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Architecturally complex model polymers in nonlinear shear flows

Frank Snijkers Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology – Hellas (FORTH), Heraklion, Crete, Greece

In the last two decades, we have witnessed a remarkable progress in the understanding of the viscoelasticity of model branched polymers. The key ingredient that catalyzed the developments in this field is the synergy of state-of-the-art chemistry (high-vacuum anionic synthesis), physico-chemical characterization (NMR, chromatography), physical experiment (rheology, neutron scattering) and modeling (tube-based, slip-links). The linear viscoelastic relaxation mechanisms of architecturally complex polymers, such as stars, H-polymers and combs, have been elucidated and, despite debatable issues dealing primarily with the extend to which some of the involved physical processes play a role, the predictions of current parameter-free tube-based models are in excellent quantitative agreement with experimental data. With linear relaxation mechanisms understood, nonlinear rheology and complex flows are gaining interest from both the experimental as well as theoretical side. Nonlinear flow experiments are however difficult for highly viscous and elastic polymer melts due to several complications, such as wall slip and the occurrence of flow instabilities. In this work, we attempted to obtain artifact-free, reliable nonlinear shear data for well-defined model branched polymers with the help of a special geometry (“cone partitioned-plate”), to increase our understanding of the behavior of model branched polymers in nonlinear shear deformations. The polymers have been carefully analyzed with interaction chromatography and their linear rheology has been studied in great detail. The obtained nonlinear data exhibits systematic variations with molecular characteristics of the polymers and are rationalized by invoking the key ideas from the linear response (hierarchical motion, dynamic dilution). The results enhance our understanding of the effects of polymer architecture, i.e. branching levels, on the nonlinear start-up and relaxation upon cessation of steady shear flow behavior of branched polymers.