NONLINEAR VISCOELASTIC CHARACTERIZATION OF A LINEAR POLYETHYLENE (HDPE) MELT IN ROTATIONAL AND IRROTATIONAL FLOWS

We analyse the rheology of a commercial linear high-density polyethyle ne (HDPE) melt with a broad molecular mass distribution in uniaxial, e quibiaxial, and planar extension as well as in shear on the basis of a single integral constitutive equation with time/deformation separabil ity. In shear, the HDPE melt is in good approximation a Doi-Edwards (I .A.A) fluid, while in extension, it is a molecular stress function (MS F) fluid with considerable strain hardening. In molecular terms this m eans that in shear flow, molecules are only orientated, but not stretc hed, while they are orientated and stretched up to a factor of 5.5 by irrotational flows. (C) 1998 Elsevier Science B.V. All rights reserved .