Smectic modifications

Useful applications have been found lor the varied effects of these crystal changes. One of the first came from the properly of selectively reflecting visible light because this is lempcraiurv-dependent. the property can be used as a temperature detector, and in gel lurm liquid crystals have been used lor the early detection of those cancers which cause hot spots in the body. Applications of the smectic modifications arise from their ferroelectric properties this phase can function as a fast-switching light-valve device with memory. This kind of application requires some... 

Figure 7.1. Categories of smectic liquid crystals. From Smectic Liquid Crystals, Textures and Structures, Gray, G.W. and Goodby, J.W.G. 1984 (Leonard Hill, Glasgow). (Reproduced by kind permission of the authors and publishers.) Side elevation and plan representation of molecular ordering in each of the smectic modifications. Triangles or arrows are used to represent tilt direction.

If isotactic polypropylene is quenched from the melt to room temperature a so-called smectic modification is formed which shows only two crystal reflexions. Cabarcos, Bbsecke, and Zachmann investigated the kinetics of the transition from this modification into the a-modification. Fig. 54 shows the change of wide angle scattering during isothermal annealing at 90 °C. One sees that the 040-reflexion of the a-modification appears after about 200 sec and continues to increase in intensity afterwards. The time until the constant temperature is reached is about 150 sec. Therefore we can say that most of the process occurs at constant temperature. If the same is done at 130 °C the process occurs so rapidly that it is almost finished as constant temperature is reached. 

However, when a laterally appended mesogen is introduced into a system where all of the other mesogens are terminally appended, as for 51, the laterally appended mesogen acts as a disruptor to the orderly packing of the molecules together. This results in the suppression of the formation of smectic modifications and support for nematic phases. Thus compound 49, which... 

In the latter two phases backbones have the spindle-like conformation, i.e., the prolate shape with (R%) > R p), the characteristic of main chain liquid crystalline polymers. Important means of investigating the conformations of side chain liquid crystalline polymers include small angle neutron scattering from deuterium-labeled chains (Kirst Ohm, 1985), or small angle X-ray scattering on side chain liquid crystalline polymers in a small molecular mass liquid crystal solvent (Mattossi et al., 1986), deuterium nuclear resonance (Boeffel et al., 1986), the stress- or electro-optical measurements on crosslinked side chain liquid crystalline polymers (Mitchell et al., 1992), etc. Actually, the nematic (or smectic modifications) phases of the side chain liquid crystalline polymers have been substantially observed by experiments. 

For compound 23 the uncharacterized phase, denoted as M, was found to be exhibited over a fairly wide temperature range. Ribeiro et al, found that the X-ray diffraction patterns associated with the intermediary phase were typical of a disordered smectic modification. The d spacing of 45.3 A was found to be a little bit shorter than the calculated molecular length which is consistent with the presence of a smectic A phase. As the material also exhibits a Grandjean plane texture it was concluded that the phase must be a TGB modification. However, the platelet/fan texture exhibited by the mesophase conversely suggests that a columnar ordering must be present as well,... 

The structure of the smectic A phase, therefore, is governed by the relative lengths of the spacer and terminal chains. For short spacers, n = 3-7, the CBOnO.lO series exhibits the SmAa phase while for long spacers, n= 10-12, the SmAc phase is observed. The disappearance of smectic behaviour for intermediate chain lengths (see Fig. 11) implies that neither smectic modification is favourable and hence nematic behaviour results. There is a strong similarity here to re-entrant nematic behaviour which is also driven by two different length scales. 

Unlimited and limited miscibility of smectic modifications are illus trated in Fig. I9. 

Over the last twenty years, however, X-ray diffraction techniques have been used to investigate the structures of the smectic modifications [8-13]. This has led to combining miscibility classification with structural observations. Thus, certain structural features, such as the extent of the positional ordering, tilt orientational ordering, packing structure, and bond orientational ordering can be associated with each individual... 

Fig. 6.12. Diffraction patterns of polymer 1 (Table 6.7) in different smectic modifications Sp (1), Sq (2), (3) in the nematic phase...

As already mentioned, mesogens embrace a diversity of structure. We shall not, however, be concerned here with mesophases of disc-, pyramid-and phasm-like molecules, but shall confine attention to thermotropic mesomorphism exhibited by LMM compounds with elongated and relatively rigid lath-like molecules. Mesophases formed on heating such compounds are classified into three types nematic, cholesteric, and smectic. There are more than ten recognized smectic modifications and these are denoted by Sa, Sb, Sc-.-Sl- A description of the structural features of these phases may be found in standard books and reviews " and it is my intention in this section to bring the information on this subject up to date and to present it in a more concise form. 

MCLCPs. Unlimited and limited miscibility of smectic modifications are illustrated in Fig. 2.30. It is to be noted that Krigbaum et al and Bosio et were unsuccessful in their attempt to identify from miscibility... 

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