This paper reviews the phase behavior of polymer blends both in equilibrium and under flow. Thermodynamic issues are discussed first. They reveal that the equilibrium behavior of polymer blends is ruled by a subtle balance of energetic and entropie effects. According to an cquation-of-stale theory the phase behavior is directed by the interplay of interaction, free-volume and sizeeffects. It results that phase separation at elevated temperatures is entropy driven whereas at low temperatures it is associated with unfavorable enthalpic effects. Finally, attention will be focused on flow-induced phase changes in polymer blends. Experimental results and theoretical approaches arc outlined. It becomes obvious that shear-induced phase changes depend on the stored clastic excess energy under steady-state conditions. Theoretical and experimental results agree qualitatively.
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