Supercooled mesophase

The way, or sequence, in which thermotropic transitions occur is defined in the following ways. The liquid crystal to isotropic liquid transition is called the clearing or isotropization point, and this transition, like those between liquid crystal phases, is essentially reversible and occurs with little hysteresis in temperature. The melting point of a material is usually a constant, but the recrystallization process can be subject to supercooling. Mesophases formed on the first heating cycle of a material are thermodynamically stable, and are called enantiotropic phases, whereas phases that are formed below the melt point on cooling cycles, and are revealed... 

Some data are given for supercooled mesophases (T< Tmeiting)- ... 

In this work we wish to report some new results on the dynamic-mechanical behaviour below the glass transition of these supercooled mesophases in thermotropic liquid crystalline polyesters based on bis(4-carbonylphenyl) terephthalate units (HTH) and alkylene diols (C3, C q) or... 

Calorimetric data also demonstrated that the low temperature first-order transition occurs in quenched samples independent of the particular chemical nature and length of the flexible spacers. Since the structure of the rigid core of the mesogenic groups is the same for all the polyesters examined, it can be reasonably assumed that such a transition involves exclusively the aromatic moiety of the repeating unit. This conclusion is also supported by the calorimetric data obtained for copolyesters HTH-C3/C5 and HTH-C5/C10, which still exhibit the double-peaked transition in their DSC curves at the same temperature as that of the corresponding homopolymers (Fig. 1). It must be noted, however, that copolymer samples quenched from the LC state had very poor crystallinity and attempts to clarify the nature and state of order of the supercooled mesophase by x-ray analysis have been unsuccessful to date. 

Calorimetric data supported the conclusion that, during the quenching of these materials from the anisotropic melt, the LC phase is prevented from crystallizing and remains as a highly supercooled mesophase. This metastable glass-like modification is subsequently transformed below 235 K into a more ordered, stable phase. 

In conclusion we assume that, basically, the first order transition in the supercooled mesophase is an order-disorder transition and that segments... 

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

Metastable mesophase that can be formed by supercooling an isotropic liquid or an enantiotropic mesophase at a given pressure to a temperature below the melting point of the crystal. 

Mesophase in which non-vibratory molecular motion is frozen by supercooling a mesophase stable at a higher temperature. 

NitroBIPS having in the 5 -position (4 -X-benzoyloxy)-benzylidenamino substituents (128), where X is cyano, methoxy, and n-hexoxy, form amorphous films exhibiting birefringence when colored. The bulk materials exhibit quasi-liquid crystalline properties, intermediate between supercooled liquids and mesophases, which are attributed to the interactions between small ordered domains of the spiropyran molecules.149... 

F. P. Shvartsman and V. A. Krongauz, Quasi-liquid crystals of thermochromic spiropyrans. A material intermediate between supercooled liquids and mesophases, J. Phys. Chem., 88, 6448-6453 (1984). 

An interesting variable in all this is the position of the melting point in the sequence (here, the term melting point indicates the temperature at which the solid melts to form a disordered phase, while clearing point indicates the temperature at which the isotropic hquid forms from a mesophase), for there is no real reason for the sohd to be less stable then the mesophase(s). Thus, Fig. 31 shows three possible examples of situations that could occur. In the flrst (A), the sample melts to the mesophase (LC), which then clears to give the isotropic liquid, and on cooling, the whole things reverses. This mesophase is termed enantiotropic. In the second example (B), the sohd melts directly to the isotropic liquid and then supercools into the mesophase (LC), which then crystallizes, here, the mesophase is termed monotropic. In the... 

The distinction between macromolecular liquid crystals and condis crystals may be helped by experience, although the differences are usually only a matter of degree. Liquid crystals seem always to convert fully from the liquid state to the liquid crystalline state, and the conversion to the mesophase on cooling shows rdatively little supercooling (few kelvins). Condis crystals, in contrast, show more often than not partial conversion from the melt and a relatively high supercoolir (10 or more kelvins), i.e. they are much closer in their behavior to fully-ordered, macromolecular crystals. The crystallization kinetics of liquid crystals has foimd little attention in the past. It seems describable by Avrami expressions with exponents of 2 to 4 >. Condis crystals, in contrast, have in the few analyzed cases shown exponents of less than 2 2 W) tjie transition entropy of the liquid crystal to the isotropic melt is... 

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