Submicron Polyethylene Particles from Catalytic Emulsion Polymerization

Abstract

Particles of linear polyethylene (Mn = (2-3)X 10000 g mol-1; Mw/Mn = 2-4) obtained by catalytic emulsion polymerization of ethylene possess a nonspherical, lentil-like shape with an average aspect ratio of ca. 10 and diameters from 30 to > 300 nm, as determined by TEM and AFM. The particle structure results from a stacking of the lamellae along the one shorter axis of the lentils (i.e., their height, by contrast to the diameter). In addition to these multilamellae particles, remarkably, a considerable number of the particles consist of only a single lamella. The thickness of the lamellae (spacing) as determined by TEM is only 9-11 nm, depending on the polymerization temperature during the synthesis of the dispersions. Crystallization in the dispersions during emulsion polymerization differs dramatically from the usual heterogeneous nucleation of bulk polyethylene samples. Each submicron droplet crystallizes individually, independently from the other droplets, resulting in large supercoolings of around 55 °C.