Extensional rheology of linear and branched polymer melts in fast converging flows

Abstract

Extensional rheology of a variety of linear and branched polymer melts is investigated using entry flow measurements and 15:1 axisymmetric contraction flow simulations. Using a Cogswell model analysis, we show that log−log plots of entrance pressure drop versus wall shear stress display three distinct power-law regimes, the intermediate one of which is observed beyond a critical stress associated with the onset of chain stretching effects. Our observations suggest that this stress threshold is a chain architecture-dependent property characteristic of entangled polymers. Converging flow methods are used to analyze the excess pressure losses to predict the uniaxial extensional viscosity. As the temperature is increased, the progressive shift of the kink to higher strain rates seen in the flow curves can be captured by a proposed Trouton ratio model, where the characteristic time of the fluid is assumed to follow the empirical William–Landel–Ferry (WLF) equation. Experimental pressure drops in converging flows for Weissenberg numbers up to about 10⁵ are used to evaluate predictions of an extended generalized Newtonian fluid (GNF-X) model, where a weighted viscosity for mixed flows has recently been derived and a weighting function classifies flows intermediate between shear and shearfree flows. Judging from its success in predicting the nonlinear extensional response of both linear and branched polymers, as well as its ability to differentiate the respective flow patterns, the GNF-X model should be useful for simulations of commercial polymer processing.

Graphical abstract