Effects of molecular weight distribution and long‐chain branching on the viscoelastic properties of high‐ and low‐density polyethylene melts

Abstract

Abstract Using the Han slit/capillary rheometer, rheological measurements were taken of several commercially available low‐ and high‐density polyethylene melts, namely, three low‐density polyethylene samples of Chemplex Corp. (CX 1005, CX 1016, and CX 3020), three low‐density polyethylene samples of U.S. Industrial Chemicals Co. (NA 205, NA 244, and NA 279), two high‐density polyethylene samples of Union Carbide Corp. (DMDJ 5140 and DMDJ 4306), and two high‐density polyethylene samples of Mitsui Petrochemical Industries, LTD. Molecular characterization of these samples was carried out by the resin suppliers. The rheological measurements included (1) entrance pressure drop, (2) exit pressure, (3) pressure gradient, (4) die swell ratio. These then permitted us to determine the shear viscosity and normal stress differences. The rheological measurements were interpreted to identify the effects of long‐chain branching and molecular weight distribution on the rheological properties of polyethylenes in the light of the existing molecular viscoelasticity theories. It was found that fluid elasticity is greater for polymers having a broader molecular weight distribution and that, for polymers having more long‐chain branching, viscosities are lower while elasticities are higher.