Isotropization temperature definition

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. 

Navard and Haudin studied the thermal behavior of HPC mesophases (87.88) as did Werbowyj and Gray (2), Seurin et al. (Sp and, as noted above, Conio et al. (43). In summary, HjPC in H2O exhibits a unique phase behavior characterized by reversible transitions at constant temperatures above 40 C and at constant compositions when the HPC concentration is above ca. 40%. A definitive paper has been recently published by Fortin and Charlet ( who studied the phase-separation temperatures for aqueous solutions of HPC using carefully fractionated HPC samples. They showed the polymer-solvent interaction differs in tiie cholesteric phase (ordered molecular arrangement) from that in the isotropic phase (random molecular arrangement). 

The above definition thus includes the classical fibrillar crazes (formed on surfaces, in thin films or in the bulk at lower or higher temperatures in preoriented or isotropic material, in the presence or absence of a solvant or diluant) as well as those crazes which contain so many voids and cross-ties within the heavily deformed craze matter that it becomes difficult to identify individual fibrils. The Editor is extremely grateful to all authors of this volume and to Dr. G. H. Michler for frequent discussions on the above subject and for their constructive comments which led to the above Jointly proposed definition. 

There are one or two exceptions to this rule, however, such as glass and pitch. These substances remain soft for a considerable interval of temperature, becoming less and less viscous as the temperature rises, until they ultimately hquefy. This property is peculiar to so-called amorphous, i.e. non-crystalline, bodies. We shall exclude bodies of this kind from consideration, and also hquid crystals and crystalhne fiuids, substances which appear to combine the characteristic properties of the sofid and of the hquid states. They have the fixed configuration of the molecules in space peculiar to sohds, as well as the mobihty of liquids. The transition from an anisotropic crystal to a perfectly isotropic fluid always takes place at one definite temperature. The softening of crystals in the neighbourhood of the melting point is not an exception to this rule, since these softened crystals are still anisotropic. 

The Flux Expressions. We begin with the relations between the fluxes and gradients, which serve to define the transport properties. For viscosity the earliest definition was that of Newton (I) in 1687 however about a century and a half elapsed before the most general linear expression for the stress tensor of a Newtonian fluid was developed as a result of the researches by Navier (2), Cauchy (3), Poisson (4), de St. Venant (5), and Stokes (6). For the thermal conductivity of a pure, isotropic material, the linear relationship between heat flux and temperature gradient was proposed by Fourier (7) in 1822. For the difiiisivity in a binary mixture at constant temperature and pressure, the linear relationship between mass flux and concentration gradient was suggested by Pick (8) in 1855, by analogy with thermal conduction. Thus by the mid 1800 s the transport properties in simple systems had been defined. 

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. 

The first indication that the surface-induced order in the isotropic phase of confined liquid crystals can be detected by deuteron NMR was the spectrum of selectively deuterated 4 -n-pentyl-4-cyanobiphenyl (5CB) in the spherical cavities of a PDLC material. Far above Sfi C, where the bulk turns into the isotropic phase, the spectral line (a single one) of the PDLC was considerably broader than in the bulk at the same temperature. An even more definite and surprisingly clear evidence of surface-induced order was later obtained from... 

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