Shear and Extensional Properties of Three-Arm Polystyrene Solutions

Abstract

Following the impressive advances in understanding the rheology of linear polymers using the basic Doi−Edwards model of reptation and its variants, rheologists are turning their attention to branched polymers. This is primarily motivated by a desire to understand the connection between chain topology and rheology. We examine the behavior of star polymers in shear and extensional flows. We use this study to illustrate the dramatic effect that the branch point has on both shear and extension. Data on steady and oscillatory shear and step strain shear are presented and analyzed using the Milner−McLeish model. Excellent agreement, using a consistent parameter set, is obtained between this theory and experiments. The filament-stretching rheometer is used to obtain the extensional viscosity of these solutions. The steady-state extensional viscosity shows strain rate softening at moderate strain rates and strain rate hardening at high strain rates. At low Weissenberg numbers (based on the longest relaxation time), the steady extensional stress depends on the number of chain entanglements. At the other extreme, at high Weissenberg number (based on the Rouse time), the steady-state stress depends only on the Weissenberg number. Qualitative similarities and some differences with linear polymers are noticed. The differences appear to be related to the different time constants for the longest relaxation times in these systems.