On Crystallization in Polypropylene-Polyethylene Blends

Blends of polypropylene (PP) and low density polyethylene (LDPE) have been crystallized to form open structures("cluster spherulites") where the droplets of the minority PP are bridged by PP lamellae which have grown in the LDPE-rich matrix. These are studied by permanganic etching followed by electron and optical microscopies. Two similar PP types are compared, one synthesized with Ziegler-Natta catalyst and one with metallocene catalyst. The metallocene-catalysed material crystallized much more slowly due to the presence of regio defects in the chains, even though the overall concentration of tacticity defects in the Ziegler-Natta material is much higher. A mechanism involving reversal of helical direction at the regio defect interfering with the regular chain packing in the crystal is suggested. Growth of "cluster spherulites" is faster in regions where low molecular weight material is concentrated. It is slower where droplets are larger, and this is attributed to competition between PP lamellar growth in the matrix and diffusion of PP to already crystallized droplets.