Effect of Short-Chain Branching on the Rheology of Polyolefins

摘要

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 G_N~0,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 delta)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,tau_e,and monomeric friction factor,zeta.