Effect of Short-Chain Branching on the Rheology of Polyolefins

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 m b, 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 m b. 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.