Berlin 2012 – scientific programme

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BP: Fachverband Biologische Physik

BP 21: Focus: Stress Relaxation in Polymers - From single molecules to biological cells (with CPP)

BP 21.1: Invited Talk

Thursday, March 29, 2012, 09:30–10:00, C 243

Stress relaxation and chain dynanics in entangled polymer melts — •Ralf Everaers — Laboratoire de Physique, ENS Lyon, 46 allee d’Italie, F-69364 Lyon, France

High molecular weight polymeric liquids display remarkable viscoelastic properties. Contrary to glassy systems, their macroscopic relaxation times are not due to slow dynamics on the monomer scale, but arise from the chain connectivity and the restriction that the chain backbones cannot cross. We use a combination of analytical theory and computer simulations to arrive at a quantitative description of the complex relaxation scenario expected from current versions of the tube model. Our data for the stress relaxation in equilibrium and step-strained bead-spring polymer melts allow us to explore the chain dynamics and the shear relaxation modulus, G(t), into the plateau regime for chains with Z=40 entanglements and into the terminal relaxation regime for Z=10. We have performed parameter-free tests of several different tube models using the known (Rouse) mobility of unentangled chains and the melt entanglement length determined via the primitive path analysis of the microscopic topological state of our systems. We find excellent agreement for the Likhtman-McLeish theory using the double reptation approximation for constraint release, if we remove the contribution of high-frequency modes to contour length fluctuations of the primitive chain. In particular, we rationalize the onset of entanglement constraints in polymeric liquids via an analysis of the short-time dynamics of (primitive) chains.