Smectic mesophase

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]). 

Among these three polybibenzoates, PTEB has a smectic mesophase stable during several days at any temperature below its isotropization point, although the transformation into a three-dimensional crystal can be attained by annealing at the appropriate temperatures, thus making it possible to analyze the effect of the thermal history on the dynamic mechanical relaxations of PTEB [27]. 

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]. 

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. 

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. 

Discotic liquid crystals arise from disk-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to the respective ealamitie mesophases, that is, the normals of the disks are oriented parallel. Instead of the smectic mesophases, diseotie columnar liquid crystals arise from eonnecting the disks to each other. The columns of the discotic columnar mesophase form a two-dimensional lattice whieh is in a hexagonal or rectangular modification. In addition, the columns may be tilted (Fig. 2f,g). 

The smectic mesophases of the thermotropic liquid crystals show a variety of textures but resemble mainly the fan-shape texture of the lyotropic hexagonal meso-phase. For further reading more comprehensive literature is recommended [11]. 

Note 1 There are several types of smectic mesophases, characterised by a variety of molecular arrangements within the layers. 

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. 

Note 5 Tilted smectic mesophases formed by chiral compounds or containing chiral mixtures are designated by the superindex (SmC, SmF, etc.). (See, for example. Definition 3.1.5.1.3.)... 

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. 

Note There are three types of hexatic smectic mesophases. smectic B (SmB), smectic F (SmF) and smectic I (SmI). Here, the term hexatic may be omitted because it is implicit for this group of smectic mesophases. 

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

Hexatic smectic mesophase the structure of which may be regarded as a C-centered monoclinic cell with a hexagonal packing of the molecules with the director tilted, with respect to the layer normals, towards the sides of the hexagons. 

Smectic mesophase that has a periodic in-plane density variation. 

Note 1 The recommended mark to designate a modulated smectic mesophase is a superior tilde ( ). 

Smectic mesophase that has a spacing between layers (smectic periodicity) of approximately one half of the molecular length. 

Note 1 The recommended subscript to designate an intercalated smectic mesophase is c Note 2 Intercalated smectic mesophases are commonly observed for liquid-crystal dimers. Note 3 At present intercalated smectic A (SmAc) and smectic C (SmCc) as well as intercalated crystal B (Be), G (Gc) and J (L) mesophases have been observed. 

Note 4 The local structure in the nematic mesophase of certain dimers exhibit an intercalated smectic mesophase. 

Note 3 Mesophases can also be induced when the free-volume between the large, irregular molecules of one component is filled by the smaller molecules of the second component. Such mesophases have been called filled smectic mesophases although the term induced is recommended. 

Note 2 For a smectic mesophase, the term monodomain also implies a uniform arrangement of the smectic layers. 

Ferroelectric mesophase that appears through the breaking of symmetry in a tilted smectic mesophase by the introduction of molecular chirality and, hence, mesophase chirality. 

Note 1 When the numbers of layers with opposite tilt directions are not the same, the smectic mesophase has ferroelectric properties. 

Note 7 When the tilt direction alternates from layer to layer, the smectic mesophase is antiferroelectric such mesophases do not possess spontaneous polarization. They can be turned into ferroelectric structures through the application of an electric field. 

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