On nucleation and isothermal changes in supercooling in a partially miscible polypropylene/polyethylene blend

The microstructure of branched polyethylene/polypropylene blends, to 30% of polypropylene, has been investigated with the transmission electron microscope (TEM) following permanganic etching. The gross structure is one of polypropylene dispersed within a polyethylene-rich matrix, reflecting a degree of liquid/liquid phase separation in the melt. For concentrations of polypropylene above approximately 20%, heterogeneous nuclei cause polypropylene to crystallize before, and independent of, the polyethylene. At lower concentration, such nuclei become too dispersed to affect the majority of the polypropylene, almost all of which then solidifies after the polyethylene. Polyethylene tends to nucleate around polypropylene spheres, but the nature of the interface differs according to which polymer crystallizes first. If it is polyethylene, the two regions do not interpenetrate, but they do when polypropylene crystallizes first because polypropylene lamellae then grow out from polypropylene droplets into the polyethylene-rich matrix, strengthening the mutual interface. When lamellae cross into the matrix, their isothermal growth continues, but more slowly. They are thicker, further apart on average, and show little or no cross-hatching. These are all consequences of growth occurring at much reduced supercooling because of the lowered equilibrium melting temperature for polypropylene in the miscible blend. The presence or absence of similar changes may be used in other systems to indicate whether or not there is a local change of composition affecting polymeric growth from the melt.