Thermotropic liquid crystals supercooled liquids

DYNAMICS OF THERMOTROPIC LIQUID CRYSTALS ACROSS THE ISOTROPIC-NEMATIC TRANSITION AND THEIR SIMILARITY WITH GLASSY RELAXATION IN SUPERCOOLED LIQUIDS... 

Dynamics of Thermotropic Liquid Crystals Across the Isotropic-Nematic Transition and Their Similarity with Glassy Relaxation in Supercooled Liquids 249... 

C. Supercooled Liquids and Thermotropic Liquid Crystals Similarity in Dynamics... 

It is beyond the scope of this review to be exhaustive in the field of supercooled liquids that has drawn intense research activities over several decades. Reviews that are exclusive for this field and deal with specific topics in considerable detail are recommended for supplemental reading [9-11]. In view of the scope this chapter, the next section provides the readers with a brief introduction to the systems of interest and associated nomenclature. Section III sets up the background by reviewing experimental results on the dynamics of thermotropic liquid crystals across the I-N transition, then introducing the central issues in the dynamics of supercooled liquids, and finally comparing the dynamics of the two systems in the light of recent experiments. Section IV presents a summary of some of the well-known theoretical approaches to liquid crystals. Section V provides a detailed account of computational efforts. Finally, we conclude in Section VI with a list of problems for future work. 

In this section of the review we restrict ourselves to those aspects of dynamics of supercooled liquids that appear to be analogous with dynamical features of thermotropic liquid crystals across the I-N transition. Power law relaxation in short-to-intermediate time scales is well known for supercooled liquids. We first review recent computational efforts that provided insights into power law relaxation in supercooled liquids. We then focus on a model system tailored specifically to study analogous dynamical features of the two seemingly different classes of soft matter systems. 

Liquid crystals can be divided into two main classes those similar to the cho-lestrayl derivatives, whose liquid crystalline phases are formed when the pure compound is heated, arc called thermotropicy and those where the hquid crystalline phase forms when the molecules are mixed with a solvent are referred to as lyotropic. The thermotropic class also includes enantiotropic types, whae the liquid crystalline phases can be seen on both the heating and the cooling cycles, and monotropic types, where the mesophase is stable only on supercooling from the isotropic melt. 

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... 

Fig. 5.24. Schematic representations of the DSC trace and change in volume on heating a thermotropic polymer liquid crystal. After the first heating, the mesophase supercools and vitrifies on subsequent heatings (lower curves) the DSC trace and change in volume reveal transformations from glass to mesophase at T the mesophase goes isotropic at T x.

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