystallinity of such compounds, for which also a nearly perfect space filling of the substituents plays an important role [134, 137]. 

Peracylated sugar derivatives of type 41, studied independently by two groups [ 134, 138], possess the required amphiphilicity, but owing to the gap around the pyranosidic oxygen the space-filling of the molecular periphery by only five substituents is not optimal and complicates the molecular situation. On the one hand [138, 139], these compounds have been described as monotropically mesomorphic whereas on the other thermomes-omorphism was not observed [134]. 

The monotropic mesomorphism is described to be very durable and allow detailed studies, for example by polarizing microscopy, DSC, X-ray diffraction, and circular dichroism spectra [139]. 

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Note 5 If the mesogenic side-groups are rod-like (calamitic) in nature, the resulting polymer may, depending upon its detailed structure, exhibit any of the common types of calamitic mesophases nematic, chiral nematic or smectic. Side-on fixed SGPLC, however, are predominantly nematic or chiral nematic in character. Similarly, disc-shaped side-groups tend to promote discotic nematic or columnar mesophases while amphiphilic side-chains tend to promote amphiphilic or lyotropic mesophases. 

For ionic amphiphiles the first formed aggregates are closely spherical. At higher amphiphile concentrations there is a tendency for the formation of rod-shaped micelles168. Also the addition of salt favours the rod-shape aggregates33. It has been suggested that disc-shaped micelles also occur160 but experimental evidence in favor of this view has only been obtained for mixed micelles of lecithin and sodium cho-late179. ... 

Figure 28. Some disc-shaped model compounds (38-42) amphiphilic and mesomorphic in character (38-41) or monophilic and not liquid crystalline (42) [9, 109, 131-137] R = alkyl, R =-CO-alkyl, R =-0-alkyl or only alkyl.

The present research and development on LC display technology is conducted primarily in industrial labs. Academic research focuses mainly on more exciting and explorative topics that can not only stimulate fundamental scientific interest, but offer tremendous potential for innovative applications beyond the realm of displays, for example, new materials and attractive properties, and new uses in optics, nano/micromanipulation, novel composites, and biotechnology [7]. Future applications depend on the increase of complexity and functionality in LC materials and phases. The past three decades have seen the discovery of complex LC molecules with a variety of new shapes for instance, disc shape (Fig. 6.1b) [8], bent-core shape (Fig. 6.1c) [9], H shape (Fig. 6.1d) [10-13], board shape (Fig. 6.1e) [14,15], T shape (Fig. 6.1f) [16], cone shape (Fig. 6.1g) [17], and semicircular shape (Fig. 6.1h) [18]. The shapes of the molecules are not exactly associated with the types of mesophases formed. Like rod-shaped molecules, each complex shape is likely to organize a nematic, Sm, Col, and 3D-ordered mesophases [19,20]. The incorporation of functionality, amphiphilicity, and nano-segregation into these molecular shapes offers different ways to increase the complexity of LC phases. 

The plethora of liquid crystal structures and phases is categorized into two main classes thermotropic and lyotropic liquid crystals. While thermotropic liquid crystals are formed by, e.g., rod- or disc-shaped molecules in a certain temperature range, lyotropic liquid crystals are liquid crystalline solutions, built up by, e.g., aggregates of amphiphilic molecules in a certain concentration range. Many liquid crystal phases are found in thermotropic as well as in lyotropic systems. In some cases, however, the lyotropic analog of a thermotropic phase has never been observed. The probably most interesting of these missing link cases is the thermotropic chiral smectic C (SmC ) phase, which has become famous as the only spontaneously polarized, ferroelectric fluid in nature. 

Under just the right conditions, a mixture of a highly polar liquid, a slightly polar liquid, and an amphiphilic molecule form micelles that are not spherical. They can be rodlike, disc-like, or biaxial (all three axes of the micelles are different). These anisotropic micelles sometimes order in the solvent just as liquid crystal molecules order in thermotropic phases. There is a nematic phase of rod-shaped micelles, another nematic phase of disc-shaped micelles, and even a biaxial nematic phase, in which the molecular axes transverse to the long molecular axis partially order. Chiral versions of these phases with the same structure as the chiral nematic phase also form. 

While flattened vesicles are sometimes observed with the more hydrophobic amphiphiles, recently however we have discovered a route to disc shaped self assemblies (Fig. 11.6(a)) and found that branched polymers because of their lower hydrophilic head group area give rise to disc shaped self assemblies in the presence of cholesterol. The difference in hydrophilic head group area is also evidenced by the lower CMC values exhibited by the branched polymer (Fig. 11.6(b)). A lower hydrophilic head group area would favour molecular aggregation in aqueous media as stabilisation of the individual molecules in water via hydrogen bonding would be less likely. 

X. Qu et al.. Polymeric amphiphile branching leads to rare nano-disc shaped planar self assemblies. Langmuir, 24,9997-10004 (2008). 

Note 1 At one time it was thought that a non-amphiphilic molecule had to be long and rod-like for mesophase formation, but it has now been established that molecules of other types and shapes, for example, disc-like and banana-shaped molecules, may also form mesophases. (See ref. 6). 

The aggregates created by amphiphiles are usually spherical (as in the case of micelles), but may also be disc-like (bicelles), rodlike, or biaxial (all three micelle axes are distinct) (Zana, 2008). These anisotropic self-assembled nanostructures can then order themselves in much the same way as liquid crystals do, forming large-scale versions of all the thermotropic phases (such as a nematic phase of rod-shaped micelles). 

The solution behavior of low molecular weight amphiphilic molecules has been intensively investigated in the past (12-16) with respect to the formation of liquid crystalline phases. In very dilute aqueous solutions, the amphiphiles are molecularly dispersed dissolved. Above the critical micelle concentration (CMC), the amphiphiles associate and form micelles (Figure 4) of spherical, cylindrical or disc-like shape. The shape and dimension of the micelles, as a function of concentration and temperature, are determined by the "hydrophilic-hydrophobic" balance of the amphiphilic molecules. The formation of spherical aggregates is preferred with increasing volume fraction of the hydrophilic head group of the amphiphile, because the... 

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. 

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...

We have seen that thermotropic liquid crystal molecules that tend to form ID and 2D structures in the form of smectic and columnar liquid crystals have weakly amphiphilic anisometric (rod-, disc-, banana-, and bowl-shaped) molecules, where the rigid cores tend to avoid the flexible aliphatic chains. These... 

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