Structures ordered smectic phases

X-ray measurements have shown that the positional and orientational ordering in highly ordered crystalline smectic phases is more comparable with the ordering in the solid state than the structures of the fluid smectic phases and the nematic phase. Therefore, a comparison of the structures of crystals and highly ordered smectic phases is possible because of their closer similarities. Up to now, only the crystal structures of few mesogenic compounds exhibiting highly ordered smectic phases have been reported in... 

That is why here we describe two groups of polymers- those. forming an ordered smectic phase (of SB type) and polymers with mesogenic groups, for which the formation of crystalline structure is proposed (Table 5). Sometimes it is difficult to distinguish between crystalline and LC states of ordered smectic phases. Thus, the interpretation of data on crystalline phases of some polymers, listed in Table 5, is also possible from the viewpoint of smectic polymorphism. 

Some liquid crystals form one or more smectic phases. These display a variety of microscopic structures that are indicated by the letters A, B, C, and so forth. Figure 23.9c shows one of them, the smectic A structure the molecules continue to display net orientational ordering, but now, unlike in the nematic phase, the centers of the molecules also tend to lie in layers. Within each layer, however, these centers are distributed at random as in an ordinary liquid. TBBA enters the smectic A phase at 200°C, before undergoing transitions to two other more ordered smectic phases at lower temperatures. 

Similarly, the two structural units of poly[ 8-[(4 -(2/ ,35)-2-fluoro-3-methyl-pentyloxycarbonyl)-3 -fluorophenyl-4"-phenoxy)octyl]vinyl ether)-co- 11-[(4 -(2/ ,3S)-2-fluoro-3-methylpentyloxycarbo-nyl)-3 -fluorophenyl-4"-phenox.y)unde-cyl]vinyl ether are isomorphic within both the SmC and SmA mesophases, but not the more ordered phases (Fig. 24 b) [139]. Therefore, copolymerization can be used to eliminate the crystalline and/or more ordered smectic phases of the corresponding homopoly mer(s). 

The palladium complexes were further studied using EXAFS. In the solid state at room temperature, the data were readily fitted to the known crystal structure, which arrangement persisted through several crystalline modifications until the ordered smectic phase was entered. In this phase, the Pd S distance increased to 3.8 A where interaction must be precluded. Further heating into the SmC phase showed only four short Pd-S distances, confirming that the all significant intermolecular interactions had now disappeared. 

In summary, TPPs show complicated phase transition behaviors. Their phase diagrms are established and various phases are identified via the thermodynamic transition properties obtained from DSC, the structural order and symmetry determined by WiOCD, and morphology and defects observed under PLM and TEM. In particular, the WAXD fiber patterns in different phases play the most important role in determining the phase structures and symmetry. It is evident that the concepts of highly order smectic phases developed in small-molecule liquid crystals can also be utilized in the main-chain liquid crystalline polymers. 

Mostly thermotropic polyesters show a nematic phase with a typical appearance that is called a marbled texture, a Schlieren texture, a threaded texture, or, a nematic droplets depending on the thickness and temperature of the polyester. Only in a few cases, they exhibit low-order smectic phases [100], which are identified through the observations of fan-shaped and broken fan textures in their LC phases. No report of higher-order smectic phases has been observed in this class of pol)nners. It is worth mentioning that nematic texture can simply be frozen into a glassy state in the case where there is no interference from the crystallization process. An intriguing property of an LC polyester is its biphase structure over... 

In a polymer, the rod-like structures can be attached as side groups—side-chain liquid crystal polymers or with the skeletal backbone—main chain liquid crystal polymers (Donald et al. 2006). The latter usually exhibit liquid crystal characteristics at elevated temperatures, while some side-chain liquid crystal polymers exhibit liquid crystalline order at room temperature. A number of more ordered smectic phases can be observed as well as chiral, nematic, and smectic phases (Donald et al. 2006). 

Smectic- (orthogonal) and smectic-C (tilted) are the least-ordered smectic phases. The layer structure of these phases actually consists of a onedimensional density wave. The shape of the density wave appears to be almost purely sinusoidal as can be concluded from the absence or very weak intensity of higher-order reflections in X-ray experiments. Thus, one should... 

X-ray studies are often carried out on unoriented samples, while the results only allow limited characterization of the structural features. X-ray studies are very much useful especially to identify the ordered smectic phases. Polarizing microscopy on the other hand can provide a determination of both phase transition temperatures and phase type [5-7]. 

The problem with applying Eq. (12) directly to experimental X-ray data is that absolute intensities are needed, and this is difficult to determine. When only two Bragg peaks are detected, as for many smectic phases, if a model structure can be assumed for the molecules, then the intensities which would be observed from a perfectly-ordered smectic phase, /qo/ can be calculated. The ratio of the first two smectic order parameters is then given by ... 

Figure 24. (a) Structures, and (b) layer stackings of the ordered smectic phases. A, B and C denote the highest symmetry positions of molecules in adjacent layers, whilst the open squares and triangles show the lower symmetry positions (ortho-F and mono-C) which have been observed in the smectic B phase. (From [15]). 

Larger differences were found for the more highly ordered smectic phases, and for ketones bearing substituents which mimic the structure of the mesogen. From an analysis of the results of a number of studies, it was concluded that the effects observed were the result of the solvent s ability to alter the equilibrium constant for trans/ gauche interconversion in favour of the nonquenching tra 5-conformer. These studies included a careful determination of the solubility limits of each solute in the various phases by deuterium nmr spectroscopy and thus also provide several clear examples of the effects and complexities of biphasic solubilization on solute reactivity. 

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