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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 40: Polymer and Molecular Dynamics, Friction and Rheology
CPP 40.4: Talk
Tuesday, March 17, 2020, 11:45–12:00, ZEU 255
Local polymer chain mobility under shear — Benjamin Kohn1, Roland Vogel1, Petrik Galvosas2, and •Ulrich Scheler1 — 1Leibniz-Institut für Polymerforschung Dresden e.V — 2Victoria University of Wellington
The NMR transverse relaxation T2 is sensitive to the slow motion of polymer chain segments. In entangled polymers at least two components are observed, a short one for chain segments with restricted motion due to entanglements and a longer component originating from the more free chain segments. Here the effect of external shear in a Searle cell as used in rheological experiments is studied. For short polymer chains little effect of shear is observed while for high molecular weight with clear signs of entanglement both components become longer. This is indicative of longer chain segments between entanglements, which results from loss of entanglements or rearrangement resulting in longer chain segments between entanglements. A new experimental setup permits measuring NMR relaxation under oscillatory shear varying both amplitude and frequency. The resulting strain-rate dependence shows that the prolongation of the relaxation time indicating longer chain segments between entanglements starts at a strain of at least 100 for entangled PDMS.The combination of PFG NMR with NMR imaging in the same setup allows to measure flow pattern. At the turning point of the oscillation for low-viscosity fluids a counterflow is observed.