Disordered conformations liquid crystals

Note 2 The term is used to describe orientationally disordered crystals, crystals with molecules in random conformations (i.e., conformationally disordered crystals), plastic crystals and liquid crystals. 

The possible transitions of plastic and condis crystal-forming materials are shown in Fig. 4. For plastic crystals, this diagram is fully based on information on low molecular weight materials. No flexible, linear macromolecules which resemble plastic crystalline behavior have been reported (see Sect. 5.2.3). Similarly, little attention has been paid in the past to conformationally disordered mesophases in small molecules. In fact, some of the plastic crystals of larger organic molecules may actually be condis crystals (see Sects. 5.2,2 and 5.3.3). Since the positional order is preserved in both plastic and condis crystals, the possible phase relations are similar. The major difference from the liquid crystals is the possibility of partial mesophase formation. 

The thermotropic mesophases are well enough understood to propose a subdivision into six types. Depending on the type of disorder, they are called liquid crystals, plastic crystals or condis crystals (positional and if applicable conformational disorder, orientational disorder, and conformational disorder, respectively). For the corresponding glasses, which represent the frozen-in mesophases, the names LC-, PC-, and CD-glasses are proposed (Fig. 2). For macromolecules not only equilibrium... 

The first ideas on the nature of liquid crystals in hpids were derived from X-ray studies by Luzzati [1]. A crucial discovery was his demonstration of the liquid character of the hydrocarbon chains, which are thus space-filling. This was evident after it was foimd that the Upid bilayer thickness decreases with temperature with a large linear thermal coefficient about 10"3/°C. Such an effect is consistent only with a highly disordered chain conformation. Also the X-ray scattering characteristics were found to be very similar to those of liquid paraffins. 

For the synthesis of side chain liquid crystal polyphosphazenes, the most important examples in Figure 2 are the alkoxy- and aryloxy-polymers. Mesophase behavior has been noted with simple side chains, such as trifluoroethoxy and aryloxy side chains (9). This mesophase behavior is not conventional liquid crystal order, but polymers which exist in a conformationally disordered state (10). 

The molecular mechanism behind the destabilization of liquid crystals was subsequently clarified [25], The specific disordering promoted by the hydrotrope in the water-surfactant-oily liquid crystal was first determined, followed by an investigation into the conformation of the diacid molecule itself [92],... 

In this discussion at attempt will be made to describe in greater detail the structure and motion for a larger number of condis crystals. A special effort will be made to point-out the differences between condis crystals on the one hand, and liquid and plastic crystals on the other. It seems reasonable, and has been illustrated on several examples, that molecules with dynamic, conformational disorder in the liquid state show such conformational disorder also in the liquid crystalline and plastic crystalline states The major need in distinguishing condis crystals from other mesophases is thus the identification of translational motion and positional disorder of the molecular centers of gravity in the case of liquid crystals, and of molecular rotation in the case of plastic crystals. 

Conformational Motion and Disorder in Low and High Molecular Mass Crystals Table 5.2, Structural formulae and transition temperatures of some liquid crystals... 

The differences between the three mesophases, liquid crystal, plastic crystal and condis crystal could be clarified and, although difficult to classify, intermediate cases remain. If the molecule permits dynamic conformational disorder in the liquid phase, the same is usually also possible in a similar temperature range in the liquid-crystalline and plastic-crystalline phases. 

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