Amorphous supercooling

The amorphous supercooled melt obtains shape stability by chemical or mechanical cross-links and then behaves like an isotropic rubber with low elastic modulus and high recoverable or even reversible strain. The cross-links transform the melt into a network of N individual chain links per volume which yield a restoring force coefficient... 

The finding that the VHDA HDA transformation is continuous [65], as opposite to the pressure-induced, apparently discontinuous LDA-HDA transformation, has implications for our understanding of the metastable phase diagram of amorphous supercooled and glassy water. It seems counterintuitive that a continuous amorphous-amorphous transition at 140K changes character into a discontinuous liquid-liquid transition when performed above Tg. On the other hand, it seems quite possible that a first-order-like amorphous-amorphous transition develops into a first-order liquid-liquid transition when performed above Tg. Since first-order... 

Between T j, and Tg, depending on the regularity of the polymer and on the experimental conditions, this domain may be anything from almost 100% crystalline to 100% amorphous. The amorphous fraction, whatever its abundance, behaves like a supercooled liquid in this region. The presence of a certain degree of crystallinity mimics the effect of crosslinking with respect to the mechanical behavior of a sample. 

Electron diffraction studies indicate that phosphoms pentoxide vapor consists of P O q molecules. The vapor usually condenses to the hexagonal crystalline modification but under rapid cooling can be condensed to an amorphous soHd (P-form). The Hquid obtained by melting the stable orthorhombic modification cools to form a glass which is the P-form. The Hquid obtained from the H modification also can be supercooled to a glass. 

Supercooling The rapid cooling of a normally crystalline plastic through its crystallization temperature, so it does not get a chance to crystallize and it remains in the amorphous state. 

Tammaun has advanced the view that amorphous solids are really liquids which have been cooled far below their freezing-points, and have thereby acquired great viscosity, but have not crystallised. They are supercooled liquids. This hypothesis is supported by the following evidence ... 

Continuous transition of state is possible only between isotropic states it may thus occur between amorphous glass (i.e., supercooled liquid of great viscosity) and liquid ( sealing-wax type of fusion ), or between liquid and vapour, but probably never between anisotropic forms, or between these and isotropic states. This conclusion, derived from purely thermodynamic considerations, is also supported by molecular theory. 

Water is a very structurally versatile molecule. Water exists in all three physical states solid, liquid, and gas. Under extremely high temperature and pressure conditions, water can also become a supercritical fluid. Liquid water can be cooled carefully to below its freezing point without solidifying to ice, resulting in two possible forms of supercooled water. In the solid state, 13 different crystalline phases (polymorphous) and 3 amorphous forms (polyamorphous) of water are currently known. These fascinating faces of water are explored in detail in this section. 

FIG. 31 Schematic diagram illustrating the transition between a supercooled liquid state (rubber) and an amorphous solid state (glass). The glass transition event is typically caused by a decrease in water content and/or temperature. The reversibility of the transition, as indicated by the dotted arrow, is material dependent (see text for further discussion of the reversibility of the transition). 

The preceding observations stimulated Olander and Rice 4> to search for a substance that is simultaneously simpler" than water yet a "good model of it. They suggested that amorphous solid water [H O/as)], first reported by Burton and Oliver 5> in 1935, satisfied these two requirements. Unlike the liquid, amorphous solid water can be studied at low temperature where the effects of thermal excitation and positional and orientational disorder can be separated. Moreover, it is plausible to accept as a working hypothesis that the amorphous solid is, essentially, extensively supercooled liquid water if so, the properties of the amorphous solid should be directly related to those of the liquid. 

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