Monolayer structures, smectics

We saw in Sect. 3.1.1 that for a symmetric dimer containing terminal alkyl chains to exhibit smectic behaviour then the terminal chain length must be greater than half the spacer length. This empirical rule effectively eliminates the possibility that the dimers form an intercalated structure simply because the terminal chains can only be accommodated within such a structure if the total length of the two terminal chains is equal to or less than the length of the spacer (see Fig. 16a). This view is supported by X-ray diffraction studies which reveal that for the overwhelming majority of symmetric dimers the smectic phases have a monolayer structure (see Fig. 16b). 

Figure 15. Heat capacity of 80PCB0B near the nematic to smectic A1 phase transition. The smooth curve represents a fit to the data with Eq. (6) based on critical parameters in agreement with the three-dimensional XY model. The index 1 in SmA refers to the monolayer structure of this frustrated smectic compound [65].

Figure 6. Bilayer and monolayer structures of the smectic A phase.

The study described above reflects a comparison for systems that would be expected to form monolayer structures in the smectic state. A similar comparison, however, can be made for systems that might exhibit bilayer smectic ordering. For example, the combination of a biphenyl core and a cyano terminal unit has been explored in depth because of its use in device applications [22]. Again one of the rings of the biphenyl unit can be systematically replaced by cyclohexyl [23] and bicyclooctyl [24] moieties. Figure 17 shows some comparative results for the 4-n-alkyl-4 -cyanobiphenyls, the trans-l- -alkyl-4-(4-cyanophenyl)cyclohexanes and the l-n-alkyl-4-(4-cyanophenyl)bicy-... 

SmAm-N [19], SmAd-N [19] and SmAa-N [22] transitions, where SmAm denotes the monolayer structure for non-polar compounds, while SmAd and SmAa imply the interdigitated bilay and the bilayer smectic structures respectively for polar compounds. For the SmAi-N transition, where SmAi denotes the monolayer smectic structure for polar liquid crystals, 3D-XY critical behaviour has been expected theoretically [23,24]. FIGURE 4 [25] shows a typical example of 3D-XY heat capacity behaviour determined by AC calorimetry for the compound 4-octyIoxyphenyl-4-(4-cyanobenzyloxy)-... 

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

L. The liquid-expanded, L phase is a two-dimensionally isotropic arrangement of amphiphiles. This is in the smectic A class of liquidlike in-plane structure. There is a continuing debate on how best to formulate an equation of state of the liquid-expanded monolayer. Such monolayers are fluid and coherent, yet the average intermolecular distance is much greater than for bulk liquids. A typical bulk liquid is perhaps 10% less dense than its corresponding solid state. 

The nematic phase of all the compounds CBn is characterized by a coherence length of about 1.4 times the elongated structure of the molecule. Based on this behaviour local associations in form of dimers with cyano-phenyl interactions were postulated. For the smectic A phase a partial bilayer arrangement of the molecules (SAd) is most likely. But there are also example for the smectic A phase with a monolayer (Sai) or a bilayer (Sa2) arrangement of the molecules as well as a commensurate structure A large number of X-ray measurements were carried out in the liquid crystalline state to clear up the structural richness and variability (see Chap. 2, this Vol. [52]). 

We have little information on the way low molecular weight molecules and oligomers adsorb (19). Apparently below DP s of about 100 they lie flat on the surface for concentrations up to a monolayer of segments, then seem to form thicker islands of smectic or nematic structure. Ordered condensed mono, -di, -or multi-layers are primarily the arrangements of smaller, especially amphipa-tic molecules on liquid-liquid interfaces. Polymers are too large to adsorb, in the ordinary sense, on micelles but segments of linear polymers may act as nucleation centers for micelles of small molecules which probably is one of the mechanisms for the lipid-, or detergent-, polymer interaction. 

If one makes use of the rather limited information available and given above one may infer that a tilt of between 20° and 30° is normal for straight chain azobenzene derivatives when deposited as LB films, even when a homeotropic phase exists. Such a structure can only be produced in a rather loosely packed film. At the moment it is an open question whether monolayers of these materials exist in the hexatic phase as is the case for fatty acids or whether the structure more nearly corresponds to the smectic-A phase. In the case of the birefringent phase described by Jones et al. [151] it was shown that, once this phase was established, further layers deposited by the LB technique go down in an epitaxial manner. 

In the presence of a suitable substrate, the resonant conjugated molecules are prone to precipitate as liquid crystals with the structure of the smectic phase. They form monolayers with the molecules aligned side-by-side. 

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