Long-Chain Branching in Metallocene-Catalyzed Polyethylenes Investigated by Low Oscillatory Shear and Uniaxial Extensional Rheometry

This paper explores shear and extensional rheological behavior of unimodal, metallocene-catalyzed polyethylenes with low contents of long-chain branching (LCB). The polymers were produced in semibatch slurry polymerizations with methylaluminoxane (MAO) activated metallocene catalysts bis(n-butylcyclopentadienyl)hafnium dichloride (1), rac-[ethylenebis(2-tert-butyldimethylsiloxy)indenyl)]zirconium dichloride (2), rac-[ethylenebis(1-tert-butyldimethylsiloxy)indenyl)]zirconium dichloride (3), and rac-[ethylenebis(1-triisopropylsiloxy)indenyl)]zirconium dichloride (4). Melt properties in low oscillatory shear, in contrast to molecular weight and molecular weight distribution data from gel permeation chromatography, suggested that the polymers prepared with the ethylene-bridged complexes 2, 3, and 4 contain small but different amounts of LCB. In the melt uniaxial elongation experiments, the long-chain branched polymers exhibited strain hardening at all extension rates (rate range was from 1.0 to 0.01 s-1) with continual increase in strain hardening toward low strain rates. Unexpectedly, the behavior in LVE regime low shear and uniaxial elongation in the nonlinear range arranged the polymers in dissimilar order of apparently increasing LCB. Even though both these rheological techniques are sensitive to the molecular structure, they evidently reveal different features of it. Variation in the distribution (topology) of the long-chain branching due to differences in catalyst systems offers a plausible explanation of the differences in uniaxial elongation.