The plastic crystalline state

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). 

Existence of a high degree of orientational freedom is the most characteristic feature of the plastic crystalline state. We can visualize three types of rotational motions in crystals free rotation, rotational diffusion and jump reorientation. Free rotation is possible when interactions are weak, and this situation would not be applicable to plastic crystals. In classical rotational diffusion (proposed by Debye to explain dielectric relaxation in liquids), orientational motion of molecules is expected to follow a diffusion equation described by an Einstein-type relation. This type of diffusion is not known to be applicable to plastic crystals. What would be more appropriate to consider in the case of plastic crystals is collision-interrupted molecular rotation. 

The rotational diffusion model was generalized by Gordon (1966) to include 

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. 

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J. N. Sherwood, ed., The Plastically Crystalline State, Wiley, New York, 1979. 

J. Sherwood, The Plastically Crystalline State Orientationally Disordered Crystals, Wiley, 1979. 

Leadbetter A.J. and Lechner R.E. "Neutron Scattering Studies" in "The Plastically Crystalline State" Edited by J.N. Sherwood J. Wiley and Sons, 285 (1979)... 

An example of globular molecules of similarly different motional bdiavior is presented by adamantane (Cj Hj ) and camphor (CjpH,gO) The symmetric adamantane shows motional narrowing of the proton NMR sp trum due to reorientational jump motion between equivalent orientations about its two and threefold axes starting at 140 K, and shows hardly any further chan at the transiticm to the plastic crystalline state that occurs at 209 K. The asymmetric camphor (d), in contrast, diows the thermodynamic transition to the plastic crystalline state and the motional NMR line-width narrowing together at T = 244.5 K. The temperatures of isotropization for the two molecules are 543 and 451.6 K, respectively. 

Sherwood N (1979) The plastically crystalline state (Orientationally disordered crystals). Wiley, Chichester... 

Plastic crystals are more closely related to the classical crystals. They have full positional order as shown in the sketch in Fig, 2.105. The plastic crystal consist, however, of molecules (their mesogens) that are almost spherical and can start to rotate within the crystal at a given transition temperature. Figure 2.107 contains a list of typical properties of plastic crystals and allows a comparison to liquid crystals. The plastic crystalline state was first recognized in the 1930 s. Most plastic crystals... 

Sect. 2.5. General discussions of the classical phases and their transitions can be found in your favorite physics, chemistry, or physical chemistry text. Liquid crystals are treated by DeGennes PG (1974) The Physics of Liquid Crystals. Clarendon Press, Oxford Brown GH (1975) Advances in Liquid Crystals. Academic Press, New York Gray GW (1962) Molecular Structure and the Properties of Liquid Crystals. Academic Press, New York. Plastic crystals by Sherwood N, ed. (1979) The Plastically Crystalline State (Orientationally-disordered Crystals). Wiley, Chichester. For the condis state see Wunderlich B,MollerM,Grebowicz J, Baur H (1988) Conformational Motion and Disorder in Low and High Molecular Mass Crystals. Springer Verlag, Berlin (Adv Polymer Sci Vol 87). 

Figure 4.5 Variation of the potential energy with rotation angle (a) the liquid state (b) the plastic crystalline state.

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