Plastic-crystal

Let us enter the world of liquid crystals built by the purely entropic forces present in hard body systems. The phase diagram of hard spherocylinders (HSC) shows a rich variety of liquid crystalline phases [71,72]. It includes the isotropic, nematic, smectic A, plastic, and solid phases [73]. In a plastic crystal the particle centers lie on lattice sites, but the orientations of the... 

In some cases there is evidence of multiple solid-solid transitions, either crystal-crystal polymorphism (seen for Cl salts [20]) or, more often, formation of plastic crystal phases - indicated by solid-solid transitions that consume a large fraction of the enthalpy of melting [21], which also results in low-energy melting transitions. The overall enthalpy of the salt can be dispersed into a large number of fluxional modes (vibration and rotation) of the organic cation, rather than into enthalpy of fusion. Thus, energetically, crystallization is often not overly favored. 

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. 

Figure 6. Vapor-liquid-solid (plastic crystal) phase diagram of adamantane. The phase transition from plastic crystal to rigid crystal phase occurs at 208.6K (l/T = 0.004794K ). This diagram is based on the data of Table II.

Figure 8. The temperature dependence of the heat capacity in the condensed state for adamantane [5] as measured by a scanning calorimeter. Tu, stands for temperature of transition from rigid crystal (fee) to plastic crystal (cubic) state of adamantane and Tfas stands for fusion temperature.

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. 

The term plastic crystal is not used if the rotation of the particles is hindered, i.e. if the molecules or ions perform rotational vibrations (librations) about their centers of gravity with large amplitudes this may include the occurrence of several preferred orientations. Instead, such crystals are said to have orientational disorder. Such crystals are annoying during crystal structure analysis by X-ray diffraction because the atoms can hardly be located. This situation is frequent among ions like BF4, PFg or N(CH3)J. To circumvent difficulties during structure determination, experienced chemists avoid such ions and prefer heavier, less symmetrical or more bulky ions. 

Plastic crystals and crystals with orientational disorder still fulfill the three-dimensional translational symmetry, provided a mean partial occupation is assumed for the atomic positions of the molecules whose orientations differ from unit cell to unit cell ( split positions ). 

MATERIALS Pocket watch with plastic crystal, small screw, electric wire, battery, electric blasting cap. 

Fontell, K., X-ray Diffraetion by Liquid Crystals—Amphiphilie Systems. In Liquid Crystals and Plastic Crystals (G. Gray and P. Winsor, eds.), Vol. 2, Ellis Horwood, Chiehester, 1974. Luzzati, V., Mustaeehi, H., Skoulios, A., and Husson, F., La structure des colloides d associa-tion. I. Les phases liquide-eristalline des systemes amphiphile-eau, Acta Cryst., 13 660-677 (1960). 

Fontell, K. "Liquid Crystals and Plastic Crystals", Ellis Horwood Ltd., 1974 p. 80. 

As = 4.8 J mol K l), at 122°C, is also characteristic of a plastic crystalline phase, as is the softness of the crystals and their sensitivity to macroscopic mechanical damage. Variable temperature X-ray powder diffraction shows a change in pattern from monoclinic to cubic upon heating the diffraction lines, sharp and numerous for the "rigid" crystals, become few and less well-defined for the plastic crystal. 

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. 

Note 4 At one time, a number of mesophases were identified as smectic on the basis of their optical textures, but they are in fact soft crystals characterised by very low yield stresses. Hence, these three-dimensionally ordered phases should no longer be called smectic mesophases. They are akin to plastic crystals with some elementary long-range order and are referred to by the letters E, J, G, H, and K. 

G. W. Gray and P. A. Winsor (Eds.). Liquid Crystals and Plastic Crystals, Ellis Horwood, Chichester (1974), Vols. 1 and 2. 

Table 4.2. Structure and thermodynamic properties of some plastic crystals"... 

All the plastic phases listed in the table possess FCC structure crystal-plastic crystal transition temperature AS, entropy change at T, AS , entropy change at T , activation energy for molecular reorientation obtained from NMR spectroscopy. 

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. 

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. 

The nature of rotational motion responsible for orientational disorder in plastic crystals is not completely understood and a variety of experimental techniques have been employed to investigate this interesting problem. There can be coupling between rotation and translation motion, the simplest form of the latter being self-diffusion. The diffusion constant D is given by the Einstein relation... 

Thermal or low-energy neutron scattering experiments have been most valuable in throwing light on molecular motion in plastic crystals. These experiments measure changes in the centre of mass of a molecule. Diffusion constants obtained from neutron experiments differ widely from those obtained from tracer experiments since neutron scattering is mainly determined by rotational diffusion. The scattering function has the form... 

Correlation times and activation energy parameters obtained from different techniques may or may not agree with one another. Comparison of these data enables one to check the applicability of the model employed and examine whether any particular basic molecular process is reflected by the measurement or whether the method of analysis employed is correct. In order to characterize rotational motion in plastic crystals properly it may indeed be necessary to compare correlation times obtained by several methods. Thus, values from NMR spectroscopy and Rayleigh scattering enable us to distinguish uncorrelated and correlated rotations. Molecular disorder is not reflected in NMR measurements to this end, diffraction studies would be essential. 

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