ON NUCLEATION AND ISOTHERMAL CHANGES IN SUPERCOOLING IN A PARTIALLY MISCIBLE POLYPROPYLENE POLYETHYLENE BLEND/

The microstructure of branched polyethylene/polypropylene blends, to 3 0% of polypropylene, has been investigated with the transmission elect ron microscope (TEM) following permanganic etching. The gross structur e is one of polypropylene dispersed within a polyethylene-rich matrix, reflecting a degree of liquid/liquid phase separation in the melt. Fo r concentrations of polypropylene above approximately 20%, heterogeneo us nuclei cause polypropylene to crystallize before, and independent o f, the polyethylene. At lower concentration, such nuclei become too di spersed to affect the majority of the polypropylene, almost all of whi ch then solidifies after the polyethylene. Polyethylene tends to nucle ate around polypropylene spheres, but the nature of the interface diff ers according to which polymer crystallizes first. If it is polyethyle ne, the two regions do not interpenetrate, but they do when polypropyl ene crystallizes first because polypropylene lamellae then grow out fr om polypropylene droplets into the polyethylene-rich matrix, strengthe ning the mutual interface. When lamellae cross into the matrix, their isothermal growth continues, but more slowly. They are thicker, furthe r apart on average, and show little or no cross-hatching. These are al l consequences of growth occurring at much reduced supercooling becaus e 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 chan ge of composition affecting polymeric growth from the melt.