Coagulation, polymer blend phase separation

Several implications can be drawn directly from Eq. (2-39). First, A // is always positive. Thus, the rule like attracts like, inferred from Eq. (2-30) for molecular mixtures, should also hold at the continuum level. Second, when dispersion forces are dominant, the Hamaker constant is small when ha= b—that is when the dispersed phase (A) has an index of refraction close to that of the medium (B), These rules also apply to molecular mixtures. Nevertheless, small molecules with a significant difference in index of refraction often mix because of the large entropy thereby gained. But particles lose too little entropy on coagulation to resist doing so when there is an attractive van der Waals interaction, and so particle-particle clumping is the norm in suspensions, unless countermeasures are taken to stop it (see Section 7.1). Analogous considerations explain the prevalence of phase separation in polymer blends (see Section 2.3.1.2). 

Recently, combinations of polymers meeting the basic concept of molecular composites have been reported. Poly-(p-phenylene terephthalamide) miscibility with PA-6 and PA-66 was reported [Kyu et al., 1989]. The blends were prepared by rapid coagulation of methane sulfonic acid solutions in water. Above 70% of the rigid rod polymer, polyamide crystallization disappears implying a level of intermixing of the blend constituents. However, thermal treatment results in phase separation thus indicating metastability for this combination. 

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