Analysis of elongational flow of star polymers
Manfred H. Wagner, Esmaeil Narimissa, Qian Huang
Abstract
Abstract Star polymers with three arms are the simplest example of branched polymers. Elongational rheology data of three well-characterized monodisperse polystyrene melts, a symmetric star, an asymmetric star, and a linear polymer with the same span molecular weight of 180 kg/mol reported by Huang et al. (Macromolecules 49:6694−6699, 2016) are analyzed by the enhanced relaxation of stretch (ERS) model (Wagner and Narimissa, J Rheol 65:1413–1421, 2021). All three melts show the same elongational stress growth coefficient and the same steady-state elongational viscosity in fast extensional flows when the stretch-related Weissenberg number $${Wi}_R=\dot{\varepsilon}{\tau}_R>1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>Wi</mml:mi> </mml:mrow> <mml:mi>R</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mover> <mml:mi>ε</mml:mi> <mml:mo>˙</mml:mo> </mml:mover> <mml:msub> <mml:mi>τ</mml:mi> <mml:mi>R</mml:mi> </mml:msub> <mml:mo>></mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> . Excellent agreement between experimental data of elongational stress growth coefficient and model predictions is obtained, based exclusively on the linear-viscoelastic characterization of the polymer systems. Stress relaxation following steady elongational flow depends on the presence of the branch point and the length of the arm, and a new process regarding relaxation of the orientation of the stars is identified.