Double stress overshoot in startup shear flow and failure of Cox–Merz rule of pom-pom polymers.

The understanding of long chain branched homopolymer melts in shear flow is of significant interest to polymer science and critical to ensure stable processing. We report on the startup shear flow at a constant shear rate of well-defined pom-pom shaped polymer melts. Pom-poms consist of two stars covalently connected by a single backbone. For low-arm numbers and short arms, a single stress overshoot followed by a steady state is observed, similar to linear and star shaped melts. For one highly branched pom-pom with entangled branches, a double stress overshoot during the startup shear flow could be observed before reaching the steady state viscosity. Additionally, we find that the Cox–Merz rule, which relates the complex viscosity η * ω to the steady state shear viscosity η γ ̇ , fails at high-shear rates, if the backbone is not self-entangled. [ABSTRACT FROM AUTHOR]