Metastable region, polymer blend phase separation

Most polymer blends phase-separate with LCST thus the miscibility region stretches from the melting point or Tg up to the binodal while the phase-separated region exists above the spinodal temperature, Ts [1, 212]. Within the region between binodal and spinodal the system is metastable, characterized by strong interrelation between the rheology and thermodynamics [211, 213-215]. 

In polymer blends, different phase behaviors can be observed as shown in Figure 27.2. It is possible to calculate the compositions at which the blend will always separate into more than one phase (two-phase region), and also those compositions in which the blend may either form a single phase or will separate into several phases by different mechanisms, either in the metastable or in the spinodal regions. The binodals separate the miscible (one phase) and the metastable regions the metastable region is... 

The line on the temperature vs. composition phase diagram for a mixture of two components, which separates the metastable region from the single-phase regions. Hence, it represents the limits of stability in a two-phase system, viz., a polymer solution or polymer blend. 

The area below the spinodal curve is the region of absolute instability of a polymer blend. The phase separation in this region is controlled by a spinodal mechanism. The region between spinodal and binodal curves is called the metastable region. Phase separation in this region is controlled by a nucleation mechanism. 

FIGURE 1.1 Phase behavior of polymer blends with the upper and the lower critical solution temperature UCST and LCST (A) single-phase miscible region between two binodals, (B) two-phase separated regions of immiscibiUty, surrounded by spinodals, (C) four fragmented metastable regions between binodals and spinodals. (Adapted from Ougizawa Toshiaki and Inoue Takashi. Polym. J., 18, no. 7, 521-527, 1986.)... 

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