Non-Newtonian Behavior of Low Density Polyethylene Melts

Abstract

Non-Newtonian behavior of low density polyethylene melts was studied at 190°C in a wide range of shear rate γ, namely from 1×10^-2 to 3×10³ sec^-1. Three types of commercial low density resins were fractionated carefully to obtain sharp fractions of branched polyethylenes, AF, BF and CF series, respectively, and one type of commercial high density resin to obtain a series of linear polyethylenes, HDPE fractions, as reference materials. The weight-average molecular weight of low density polyethylene fractions was determined by light scattering. The melt viscosities η of low and high density polyethylene fractions were measured by using a cone-plate type rheometer in the range of low shear rate and a capillary type rheometer in the range of high shear rate. The zero shear viscosity η₀ of the high density polyethylene fractions was proportional to M_v^3.53. The zero shear viscosities of two series (AF and BF series) of branched polyethylene fractions were higher than that of the linear polyethylene fractions of the same molecular weight in the range of molecular weight higher than 5×10⁴, while η₀ of the third series (CF series) of branched polyethylene fractions was always lower than that of the linear polyethylene fractions of the same molecular weight. The reduced viscosity η/η₀ of branched polyethylene fractions was not a unique function of τ_Bγ, but was a function of g₁τ_Bγ. Here τ_B is the Bueche relaxation time and g₁, is the branching index calculated by the Zimm-Kilb equation. The dependence of η/η₀ on the quantity g₁τ_Bγ was given by the modified Bueche-Harding equation, η/γ₀=1/[1+0.6 (g₁τ_Bγ)^0.75] for AF and BF series, and by a slightly different form for CF series.