Amorphous equilibrium supercooled liquid state

Calculation of the thermodynamic quantities requires the system under investigation to be in equilibrium. For an amorphous system, this is achieved in the temperature range above the Tg, in the supercooled liquid state. Theoretically, the calculations of the configuration thermodynamic properties are not valid for the glass however, the behavior of the amorphous form above the Tg may give an indication on the stability behavior below the Tg and calculation of these parameters provides further information on the behavior of the amorphous form. 

In 1931 Simon reported that small molecules in their amorphous solid state are not in thermodynamic equilibrium at temperatures below their glass transition u. Such materials are in fact supercooled liquids whose volume, enthalpy, and entropy are greater than they would be in the equilibrium glass. (See Fig. 1). 

Amorphous solid substances have no fixed arrangement of their molecular bulks and are not in a definable state, either in energy content or structurally. One view is to consider an amorphous substance as an entanglement of molecules of many shapes and sizes. This is particularly true of phosphate and silicate glasses when considered as supercooled liquids. Their properties depend upon much history. They are not in a thermodynamically defined state. This means that they are not in equilibrium with their environment under existing conditions. Then there must be some other state of matter made of these same molecules (or ions) that has less energy than the amorphous system. These lower-energy systems are usually crystalline. 

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