Plastic crystal crystalline phase structure

A number of other thermodynamic properties of adamantane and diamantane in different phases are reported by Kabo et al. [5]. They include (1) standard molar thermodynamic functions for adamantane in the ideal gas state as calculated by statistical thermodynamics methods and (2) temperature dependence of the heat capacities of adamantane in the condensed state between 340 and 600 K as measured by a scanning calorimeter and reported here in Fig. 8. According to this figure, liquid adamantane converts to a solid plastic with simple cubic crystal structure upon freezing. After further cooling it moves into another solid state, an fee crystalline phase. 

The preparation of single crystals is difficult, but is successful in some case, so that we are well informed about the structures (1,6,17,21,22,23). The structures of the plastic phases are related to the well-known intermetallic phase Li3Bi, where the centres of the polycyclic P7 or Pn anions surround the positions of the Bi atoms in LisBi. The orientation of the polyanions is disordered (dynamically ). For these structures this orientation leads to a typical electron density distribution of a seemingly octahedral unit. In contrast the orientation of the anions is fixed for the crystalline phases. The symmetry of the unit cells as well as the distribution of cations and anions in these M3P7 and M3P11 type structures reflect the direct relationship to the structures of the plastic phases. 

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]. 

In the presence of excess fatty acid, different soap crystalline phase compounds can form, commonly referred to as acid-soaps. Acid-soap crystals are composed of stoichiometric amounts of soap and fatty acid and associate in similar bilayer structures as pure soap crystals. There are a number of different documented acid-soap crystals. The existence of crystals of the composition 2 acid-1 soap, 1 acid-1 soap, and 1 acid-2 soap has been reported (12-14). The presence of the acid-soaps can also have a dramatic impact on the physical and performance properties of the finished soap. The presence of acid-soaps increases the plasticity of the soap during processing and decreases product firmness, potentially to the point of stickiness during processing. Furthermore, the presence of the acid-soap changes the character of the lather, decreasing the bubble size and subsequently increasing lather stability and creaminess. 

Possible distinctions among liquid, plastic, condis and fully organized crystals [1] and among the various types of liquid crystalline phases [2] based on thermodynamic considerations have been discussed. The consequences of those distinctions will be emphasized here using anecdotal but related examples. Selected molecules, grouped by shape, functionality, etc., will be examined (and when possible compared) to discern the salient structural and electronic features that may be key to whether they form thermotropic mesophases and, in a few cases, lyotropic mesophases. Since the common structural features of molecules which become plastic crystalline [21-27] and liquid crystalline [12] are well known, an emphasis will be placed upon more exotic characteristics. Questions to be addressed include the following. 

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). 

To begin with, and contrary to a still widely held belief, the words liquid-crystalline and mesomorphic are not synonymous. The term mesomorphic phases was introduced by Friedel in 1922 [1] it is now often abbreviated to meso-phases. He defined them as phases with microscopic structures between solids and ordinary isotropic liquids. Not much happened in this area until 1955 when Kast [2] tried to characterize such phases in terms of lateral, longitudinal, and steric disorder. The next step occurred in 1984 when Wunderlich and Grebowicz [3] defined condis crystals for the first time. Following them [3,4] we now distinguish three kinds of mesophases liquid crystals, plastic crystals, and condis crystals. 

PE is therefore a semi-crystalline polymer. Its Tg does not determine whether it is a rubbery material or a plastic, but rather its crystallinity does. For PE alone, its crystallinity ranges from 20% up to 80% depending both on its chemical structure, branched or linear, and on processing conditions. It is the crystals (hard phases) together with the mbbery soft phases that make the PE polymer function as a plastic with good toughness, and not a mbber. 

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