Miscibility Studies on Heterogeneous Polymer Blends by Radioluminescence Spectroscopy and Particle Coarsening Measurements

Interest in the subject of polymer miscibility has been stimulated by the investigation and common use by industry of heterogeneous polymer systems. Particularly, in recent years we have seen the development of materials, such as ABS, high-impact polystyrene, block copolymers, thermoplastic elastomer blends (TPR, etc.), and many more, which owe their unique properties to a certain critical degree of immiscibility of the polymeric constituents. This subtle difference in miscibility contributes to the formation of morphological features in the above-mentioned materials. More importantly, it governs the adhesion between the domains of the phase-separated polymeric composite. The latter property provides for stress transfer across the interface and thus is needed for the attainment of physical strength. Thus the questions posed by researchers are not so much concerned with whether two polymers are fully miscible on a molecular scale, a rare event indeed, but rather in the degree of miscibility of the two materials. In spite of recent advances made, the bulk miscibility of polymers cannot be predicted by theory. The lattice theory of Flory leads to unsatisfactory conclusions. It can, however, be used for an after-the-fact representation of experimental findings by use of an empirically determined concentration- and temperature-dependent polymer interaction coefficient. More insight and qualitative postulates on polymer miscibility are provided by the equation-of-state theory. It correctly predicts the marked influence of the degree of polymerization and of the thermal expansion and pressure coefficients and, more importantly, it anticipates the lower critical solution temperature observed in a number of polymer systems. However, the equations are complicated and contain many generally unknown, but experimentally accessible, parameters, and thus this theory too is of little help to the investigator seeking miscibility data for a specific pair of polymers.