Effect of Short-Chain Branching on the Rheology of Polyolefins

Abstract

A series of carefully synthesized ethylene/butene copolymers (0.2−0.85 mole fraction butene) were synthesized using metallocene catalysts to probe the relationship between the chemical architecture of polyolefins and their rheology. The dependence of , the plateau modulus, on mb, the molecular weight per backbone bond, which we have seen previously for polyolefins is obeyed by these copolymers. Moreover, the modulus at the frequency at which G‘ = G‘ ‘ also scales with copolymer composition. We also show that the van Gurp−Palmen plots (|G*| vs δ) for these copolymers show a universal behavior, which could allow for the characterization of copolymer composition by rheology. We demonstrate that combining small-amplitude oscillatory shear data with stress relaxation experiments the theory of linear viscoelasticity allows us to determine low-frequency behavior much more rapidly. The zero shear viscosity of these ethylene copolymers also shows a regular dependence on the copolymer composition, and we propose a new scaling relationship for zero shear viscosity in terms of mb. We show that this applies to a wide range of polyolefins and show similar relations for the equilibration time, τe, and monomeric friction factor, ζ.