Structure of the Smectic I Phase

The molecules in the smectic I phase, like the smectic C and hexatic B phases, are expected to be in dynamic motion about their long axes, presumably on a similar time scale. The rotation is expected to be of a cooperative nature as the molecular centres are separated by only 0.4-0.5 nm. 

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The structure of the smectic A phase when it is composed of optically active material (i.e., smectic A ) remains the same as that for the achiral phase. The molecules are arranged in diffuse disordered layers, and there is no long-range periodic order. However, because of the molecular chirality, the environmental symmetry is reduced to [10]. As a consequence, when an electric field is applied to a chiral smectic A= phase there will be a coupling of the electroclinic susceptibility to the field and the long axes of the molecules will tilt with respect to the layer planes. The tilt angle, for relatively low applied fields, varies linearly with the field. This linear electrooptic phenomenon is called the electroclinic effect. 

In addition to the homopolyesters 80a-i, 81a-i and 82a-i, three classes of co PEIs were studied 83a-g [24], 84a-d and 85a-f [84]. The combination of two different alkane spacers did not significantly change the properties of the co PEIs 83a-d, when compared to 82i. However, when the difference on their length increased, a destabilization of the smectic layer-structures became evident, with the consequence that a nematic phase was formed on top of the smectic-C phase... 

Figure 5 Photomicrographs of liquid crystal textures seen in the polarizing microscope. (A) The Schlieren texture of a smectic I phase. (B) The focal conic texture of a chiral smectic C phase, which has a helical structure, forming at a transition from the liquid. The pitch of the helix shows up as parallel lines that are parallel to the molecule layers (each line corresponds to about a thousand molecular layers). The pitch lines reflect accurately the layer structure in the focal-conic domain. (A) Courtesy of JW Goodby, University of Hull, UK, with permission. (B) Reproduced with permission from Gordon and Breach, Switzerland.)...

Smectic liquid crystalline polymers have more ordered structures than nematic liquid crystalline polymers, as their molecular arrangements have not only long-range orientational order, but also positional order. The positional order refers to the layer packing structures of the polymers. The less ordered smectic liquid crystals, such as smectic A, are true one-dimensional crystals. The packing structure of the smectic A is illustrated in Figure 5.6. The smectic A phase can be considered as convolution of a layer of two-dimensional liquid, i.e., a layer of randomly packed hard rods that are uniaxially oriented in the direction of the layer normal, and a one-dimensional lattice as shown in Figure 5.7. 

In Section 5.7.2 we discussed a general problem of stability of one, two- and three-dimensional phases. Here, we shall analyze stability of the smectic A liquid crystal, which is three-dimensional structure with one-dimensional periodicity. The question of stability is tightly related to the elastic properties of the smectic A phase. Consider a stack of smectic layers (each of thickness Z) with their normal along the z-direction. The size of the sample along z is L, along x and y it is L, the volume is V = Lj L. Fluctuations of layer displacement u(r) = u(z, r i) along z and in bofli directions perpendicular to z can be expanded in the Fourier series with wavevec-tors q and q (normal modes) ... 

On a macroscopic scale, the spontaneous polarization vector in the optically active phase spirals about an axis perpendicular to the smectic layers (Fig. 20), and sums to zero. This macroscopic cancellation of the polarization vectors can be avoided if the helical structure is unwound by surface forces, by an applied field, or by pitch compensation with an oppositely handed dopant. The surface stabilized ferroelectric liquid crystal display utilizes this structure and uses coupling between the electric field and the spontaneous polarization of the smectic C phase. The device uses a smectic C liquid crystal material in the so-called bookshelf structure shown in Fig. 21a. This device structure was fabricated by shearing thin (about 2 i,m) layers of liquid crystal in the... 

A sub-phase of the smectic C phase also exists which is called (for the moment) the alternating smectic C (SmCai,) phase [55-57]. This phase was originally discovered by Levelut et al. [55] and given the code letter smectic O. However, the chiral version of this phase was labelled as being an antiferroelectric smectic C phase by Fuku-da [58], and it is this descriptor that is in current, general use. Consequently, the achiral modification of the antiferroelectric phase requires a matching code letter, and therefore, for the present we have opted to use smectic C i, as this term best describes the structure of the phase in relation to the antiferroelectric label. 

To determine whether the 8CB droplets condensed above 41°C (trapped in the isotropic phase) sit on a trilayer or on bare silicon, we used the ATM tip to mechanically spread the droplets and thus accelerate their conversion to a stable configuration. The SPFM images shown in Fignre 15 were obtained after such tip-induced spreading. A layered structure with 32-A-high steps typical of the smectic phase is obtained. The first, or bottom, layer is 41 A thick, while the layers above it are all 32 A thick. This indicates that the bottom layer of the film is a trilayer and that the remaining snbstrate is dry silicon, i.e.. 

While the existence of the anticlinic minimum in conventional SmC materials has been suggested by experiments with DOBAMBC,17 by far the majority of tilted smectics only exhibit the synclinic structure of the SmC or SmC under normal conditions. When the mesogenic compound is unichiral (i.e., enantiomerically pure) or enantiomerically enriched, a macroscopic polarization with the same orientation in each layer is produced, as discussed above. In the SmCA phase of MHPOBC, however, the anticlinic structure is the global minimum structure, as shown at the bottom of Figure 8.9. In this case, given that the chirality of the molecules is fixed, the polarization must alternate... 

Further studies by Nishiyama et al. [34-45] showed that when taken in isolation, only one of the aromatic units within a supermolecular system has a propensity to exhibit liquid crystal phases, then the supermolecular material itself could be mesomorphic, see Fig. 5. For example, for the top molecular structure, 5 [45], in Fig. 5, only the biphenyl unit at the center of the structure supports mesophase formation, whereas the benzoate units are too isolated from the biphenyl moiety in order to affect mesomorphic behavior. The second material, 6 [45] has terminal phenyl units, which are only connected by aliphatic chains to the benzoate units. Thus in this case, the material has four aromatic units out of six which are not in positions that can enhance mesophase formation. However, the second material has similar transition temperatures and phase sequences to the first, i.e., both materials exhibit an unidentified smectic phase and a synclinic ferroelectric smectic C phase. If the third material, 7 [38], is examined, it can be seen that the mesogenic unit at the center of the supermolecule is an azobenzene unit which is more strongly supportive of mesophase behavior than the simple biphenyl moiety. Thus the clearing point is higher for this material in comparison to the other two. The attachment of the terminal phenyl unit is by a methylene spacer of odd parity, and as a consequence the smectic C phase has an anticlinic structure rather than synclinic. 

The smectic order parameter provides a quantitative measure of the onedimensional translational order, which is a characteristic of the smectic phase. In Fig. 30, we show the evolution of the average smectic order parameter I/ of the inherent structures with temperature. A steady increase in with the concomitant growth of S for the underlying inherent structures is apparent across the nematic phase. 

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