Immiscibility in Glass Forming Systems

So many glass forming melts exhibit liquid-liquid immiscibility that it might be argued that the occurrence of phase separation in glasses is the norm rather than the exception. The extent of phase separation can range from the sub microscopic level, which requires detection by use of electron microscopes, to the macroscopic level, represented by complete separation into two layers in the melt. These differences in the effect of phase separation on the physical appearance of glasses are usually the 

Figrue 4.3 Micrograph of a phase separated glass with a sphere in a matrix morphology 

Thermodynamic factors indicate that a system which exhibits liquid-liquid immiscibility should be phase separated at any temperature between absolute zero and T. Pragmatically, however, we realize that phase separation will not occur if the viscosity of the melt is too great. For most purposes, this means that no changes in morphology occur below the glass transformation region. On the other hand, phase separation will never occur in a melt held at a temperature greater than 

It appears, then, that a temperature window exists between T, and =Tg where a melt can change its microstructure through phase separation. 

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