Characteristics of bimodal polyethylene prepared via co-immobilization of chromium and iron catalysts on an MgCl2-based support

Bimodal polyethylenes comprising varying proportions of high- and low-molecular-weight fractions are synthesized in a single polymerization stage, via the co-immobilization of a chromium and an iron catalyst on an MgCl2/AlEtn(OEt)3-n support. Changes observed in the viscoelastic response of the polymer melt with increasing content of the high-molecularweight fraction indicate effective mixing in the bimodal blend. In flow, chains in the high-molecular weight fraction tend to orient and stretch under shear. Due to the longer relaxation time of the high-molecular-weight component, X-ray diffraction and scattering reveal that shear-induced crystallization takes place at temperatures close to the equilibrium melting point of linear polyethylene. The so-crystallized high-molecular-weight component suppresses the nucleation barrier for further crystallization, leading to the formation of a "shish-kebab" polymer morphology. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.