Plastic crystals mesophases

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. 

The various types of mesophases have usually been treated separately, and rarely did researchers review more than one mesophase at a time. One of the few descriptions of liquid and plastic crystals was given by Smith7). We will expand on this attempt and try to combine the description of all three major types of mesophase order, namely ... 

Next is a group of materials with a mesophase which shows orientational disorder , but positional order. These materials are widely known as plastic crystals 16) because of the ease of deformation of such crystals. Again, this name is well accepted and much less cumbersome than orientationally disordered crystals7). 

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. 

The crystal-mesophase transitions at Td are first order transitions. In the case of plastic crystals, they show usually the major heat of transition. In the condis crystals the magnitude of the transitions depends on the number of bonds which gain con-... 

The thermotropic mesophases are well enough understood to propose a subdivision into six types. Depending on the type of disorder, they are called liquid crystals, plastic crystals or condis crystals (positional and if applicable conformational disorder, orientational disorder, and conformational disorder, respectively). For the corresponding glasses, which represent the frozen-in mesophases, the names LC-, PC-, and CD-glasses are proposed (Fig. 2). For macromolecules not only equilibrium... 

Winsor, P. A. Non-amphiphilic cubic mesophases plastic crystals . Chapter 2.2 in Ref. 15b... 

Relationship of Cubic Mesophases ("Plastic Crystals ) Formed by Non-Amphiphilic Globular Molecules to Cubic Mesophases of the Amphiphilic Series... 

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. 

Non-Amphiphilic Cubic Mesophases Plastic Crystals. The cubic plastic crystals of the non-amphiphilic series, to which the amphiphilic... 

Table II. Crystalline Forms of Some Non-Amphiphilic Plastic Crystals (23,24) and of Some Amphiphilic Cubic Mesophases...

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). 

K by the forcible pressure swing adsorption method (ca. 13 MPa). The adsorbed methane molecules are located in the pocket-like narrow corners of the necks of the ID channel [20]. Because the thermal motion of the pseudo-spherical methane molecules seems to be effectively suppressed in its translation mode but rotation is allowed, the forcible adsorption of methane gas produces an inclusion plastic crystal [20], which can be regarded as a mesophase between the fluid and solid state of the phase of a guest incorporated in a crystal host the guest molecules are randomly oriented, but their alignment follows the crystal periodicity. 

Gray, G.W. and Winsor, P.A. (1976) Generic relationships between non-amphiphilic and amphiphilic mesophases of fused type. Relationship of cubic mesophases (plastic crystals) formed by non-amphiphilic globular molecules to cubic phases of amphiphilic series. Adv. Chem. Ser., 152, 1-12. 

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. 

The differences between the three mesophases, liquid crystal, plastic crystal and condis crystal could be clarified and, although difficult to classify, intermediate cases remain. If the molecule permits dynamic conformational disorder in the liquid phase, the same is usually also possible in a similar temperature range in the liquid-crystalline and plastic-crystalline phases. 

A tentative classification of mesophases was reported by Wunderlich [2], who divided mesophases into six different types of phases hquid crystals (LC), condis crystsds (CD), plastic crystals (PC) and the corresponding LC, CD and PC glasses. 

Let us point out that according to Wunderlich [2], plastic crystals may also be considered as mesophases. They are characterized by positional order but orientational disorder of the structural motif. Molecules of plastic crystals are generally close to spherical so that there is no high-energy barrier to their reorientation. Of course, the condition of a spherical shape of the molecules may not be fulfilled by the macromolecular chains of linear synthetic polymers, which generally crystallize in extended chains or helical conformations [2]. However, there is at least one case of crystals of macromolecules presenting orientational disorder of the structural motif as in plastic crystals. 

A listing of the typical properties that distinguish the different mesophases is also given in Sect. 2.5 with Fig. 2.107. Furthermore, Sect. 2.5 contains examples for low-molar-mass liquid crystals and plastic crystals in Figs. 2.108-111, important model compounds for polymers. 

Chapter 5 Structure and Properties of Materials covers the solid states (glasses and crystals), mesophases (liquid, plastic, and condis crystals), and liquids. Also treated are multiphase materials, macroconformations, morphologies, defects and the prediction of mechanical and thermal properties. 

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. 

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