Three-Dimensional Smectic Phases

In three dimensions the situation is different, because there are interactions between the layers that may stabilize more ordered phases. Now the in-plane ordering and 

On the other hand, the experiments with very thin free-suspended films of smectics show that the crystalline order in certain substances with weak interlayer interactions may exist only in the surface layers [11]. In thick films the smectic layers are mostly liquid. However, within the same thin film one may observe the layer-by-layer crystallization. For example, the entire sequence of phase transitions SmA-SmBhex-SmBcr is shifted downward as one advances into the bulk from the 

Order/uniaxial phase SmA Hexatic-Bhex Crystalline-Bcr 

In-plane positional order liquid-like Liquid like Long-range 

In-plane bond orientation order exp(-r/y Long range Long range 

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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. 

Fig. 4.41 Structure of single non-polar (a) and polar (b) smectic layers formed by bent-shape molecules the longitudinal axes are aligned upright and the plane of the figure is mirror plane. Possible polar three-dimensional smectic biaxial phase (c)...

The principal physical difference between hexatic and three-dimensional crystal phases is their ability to support or not to support a shear parallel to smectic layers. Like a fiuid, the hexatic smectic Bh phase cannot support such a shear [8, 9]. The relevant order parameter... 

Figure 8.6 Three-dimensional slice of C2 symmetrical SmC phase, showing tilt cone, polar axis (congruent with twofold symmetry axis), smectic layer planes, tilt plane, and polar plane.

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

In any case, both models have in common that owing to the positional ordering of the mesogenic side chains, the polymer backbone no longer exhibits a statistical three dimensional coil conformation. Therefore at the phase transformation isotropic to smectic or nematic (cholesteric) to smectic, in addition to the change of the anisotropic packing of the side chains, the main chain has to change its conformation, which must be consistent with the layered smectic structure. A direct interaction... 

The second group involves polymers with three-dimensional ordering of side branches (e.g., those forming Mj-phaseXTable 5). On X-ray patterns of these polymers 3-4 narrow reflexes at wide angles are observed. As a rule, the authors define this type of structure as crystalline, or ascribe a smectic type of structure, characteristic for ordered smectics in SE or SH phases. The heats of transition from anisotropic state to isotropic melt are usually small and do not exceed the heats of transition smectic liquid crystal — isotropic melt . The similarity of structural parameters of three-dimensionally ordered smectics and that of crystalline polymers of the type here considered, make their correct identification quite a difficult task. 

A conglomerate in real liquid crystalline phases was first observed in the smectic phase of a rod-shaped mesogen with two stereogenic centers in its tail [42], We used a racemic mixture which was supposed not to electrically switch. Evidence for conglomerate formation was provided by clear electro-optic switching and texture observation under a polarizing microscope domains with stripes, which themselves display fine stripes. These stripes are tilted in two different directions with respect to the primary stripes. This is a still very rare example now that fluid soft matter is known to resolve spontaneously into a three-dimensional conglomerate. 

Within the family of untilted smectics, there is a hierarchy of phases with order ranging from that of smectic A, in which there is no positional order within each layer, to the crystalline smectics B and E, which have long-range positional order within each layer, namely hexagonal and orthorhombic for the B and E phases, respectively. This inplane order, represented by the dots within circles or ellipses of Fig. 10-27, propagates from layer to layer, producing fully three-dimensional crystalhne order. The family of tilted smectics includes similar ordered phases J, G, K, and H. 

New chiral phases, called smectic blue phases (BPSm), have been discovered in a small temperature range with the following phase sequence TGB-BPsml-BPsm2-BPsm3-Iso, without any intermediate chiral nematic phase between the BPSm and TGB phases [27,28]. X-ray scattering studies showed that unlike the traditional blue phases, the smectic blue phases possess the quasi-long-range smectic order in addition to three-dimensional orienta-... 

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). 

The question of whether a solid smectic phase should be labelled mesophase or crystalline phase is easy to answer from the viewpoint of a theoretical definition, but more difficult to answer by experimental techniques. When the mes-ogens in subsequent layers (i.e. in neighbouring two dimensional crystals ) possess exactly the same orientation in space, they meet the requirement of a three-dimensional long range order and such a layer structure may be called... 

Liquid crystals are broadly classified as nematic, cholesteric and smectic (I)- There are at least nine distinct smectic polytypes bearing the rather mundane labels smectic A, B, C,... I, by the chronological order of their discovery. Some of the smectics are actually three-dimensional solids and not distinct liquid-crystal phases at all. There are three t s of liquid crystals. Thermotropic liquid-crystal phases are those observed in pure compounds or homogeneous mixtures as the temperature is changed they are conventionally classified into nematic, cholesteric, and smectic phases in Fig.2. Lyotropic liquid-crystal phases are observed when amphiphilic molecules, such as soaps, are dissolved in a suitable solvent, usually water. Solutions of polymers also exhibit liquid-crystalline order, the polymeric phases. Most of our knowledge about liquid crystals is based on the thermotropic phases and much of this understanding can be transferred to elucidate polymeric and lyotropic phases. 

Cubic lattice with about 10 molecules per unit cell.< " The detailed molecular arrangement is not known, but is generally assumed to be of the micelle type. Thus this phase should probably be labelled as D rather than Smectic D. At present only four compounds are known to exhibit this phase, 4 -n-hexadecyloxy- and 4 -n-octadecyloxy-3 -nitrobiphenyl-4-carboxylic add and two similar acids with CN replacing NO. Interestingly, D occurs between S, and S, or between S and the isotropic phase. The manner in which such a structural rearrangement takes place is yet to be resolved. Three-dimensional crystal, orthorhombic with interlayer herringbone arrangement of the molecules. ... 

Meso phases can be recognized and evaluated by X-ray measurements. Three-dimensionally crystalline polymers have a meridian at the equator, and also exhibit sharp diffraction reflexes in all other directions (see Figure 5-6). Other polymers only give sharp diffraction reflexes at the meridian. Examples of this behavior are poly(trimethylene terephthalamide) and poly(oxydi-ethylene-4,4 -dibenzoate). Such behavior is caused by longitudinal order in the chain direction, and lateral disorder in both other dimensions the crystals have one-dimensional order and two-dimensional disorder, which is characteristic of nematic meso phases (Figure 5-29). Smectic and cholesterinic meso phases are other one-dimensional oriented ordered systems. 

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