Effect of Phase Separation on Viscosity

Phase separation can radically alter the viscosity of a melt. If stable immiscibility leads to complete separation into two layers of liquid, 

The effects of phase separation on viscosity are more subtle for compositions exhibiting metastable immiscibility. In many cases, the fine scale of the morphology is such that no indication of phase separation is observed by the naked eye. The measured viseosity will be a funetion of the compositions of the equilibrium liquids, the connectivity of each phase, and the scale of the morphology. If the phase with the higher viscosity has a connected structure, the viscosity of the lower viscosity phase will be of little importance since the measured viscosity will be determined by the less mobile phase. On the other hand, if the phase with the higher viscosity exists only as isolated spheres within a matrix of the less viscous phase, the measured viscosity will be near that of the more fluid phase. Any thermal treatment which alters the connectivity 

If we heat a sample which has a morphology containing a connected, high viscosity phase, we can, if the composition lies in the appropriate region of the immiscibility dome, pass the upper limit for connectivity of the high temperature phase. If this occurs, the connectivity of that phase will rapidly decrease and the measured viscosity will revert to that of the lower viscosity phase. Since that viscosity is typically several orders of magnitude below that of the more viscous phase which had been controlling the measured viscosity, the viscosity of the sample will appear to abruptly decrease and the melt will suddenly become much more fluid. This effect can occasionally have unfortunate side effects if the sample now flows much more rapidly than anticipated. 

The relation between phase separation and viscosity has been studied as a function of time during which the scale of phase separation increases. Under these circumstances, the viscosity has been observed to increase by as much as five orders of magnitude during an isothermal heat treatment for times of several hundred hours. The viscosity initially changes rapidly as the connectivity of the structure and the compositions of the equilibrium phases approach their final values, and then more slowly as coarsening, or growth in the scale of the microstructure, occurs. 

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