Phase plastic crystal

Liquids are systems devoid of both long-range translational and orientational order whereas short-range order still remains at molecular scales resulting from the subtleties of forces acting on their constituent molecules. In turn, rotator-phase (plastic) crystals are liquid-like in the sense that molecules... 

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. 

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.

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). 

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 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. 

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. 

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. 

Turning to the low temperature transition of the homopolymer of PHBA at 350 °C, it is generally accepted that the phase below this temperature is orthorhombic and converts to an approximate pseudohexagonal phase with a packing closely related to the orthorhombic phase (see Fig. 6) [27-29]. The fact that a number of the diffraction maxima retain the sharp definition at room temperature pattern combined with the streaking of the 006 line suggests both vertical and horizontal displacements of the chains [29]. As mentioned earlier, Yoon et al. has opted to describe the new phase as a smectic E whereas we prefer to interpret this new phase as a one dimensional plastic crystal where rotational freedom is permitted around the chain axis. This particular question is really a matter of semantics since both interpretations are correct. Perhaps the more important issue is which of these terminologies provides a more descriptive picture as to the nature of the molecular motions of the polymer above the 350 °C transition. As will be seen shortly in the case of the aromatic copolyesters, similar motions can be identified well below the crystal-nematic transition. 

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. 

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). 

A transition to the plastic crystal is, however, not only geometry determined. This is illustrated by a comparison of adamantane (symmetric tricyclodecane) and the geometrically similar hexamethylenetetramine. The former is a typical plastic crystal, the latter not, because of its local polarity which hinders reorientation 16c). More detailed theories and discussions of the phase changes have been given in Refs, 8> 16c and102). 

Plastic crystals seem to have been only observed for small molecules. Their positional order is rather perfect, as can be derived from their full positional entropy on transition to the isotropic phase (Table 7). PC-glasses have been observed. Some of the plastic crystals of more flexible organic molecules may better be described as condis crystals. 

The cubic amphiphilic mesophases (Sic, Vi, and V2) from their interposition in the succession of mesophases Sic, Mi, Vi, G, V2, and M2, have generally been termed liquid crystalline like the optically anisotropic amphiphilic mesophases Mi, G, and M2. The cubic mesophases formed by non-amphiphilic globular molecules have however usually been termed plastic crystals. This nomenclature has obscured the fact that these plastic crystals are fundamentally liquid crystals rather than solid cyrstals and bear a relationship to the optically anisotropic non-amphiphilic smectic and nematic liquid crystals similar to that born by the amphiphilic cubic mesophases to the optically anisotropic neat (G) and middle (Mi and M2) liquid crystalline phases. 

Figure 6. Phases for systems which give I, crystal, anisotropic liquid, and isotropic liquid II, crystal and isotropic liquid III, crystal, cubic plastic crystal, and isotropic liquid (8)...

The aggregates discussed above are all anisodimensional, which is the reason for the anisotropic character of the mesophases. In some systems it has been possible to prove the existence of isotropic highly viscous phases of similar structure but which clearly consist of almost isodimensional aggregates. The exact structure of these phases is still the subject of discussion, as is also the case with the complex mesophases. The relation between the isotropic phases and globular proteins and plastic crystals of non-amphiphilic substances has been discussed by Gray and Winsor (5). 

In the solid state the translational motion of the molecules is slow and the molecules are arranged with long-range orientational and positional order. However, for compounds with long hydrocarbon chains the molecules may rotate in their lattice sites at the same time as they maintain full positional order, forming so-called plastic crystals (Evans and Wennerstrom, 1994, p. 412). The stability of these plastic crystalline phases (cy-forms) increases with chain length and with the presence of impurities (e.g., broad chain-length distributions) (Larsson, 1994, p. 27). 

Hexamethylbenzene, CeMe6, exists as a plastic phase I above 383 K, a room-temperature phase II, and a low-temperature phase III below 117.5 K. Both phase II and phase III crystallize in space group PI with Z = L The molecule is therefore located at an inversion center, and the site symmetry I is much lower than the idealized molecular symmetry of >6h- The asymmetric unit consists of one-half of the molecule. 

A measurement of the Kerr relaxation times in succinoni-trile(SN)as a function of temperature is shown in Fig. 2. The Kerr relaxation times measured show the effect of temperature on the rotational motion of the SN molecules as they undergo a change from the liquid to the plastic crystal phase. The data obtained from the Kerr gate measurement is shown along with a best fit curve from depolarized Rayleigh scattering (dotted line), and a best fit curve from dielectric relaxation measure-... 

Some compounds show meso-phases between the solid and liquid phases. These phases are classified into two kinds, namely liquid crystals in which the molecules have orientational order and disorganized position in one or more dimensions, and plastic crystal in which the molecules have organized positions and orientational disorder. Although the component ions in ILs are largely disordered, the appearance of liquid crystalline or plastic crystal phases could be the function of ion structures, when component ions have a tendency towards orientational or positional ordering by alignment of the ions and/or interaction among ions. Onium salt-type plastic crystals have been reported by MacFarlane [12,39]. 

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... 

The four librations (torsional oscillations or rocking motions) arise because the crystal-field potential prevents the I2 molecule from rotating as it would in the gas phase. There are some special crystals, called plastic crystals, in which symmetric molecules that interact weakly can still undergo hindered rotation in the solid phase, but l2( ) is not one of these. The librational motions for each I2 occur about two axes (a, /3) perpendicular to the 1—1 bond direction. The librations of the two I2 molecules in the same unit cell are coupled—giving rise to SL, AL and SL, AL vibrations, where SL denotes symmetric libration (angle displacements in phase) and AL denotes antisymmetric libration (angle displacements out of phase). 

Plastic crystal phases in organic materials have been known since the time of Timmermans [1], and these phases are often reached via a solid-solid transition below the final melting point of the crystal. These transitions often represent the onset of rotational motions of the molecules within the crystalline lattice and the resultant phases are sometimes referred to as rotator phases [18-23]. Timmermans proposed a general rule that plastic phases have a low final entropy of fusion (A5f < 20 J K mol because the rotational component of the entropy of fusion of the fully ordered phase is already present in the plastic phase. The bulk of the... 

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