Mesophase state

Pitches can be transformed to a mesophase state by further chemical and physical operations. Heat treatment of conventional pitches results in additional aromatic polymeriza tion and the distillation of low molecular weight components. This results in an increase in size and concentration of large planar aromatic molecular species whereupon the precursor pitch is transformed to a mesophase state exhibiting the characteristics of nematic Hquid crystals (1). Additional heat treatment converts the mesophase pitch to an infusible aromatic hydrocarbon polymer designated as coke. 

Mesophase state, 20 78 Mesophiles, in composting, 25 873 Mesophilic digestion, 3 702 Mesophilic enzymes, 3 669 Mesoporous molecular sieves, 16 847—849 Mesostructured hybrid materials, 13 548-549... 

The disk-shaped molecules are not rigorously oriented to parallel arrays the mesophase state constitutes only a preferred orientation, but with virtually all molecular layers lying within 25° of the director representing the average orientation (18). 

The effect of methylcyclosiloxane fragments in dimethylsiloxane backbone on properties of copoly-mers and formation of the mesophase state was studied [74] by HFC reaction of l,7-dichloro(dihyd-roxy)-l,7-diorganooctamethyl cyclohexasiloxane with... 

Others NH4SCN/NH3/water Dissolves cellulose by heating at 90 °C Forms mesophase states... 

From that point of view it seems natural that special consideration should be given to the composite membranes in which one component consists of liquid crystal. The transitions from crystalline to mesophase state are connected with mobility increase thus influencing the transport properties of these composite systems. However, it was only recently that a discontinuous jump of permeability to liquids, gases and vapours in the vicinity of transition temperature of the liquid crystal phase has been discovered. Membranes of this type form a new class of composites which deserve special consideration due to their particular properties. 

In contrast to the isotropic mesophase transition, complete supercooling to the glassy mesophase state is possible. The crystallization from the mesophase may be irreversible. 

The mesophase state of liquid crystals is normally opaque due to relatively large sizes of ordered domains. Its transition point to the isotropic melt state is called the clear point T,. The DSC scanning curves of liquid crystals can exhibit either enantiotropic or monotropic phenomena. For the thermodynamically stable mesophases of liquid crystals, they occur between the melt and the crystal states during both cooling and heating processes, as illustrated in Fig. 10.5. When both the cooling and heating curves show two symmetric consecutive phase transitions, it is known as the enantiotropic phenomenon. In contrast, for the metastable mesophase... 

Some small molecules possessing a Sfdierical (or close to spherical) structure such as Nj, CCl, CH SiH, and some others can exist in the mesomorphic state known as the plastic crystd This mesophase state is characterized by the positional order of the centres of gravity of molecules with an absence of orientational order. Although there are attempts in the literature to consider some mesophases in polymers as the state of plastic crystal Wunderlich and Grebowicz concluded recently that this mesc hase state really exists only in the case of small molecules. 

Thus, the subject of the review are linear and cyclolinear etement-organic polymers forming a spedfic type of the mesophase state. The structure of the mesoj iases and the formation of liquid crystalline macromolecules, in wiiidi classical mesogens in the main or side chains are linked by flexible polysiloxane or poly-phosphazene spacers are similar to the liquid-crystalline polymers with organic spacers. Structure and properties of these polymers have been reviewed recently and, therefore, will not be considered in our article. 

Recently, it has been established that the mesophase state is much more sensitive to molecular mass changes than the crystalline state The behaviour of nonfrac-... 

Lately, thermotropic mesophases have also been found in hexakis(trimethylsiloxy) disiloxane [(CH3)3SiO]3Si jO and in 1,3-dioxytetramethyldisiloxane (DOTES) and 1.3-dioxytetra-n-propyldisiloxane (DOTPS) The former is interesting in that the methyl substituents are not bulky. The transition to a mesophase state occurs in the temperature range 233-245 K with the thermal effect 8.4 J/g. This mesophase is isotropic. Above 373 K this substance is able to flow but the crystalline reflexes in X-ray pattern remain below 500 K. It has been suggested that this mesophase can be interpreted as a plastic crystal. 

Studied effect of molecular composition leading to changes in the mesophase state on flexural modulus. Revealed thickness and test-direction dependence of mechanical properties. 

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