Phase Behavior of Polymer Blend Systems

There are several criteria for determining the miscibility in polymer blends. The thermodynamic criteria date back to the seminal studies of Flory (1941, 1942) and Huggins (1941, 1942) in the 1940s. Since then, other thermodynamic theories have been developed (Flory 1965 Flory et al. 1968 Orwoll and Flory 1967 Sanchez and Lacombe 1978) for investigating the phase behavior of polymer blends. As the discussion of thermodydnamic criteria for miscibility in polymer pairs is beyond the scope of this chapter, here we illustrate briefly different phase behaviors of polymer blends in general terms, using examples that will facilitate our presentation in the rest of this chapter. 

What is clear from the observations made above is that miscibility in polymer blends depends on temperature and blend composition. Thus it is essential for one to have information on the miscibility of a polymer blend system before taking rheological measurements. Needless to say, the rheological behavior of a polymer blend in the single-phase region (miscible polymer blend) will be quite different from that in the two-phase region (immiscible polymer blend). 

The equilibrium phase diagrams (binodal curves) enable us to describe the extent of miscibility (or immiscibility) of a polymer blend system. A miscible polymer blend 

The locations of binodal curves and thus values of x for a blend system depend on temperature T, molecular weight Af, and blend composition (f). Thus values of x must be expressed in terms of T, Af, and p in the most general situation. However, there is scarcely any experimental data yet available for obtaining such a general expression for X in terms of T, Af, and (p. Certainly, the Rory-Huggins lattice theory does not take into account the effects of Af and f . 

Extending the lattice cluster theory (Dudowicz and Freed 1991a, 1991b), Freed and Dudowicz (1992) and Dudowicz and Freed (1993) have shown that the molecular weight dependence of x for binary blends can be expressed by 

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