Thermotropic transitions definition

Further theoretical studies by Floryconcerned the clarification of phase transitions for the systems with a definite distribution of macromolecules over the length and also the analysis of equilibrium for a model system composed of macromolecules in which rigid blocks are separated by flexible units. An example of such systems are copolyesters exhibiting the thermotropic transition into the liquid crystalline state in the absence of a solvent... 

This implies a fast, and complete, reorientation of the side groups, as in the molten state. Nevertheless, local anisotropy of motion is retained up to the much higher isotropization temperature (Ti). Great heat absorption is involved in the Tc transition, while a minor DSC signal is detected at the melting. Such a dramatic transformation of a large mass of the solid at Tc temperature supports the definition of a crystalline-to-mesophase thermotropic transition. 

A Hquid crystal compound in more cases than not takes on more than one type of mesomorphic stmcture as the conditions of temperature or solvent are changed. In thermotropic Hquid crystals, transitions between various phases occur at definite temperatures and are usually accompanied by a latent heat. 

Liquid crystals is one of the names by which certain mesomorphic phases or mesophases are known, and as the name implies refers to materials which show a high degree of structural order, yet are liquid. The topic has been reviewed recently. " Those most used in NMR work have been the thermotropic variety which have a definite meso-phase range between the melting point and a second transition producing the isotropic liquid. Both pure substances (e.g. p-azoxyanisole) and mixtures (e.g. butyl-/7-(p-ethoxybenzoyl)phenyl carbonate and p-(/7-ethoxyphenylazo)phenyl heptanoate) which provide a mesophase at room temperature, have been used. 

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