Smectic mesophase/order

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]). 

PTEB-Q) to the annealed ones, owing to the presence of the crystalline phase. Moreover, the temperature of the peak increases with the annealing, as well as the broadness of the relaxation. These results suggest that the liquid crystalline phase gives raise to an a relaxation similar to that of amorphous polymers despite the existence of the two-dimensional order characteristic of smectic mesophases, and it changes following the same trend than that of semicrystalline polymers. 

In the smectic mesophases the molecules are oriented, as in a nematic mesophase, with their principal axis roughly parallel to the director, but they are also defining layers. These layers can be perpendicular to the director, as in the smectic A mesophase (SmA), or tilted, as in the smectic C (SmC). The SmA and SmC mesophases are the less ordered and more common smectic mesophases. Other less common types of smectic mesophases are known, which differ in the degree or kind of molecular ordering within and between the layers [2]. 

Preliminary room temperature x-ray data of 0.65 Me4C00-PECH indicates that the sample presents a highly ordered smectic mesophase which was not yet completely assigned. The textures seen by polarized optical microscopy are also typical of smectic phases. Due to the very high molecular weights involved, textures specific to mesophase in thermodynamic equilibrium could not be developed within a reasonable amount of time by annealing. 

Note 2 Although the total number of smectic mesophases cannot be specified, the following types have been defined SmA, SmB, SmC, SmF and SmI. The alphabetical order of suffixes merely indicates an order of discovery. 

Note 4 At one time, a number of mesophases were identified as smectic on the basis of their optical textures, but they are in fact soft crystals characterised by very low yield stresses. Hence, these three-dimensionally ordered phases should no longer be called smectic mesophases. They are akin to plastic crystals with some elementary long-range order and are referred to by the letters E, J, G, H, and K. 

Smectic mesophase involving a parallel arrangement of the molecules within layers in which the long axes of the molecules tend to be perpendicular to the layer planes and the molecular centers of mass have no long-range positional order parallel to the layer planes. Note 1 See Fig. 5 for the molecular organization in a smectic A mesophase Note 2 Each layer approximates to a true two-dimensional liquid. The system is optically uniaxial and the optic axis, Z, is normal to the layer planes. 

Smectic mesophase with in-plane short-range positional molecular order, weakly coupled two-dimensional layers and long-range bond orientational molecular order. 

Hexatic smectic mesophase in which the director is perpendicular to the layers with the long-range hexagonal bond-orientational order. 

Complexes with the simplest alkoxyphenylisocyanide and several halides are prepared by metathetical reactions of [AuCl(CNR)] with KX salts (Figure 7.19) [17]. The chloro-derivatives (n > 4) andthebromo-complexes (n > 6) display SmA phases. However, the ligands and the iodo-complexes are not liquid crystals. The transition temperatures decrease in the order Cl > Br > I, according to the decrease in polarity of the Au—X bond. It is important to note that the coordination of a very simple non-mesomorphic isocyanide (only one alkoxy chain and one aromatic ring) to Au—Cl allows the formation of a quite ordered and stable smectic mesophase. 

Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

Thus, polymers with mesogenic groups in side chains form structural mesophases of the same types as low-molecular liquid crystals. This makes it possible to apply traditional mesophase classification for the description of the structure of LC polymers. At the same time, the structure of some of comb-like polymers (see Table 5) considered as crystalline, may probably be treated as one of highly-ordered smectic mesophases (SH or Sj), whose study is only started74). 

FIG. 2.16 Schematic representation of the four main types of mesophases. Smectic with ordered (a) and unordered (a ) arrangement of the molecules in layers b) nematic c) cholesteric and d) discotic (from Plate and Shibaev (1987) Courtesy Plenum Press). 

II-6) (Table 6, entries 1 and 3). This indicates an enhanced stability, and consequently a higher state of order for the liquid crystalline phase. This is confirmed by the fact that polymers with a higher number of Z-double bonds (Table 6, entries 4 and 5) show a smectic mesophase. Smectic mesophases were also found in polymers with poly(acrylate) A, poly(methacrylate) B, poly(silox-ane) C, and poly(vinylcyclopropene) D backbones (Table 6 entries 13,15,17, 19). The isotropization temperatures of these polymers were approximately the same. 

Such a mobility does not exist in layered LC-main chain polymers (Fig. 1 IB). The mobility required for the definition and existence of a molten state results in LC-main-chain polymers exclusively from a gliding of chains along each other. Such a motion is only possible when the intermolecular forces between the mesogens are relatively weak. Therefore only smectic -A and smectic -C phases are true LC-phases. In contrast to small molecules smectic -B (and higher ordered smectic phases) are solid mesophases. The difference between a solid smectic mesophase and a smectic crystalline phase lies in the extent of the three dimensional order and is usually difficult to determine experimentally (see Sect. 7). 

Smectic mesophases have been claimed for palladium(n) 88a and 88b " carbene complexes based on 1,3-dialkylbenzimidazol-2-ylidene and l,3-dialkylimidazol-2-ylidene. However, examination of the X-ray diffraction data in combination with the large clearing enthalpies suggest that these materials are highly ordered and may be better described as crystals. 

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