Disc-like

In summary, a vast number of materials has been used to fonn LB films. However, in the majority of cases an effort to characterize the film stmcture or even to show that a regular layer stmcture has been achieved is lacking. Work on the stmcture of films of disc-like molecules such as porjDhyrins and phthalocyanines is especially limited. Some references can be found in [29]. 

Disc-like particles can also undergo an Onsager transition—here tire particles fonn a discotic nematic, where tire short particle axes tend to be oriented parallel to each other. In practice, clay suspensions tend to display sol-gel transitions, witliout a clear tendency towards nematic ordering (for instance, [22]). Using sterically stabilized platelets, an isotropic-nematic transition could be observed [119]. 

In an inert atmosphere, the decomposition at 573—623 K of uranyl(VI) oxalate [1101] obeys the Prout—Tompkins equation [eqn. (9)] with E = 261 4 kJ mole-1. The residual product is U02 and, under low pressure accumulatory conditions, the final CO2/CO ratio is 9. In air, the reaction proceeds in two stages. The initial process obeys the Prout—Tompkins equation and is identified as a surface reaction. Thereafter, decomposition fits the Avrami—Erofe ev equation [eqn. (6), n = 2], involving isolated disc-like grains of reactant, to yield amorphous U03 as the final product. Values of E for both stages of reaction are close to that found for reaction in the inert atmosphere ( 260 kJ mole-1) and comparable with theoretical predictions [88],... 

The liquid crystalline (LC) state is an intermediary state between isotropic liquid and crystalline solid. It arises from the tendency of rod like or disc like... 

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]. 

Fig. 10a,b. The distance dependence of the Gay-Berne potential for disc-like molecules... 

Since the discovery of discotic liquid crystals [121], the mesophases formed by rod-like and disc-like molecules have been considered as belonging to different liquid crystalline classes. Indeed, the conventional rod-like and disc-... 

The mesophases of thermotropic liquid crystals are described as calamitic if the constituent molecules are rod-like and columnar, if the constituent molecules, which often have a disc like shape(discotic), stack into columns. 

There are now three major shape classifications of low molar mass liquid crystals - rod-like (calamitic), disc-like (discotic) and bent-core. The last of these is the most recent, and while examples of bent mesogens have been known for some years, it is only since the mid-1990s that the area has attracted widespread attention [2],... 

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]. 

Probably the best example of the manifestation of self-assembly processes in dendritic systems via H-bonds is seen in the recent work of Zimmerman et al. [156]. Dendritic wedges possessing tetraacid moieties (67) self-assembling into a hexameric, disc-like framework (68) was confirmed by SEC and -NMR studies. The tetraacid unit (69) is known to form cyclic as well as linear aggregates in solution via carboxylic acid dimerization (Fig. 30). However, with incorporation of larger dendritic wedges on 69, the hexamer form is preferred. 

Steck E et al (2005) Induction of intervertebral disc-like cells from adult mesenchymal stem cells. Stem Cells 23(3) 403 f11... 

The Gould Belt is a structure consisting of clusters of massive stars. The Sun is situated not far from the center of that disc-like structure. The Gould Belt radius is about 300 pc. It is inclined at 18° with respect to the galactic plane. 

The amphipathic compounds shown in Scheme 2 can form a disc-like micelle(7). The shape of a molecular aggregate depends on the shape of the constituent molecules(8). For instance, conical molecules with large polar head groups prefer to form spherical micelles while cylindrical molecules tend to give flat aggregates. Trans-azobenzene is a rod-like molecule whereas the cis-form is bent. 

Note 3 The majority of mesogeuic groups consist of rigid rod-like or disc-like molecular moieties. 

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). 

Note 2 The adjective diseotic is also employed to describe the nematic mesophases formed by diseotic mesogens. The mesophases formed by a columnar stacking of disc-like molecules are described as columnar mesophases. 

Note 2 The symmetry and structure of a nematic mesophase formed from disc-like molecules is identical to that formed from rod-like molecules. It is recommended therefore, that the subscript D is removed from the symbol No , often used to denote a nematic formed from disc-like molecules. 

Note 6 Sanidic structures are analogous to the columnar mesophases formed by disc-like molecules. 

FIGURE 3.11 Different types of micellar aggregates (a) spherical (b) disc-like (c) cylindrical (d) lamellar (e) vesicular (schematic). 

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 term mesophase is used in reference to the liquid crystalline phase, and the molecular (sub)structures that give rise to this phase are termed mesogens. Macromolecules with rod-like or a flat, disc-like geometries tend to form such... 

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