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Interfacial agents are often used to compatibilize immiscible polymer blends. They are known to reduce the interfacial tension, homogenize the morphology, and improve adhesion between phases. In this study, two triblock copolymers of styrene/ethylene−butylene/styrene (SEBS), of different molecular weights, were used to compatibilize a blend of 80 vol % polystyrene (PS) and 20% ethylene−propylene rubber (EPR). The emulsification curve, which relates the average minor phase particle diameter to the concentration of interfacial agent added, was used to quantify the effect of the interfacial agents on the blend morphology. Charpy and Izod impact tests were performed to determine the effect of the compatibilization on mechanical properties of the blend and to establish links between morphology, interface, and properties. Results suggest that for the lower molecular weight interfacial agent, a transition in fracture mechanisms, from fragile to ductile, occurs around 20% interfacial agent (based on the volume of the minor phase). This transition, however, is not observed with the high molecular weight interfacial agent.

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The size of domains in a series of compatibilized polystyrene−(ethylene-propylene rubber) blends were measured by solid-state NMR spin diffusion measurements. The average diameter of ethylene propylene rubber (EPR) particles in the blends was observed to decrease as the concentration of interfacial agent was increased up to ca. 15% (weight/volume of EPR) and remained constant upon further addition of interfacial agent. Comparison of the domain sizes obtained from NMR measurements with those obtained from scanning electron microscopy measurements suggests that the NMR technique can be used to confirm large-scale phase separation and investigate the trends in domain sizes in immiscible blends. It was found, however, that in these blends the spin diffusion is slow due to the large size of the domains; as a consequence the uncertainty of the size obtained from 1H spin diffusion is fairly large. It was also found that mechanical grinding of the NMR samples can produce changes in the phase structure of the blends.