How nucleating particles migration affects the fractionated crystallization of isotactic polypropylene/polystyrene immiscible blends

We investigate the inter-phase migration of nucleating particles and their effect on the fractionated crystalli-zation of polypropylene droplets dispersed in an immiscible polystyrene matrix by preparing melt-mixed 90/10 wt% PS/PP blends with selected nucleating agents. Different concentrations of nucleating agents (NAs): Phta-locianene Blue, NA11, Talc, and sodium benzoate, were initially incorporated either in neat PP or in neat PS. Then the different PS/PP/NAs blends were prepared. Contact angles were determined for the neat polymers and the NAs, and the results indicated that, from a thermodynamics point of view, all NAs should remain or migrate to the PS phase. According to SEM observations, a sea-island morphology was obtained that did not vary with NA addition. Hence the results should be independent of differences in the morphology. PP droplets crystallized in neat blends in three well-differentiated crystallization peaks (i.e., fractionated crystallization). The NAs induce a concentration-dependent increase in the high-temperature crystallization peak at the expense of reducing the low-temperature peaks, as droplets become in contact with or contain the NAs. When compared at a constant concentration in one of the phases, the nucleating agents have efficiencies that decrease in the order: Pht Blue approximate to NA11>Talc>Sod Benz. Surprisingly, for the three most effective NAs, it was found that adding them initially to the PS phase led to a much higher nucleation efficiency (evaluated in terms of increases in peak crystallization temperature) than adding them to PP. This indicates that these NAs, initially present in the PS phase, migrate to the interface or to the bulk of the PP phase (against thermodynamics). Sod Benz was the only exception, as it shows a higher thermodynamic affinity to the PS phase. We explain the migration of most particles from the PS phase to the phases' boundary or to the bulk of the PP droplets by considering that kinetic factors dominate the behavior, as, from a rheological perspective, the minor component (PP) always displays a lower viscosity than the matrix component (PS) under the chosen mixing conditions.

Automated summary

The inter-phase migration of nucleating particles and their effect on the fractionated crystallization of polypropylene droplets dispersed in a polystyrene matrix were investigated. The nucleating agents show a concentration-dependent increase in the high-temperature crystallization peak at the expense of reducing the low-temperature peaks. Surprisingly, the most effective nucleating agents have a higher nucleation efficiency when initially added to the polystyrene phase rather than the polypropylene phase. The migration of particles from the polystyrene phase to the phases' boundary or the polypropylene droplets is explained by considering the dominance of kinetic factors.