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Disordered Crystal Mesophases

Two basically different types of mesophases have been observed. First, there are those that retain a 3-dimensional crystal lattice, but are characterized by substantial rotational disorder (i.e., disordered crystal mesophases), and second, there are those with no lattice, which are therefore fluid, but nevertheless exhibit considerable rotational order (i.e., ordered fluid mesophases). Molecular structure is in fact important and, generally speaking, molecules comprising one of these two types of mesophase are distinctly different in shape from molecules comprising the other. Indeed, with the possible exception of some polymorphous smectic materials, there are no known substances that show both disordered crystal and ordered fluid mesophases. [Pg.3]

Experimental Data from Polymer Thermotropic Mesophases and Conformationally Disordered Crystals... [Pg.108]

The term condis crystal , which is a contraction of the term conformationally disordered crystal , was coined to designate the most important mesophase for flexible, linear macromolecules. We are not aware of prior naming of this class of mesophases4. [Pg.4]

Based on this simplified description of the melting and glass transitions, it is possible to propose six major types of mesophases. Figure 2 shows these mesophases in relation to the glass, crystal, and melt. First, it is possible to keep orientational order, but lose positional order. These positionally disordered crystals or orientationally ordered liquids 7) are widely known as liquid crystals 15). The name liquid crystal was given because of the obvious, liquid-like flow of these materials. By now it is too late to try to change the nomenclature, especially when the possible new names would be cumbersome 7). [Pg.6]

The third group of mesophase materials represents the conformationally disordered crystals, called condis crystals. The physical properties of condis crystals, which largely maintain positional and orientational order, change in much too subtle a way from the fully ordered crystals so that a common property could be attached to their name. [Pg.6]

The crystallization of 3D-ordered crystalline phases from thermotropic mesophases, envisaged as stable pre-crystalline partially ordered intermediates, is an additional interesting issue which should be considered with care experimentally, theoretically, and with appropriate simulation approaches. Depending upon the nature of the mesophase it can be seen as a crystal-crystal transition or, for conformationally disordered, columnar mesophases, it approaches a true crystallization process. It is quite clear that the preexisting order will play a major role for example if the mesophase is chain-extended, bundle equilibria and chain-folding should not play any role. Indeed available experimental evidence supports this idea. Mechanistic and kinetic features should in general differ widely from the standard chain-folded crystallization processes yielding thin lamellar structures. In a number of cases (polyphosphazenes, polysiloxanes, see below) the crystalline polymorphs obtained from the chain-extended precursor differ from those obtained from solution. [Pg.114]

These surfactants are found to exhibit a solubilization ability higher than those of their purely anionic and nonionic counterparts. It is found that for such surfactants the transition temperature from a liquid crystal mesophase to a (disordered) microemulsion structure is inversely related to the amount of alcohol cosurfactant, a quite useful feature in... [Pg.267]

Conformationally disordered crystals (condis crystals) were discovered in the 1980 s. They show positional and orientational order, but are partially or fully conformationally mobile. The condis crystals complete the comparison of mesophases in Figs. 2.103 and 2.107. Linear, flexible molecules can show chain mobility that leaves the position and orientation of the molecule unchanged, but introduces large-amplitude conformational motion about the chain axis. Again, the symmetry of the molecule is in this case increased. Condis crystals have often a hexagonal, columnar crystal structure. Typical examples of condis crystals are the high-temperature phase of polyethylene, polytetrafluoroethylene, frawj-1,4-polybutadiene, and the low-temperature phases of soaps, lipids and other liquid-crystal forming, flexible molecules. [Pg.171]

The final set of curves in Figure 4.10 refers to a possible mesophase of different lamellar thickness, i.e. a crystal phase that shows an intermediate degree of order. Most common in polymer crystals are the conformation-ally disordered crystals or condis crystals [22]. Since the entropy of the mesophase is intermediate between the crystal and melt, the slope of the free enthalpy curve, which is given by the expression d G/d T = A5, is also intermediate. The illustrated case has the proper enthalpy level, so that the mesophase has a small temperature range of stability. [Pg.232]

Figure 4.70 addresses the question of disordering of mesophase polymers [76]. An example of MTDSC of a liquid crystal and a condis crystal is shown. The analysis of low molar mass liquid crystals was discussed in Figures 4.42-4.44 as an example of a sharp, reversible transition and of low latent heat of transition of a sample of low thermal conductivity (Section 4.1). The liquid crystal-forming polymer has a much broader isotropisation... [Pg.289]

In addition to the well known liquid and plastic crystals a third mesophase the "conformational disordered crystal" exist. This classification results from the recognition that in the fully ordered crystal three kinds of order exist, namely positional, orientational and conformational. Loosing one kind of order and gaining the corresponding mobility leads respectively to liquid, plastic and "conformational disordered" crystals. On quenching in principle, the... [Pg.308]

Liquid crystals represent an intermediate state of order (mesophase) between crystals and liquids. Crystals have a three-dimensional long-range order of both position and orientation (Fig. 5.1-la). Liquids, in contrast, do not show any long-range order (Fig. 5.1-lb). In plastic crystals (disordered crystals, Fig.5.1-lc), positional order is maintained, but orientational order is lost. In mesophases, imperfect long-range order is observed, and thus they are situated between crystals and liquids. The reasons for the formation of a mesophase can be the molecular shape or a microphase separation of amphiphilic compounds. [Pg.941]

The phase transitions in PDES are accompanied with jump-like changes of the molecular motion, as seen from Table 2, where the data or pul% NMR concering the type and frequencies of the molecular motion are listed The data show indirectly that a considerable conformational disordering takes place not only at the crystal-mesophase transiticoi but also at the low-temperature solid- lid transitions. [Pg.139]

The most rigid chains are those of ladder polyorganosilsesquioxanes. Together with esters of cellulose, these polymers are an example of the lyotropic liquid crystalline system which is formed by semirigid macromolecules. ITie structure of their solid mesophasese has not been studied sufficiently enough, therefore, at present it is not clear whether it can be considered as a disordered crystal or as a liquid crystalline phase. Further study of the structure of the mesophases in element-organic polymers and low molecular substances would undoubtedly be very useful for the development of the concept of disordered crystals. [Pg.177]

The terms liquid crystal, mesophase, or mesomorphic state are used synonymously to describe a number of different states of matter in which the molecular order lies between the almost perfect long-range positional or orientational order of solid crystals and the long-range disorder found in ordinary isotropic liquids. Two main classes of liquid crystals are usually distinguished lyotropic and thermotropic. In lyotropic mesophases, the combination of order and mobility can be achieved by using a solvent thermotropic mesophases are based on the temperature-induced mobility of form-anisotropic molecules in the melt. Surfactants can often form both thermotropic and lyotropic liquid crystals i.e., they possess amphitropic properties [2]. [Pg.452]


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