Plastic crystal examples

In practice, the phenomenon of creeping flow at x < Y can usually be neglected. Thus, certainly, it is insignificant in the treatment of filled polymers, though it may be important, for example, in the discussion of the cold flow of filled elastomers. However, we cannot forget the existence of this effect, to say nothing of the particular interest of the physist in this phenomenon, which is probably similar to the mechanism of flow of plastic crystals. 

In plastic crystals all or a part of the molecules rotate about their centers of gravity. Typically, plastic crystals are formed by nearly spherical molecules, for example hexafluorides like SF6 or MoF6 or white phosphorus in a temperature range immediately below the melting point. Such crystals often are soft and can be easily deformed. 

Because of the orientational freedom, plastic crystals usually crystallize in cubic structures (Table 4.2). It is significant that cubic structures are adopted even when the molecular symmetry is incompatible with the cubic crystal symmetry. For example, t-butyl chloride in the plastic crystalline state has a fee structure even though the isolated molecule has a three-fold rotation axis which is incompatible with the cubic structure. Such apparent discrepancies between the lattice symmetry and molecular symmetry provide clear indications of the rotational disorder in the plastic crystalline state. It should, however, be remarked that molecular rotation in plastic crystals is rarely free rather it appears that there is more than one minimum potential energy configuration which allows the molecules to tumble rapidly from one orientation to another, the different orientations being random in the plastic crystal. 

As the name suggests, plastic crystals are generally soft, frequently flowing under their own weight. The pressure required to produce flow of a plastic crystal, as for instance to extrude through a small hole, is considerably less (2-14 times) than that required to extrude a regular crystal of the same substance. r-Butyl alcohol, pivalic acid and d-camphorprovide common laboratory examples of plastic crystals. The subject of plastic crystals has been reviewed fairly extensively (Aston, 1963 Sherwood, 1979) and we shall restrict our discussion to the nature of the orientational motion (Rao, 19856). 

No single model can exactly describe molecular reorientation in plastic crystals. Models which include features of the different models described above have been considered. For example, diffusion motion interrupted by orientation jumps has been considered to be responsible for molecular reorientation. This model has been somewhat successful in the case of cyclohexane and neopentane (Lechner, 1972 De Graaf Sciesinski, 1970). What is not completely clear is whether the reorientational motion is cooperative. There appears to be some evidence for coupling between the reorientational motion and the motions of neighbouring molecules. Comparative experimental studies employing complementary techniques which are sensitive to autocorrelation and monomolecular correlation would be of interest. 

Indeed for a given mixture the fluid composition changes continuously from A to B, whereas the solid composition varies correspondingly from A to B, as the temperature decreases from TA to TB. So unless diffusion is extremely efficient (as in plastic crystals, for example), the two phases are likely to be fairly inhomogeneous. 

Although polymorphism in plastic crystals is less frequent than in liquid crystals, it does exist. Tetrakis(methylmercapto)methane, C(SCH3)4, for example, has four crystal modifications of which the three high temperature forms have a high degree of plasticity 100). Also, it has been observed that plastic crystals are frequently mutually soluble 16b), a consequence of the less restrictive crystal structures. Phase separation of these solutions occurs often on transition to the fully ordered crystal, giving rise to quite complicated phase diagrams102). 

The examples of the extended-chain crystals, which from their thermal properties and mechanical properties have some similarities to plastic crystals, will probably... 

No general rules about the entropy of transitions, as were found for liquid and plastic crystal transitions, can be set up for condis crystals. Two typical examples may illustrate this point. Polytetrafluoroethylene has a relatively small room-temperature transition-entropy on its change to the condis state and a larger transition entropy for final melting. Polyethylene has, in contrast, a higher condis crystal transition entropy than melting entropy (see Sect. 5.3.2). 

Small molecules may also form condis crystals, provided they posses suitable conformational isomers, It is of interest to note that several of the organic molecules normally identified as plastic crystals are probably better described as condis crystals. Their motion was, as already shown in Sect. 5.2.2, not the complete reorientation of the presumed rigid molecule, but rather an exchange between a limited number of conformational isomers. The examples treated in Sect. 5.2.2 are 2,3-dimethyl-butane, cyclohexanol and cyclohexane. 

Plastic crystals are almost crystalline solids, except that the molecular constituents in the primitive unit cell rotate freely in place this confers to them a certain degree of plasticity. Examples are certain cage-shaped boranes (e.g., B10H14), carboranes (e.g., Bi0C2Hi2), and buckminsterfullerene (Ceo) at room temperature. These plastic crystals usually order into normal crystalline solids at low temperatures. 

As mentioned above, polymorphs may also be related by order-disorder transitions, e.g. the onset of free rotation of a group of atoms, or local tumbling in semi-plastic or plastic phases. This may be due to random orientation of the molecules or ions, but is also diagnostic of the onset of a reorientational motion. Roughly spherical molecules and ions are likely to show order-disorder phase transitions to a plastic state. In the cases of co-crystals or of crystalline salts this process may affect only one of the components, leading to semi-plastic crystals (an example will be discussed below). Order-disorder phase transitions have often... 

A further interesting effect discovered in our laboratories is that the addition of low levels of a second component, or dopant ion, can lead to significant increases in the ionic conductivity [6, 30, 31]. Typically these dopant species, for example, Li, OH , and H" ", are much smaller than the organic ions of the matrix, and since the relaxation times characterizing the motion of these ions are more rapid than those of the bulk matrix itself, these materials may represent a new class of fast ion conductor. The dopant ion effect can be used to design materials for specific applications, for example, Li+ for lithium batteries and H /OH for fuel cells or other specific sensor applications. Finally, we have recently discovered that this dopant effect can also be apphed to molecular plastic crystals such as succinonitrile [32]. Such materials have the added advantage that the ionic conductivity is purely a result of the dopant ions and not of the solvent matrix itself. 

Figures 24.1 and 24.2 present the thermal analysis traces for a range of typical plastic crystal materials based on the pyrrolidinium cation. The highest temperature phase is denoted as phase 1, with subsequent lower temperature phases denoted as phases II, III, and so on. It can be seen that the nature of the anion plays a significant role in the thermal behavior of these compounds. For example, PnTFSA has a barely detectable solid-solid transition at 20°C, followed by a melt at above...

A number of two-dimensional NMR experiments have been introduced in high-resolution solid-state NMR studies, designed, for example, to investigate chemical exchange processes [64], to retrieve chemical shift anisotropies [65] and dipolar couplings [66], and to probe spin-diffusion proces.ses 67j. Opella has proposed an intemuclear distance-determined, spin-diffusion mechanism in molecular crystals [68], and Benn and coworkers have demonstrated C/ C connectivities using the INADEQUATE sequence for the plastic crystal camphor and have used the COSY sequence for Si/ Si connectivities in the reference molecule Q Ms [691. 

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. 

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