Molecular Weight Effects in Die Swell and in Shear Rheology

Abstract

Die swell was measured in an isothermal chamber for a series of well‐characterized polystyrene samples of differing molecular weight and molecular weight distribution (MWD). The data were organized around Tanner's die swell theory, which calls for a knowledge of the first normal stress function (N1), the shear stress function (σ), and an elastic modulus function (G). Independent rheogoniometric data for N1 and σ and certain assumptions about G were introduced. Many of the die swell data could be explained but large differences were noted for samples with an appreciable low‐molecular‐weight component; these small molecules seem to “lubricate” the swelling process and allow the “potential” die swell stored by the larger molecules to show itself. Also, the low shear asymptote for the die swell, usually reported to be about 1.1, was found to depend significantly on MWD. The independent rheogoniometric data showed distinct trends with MWD, with the exponent in N1=A σb varying from about 1.8 for narrowly distributed samples to about 1.4–1.6 for broadly distributed ones. These effects can be explained by combining a quadratic blending law with a viscoelastic constitutive equation.