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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 20: Polymer and Molecular Dynamics, Friction and Rheology

CPP 20.8: Vortrag

Dienstag, 28. März 2023, 11:45–12:00, ZEU 255

Polymer chain dynamics under shear studied by rheological NMR — •Ulrich Scheler — Leibniz-Institut für Polymerforschung Dresden e.V., Dresden, Germany

The NMR spin-lattice relaxation time T2 is particularly sensitive to slow chain-segment motion in polymers, at least two components in the signal decay are observed, a faster relaxing component from chain segments of motion restricted by crosslinks or entanglements and a slower component associated with parts moving more freely. Ordering of the polymer chains under shear would result in restrictions for the segment motion and thus a shortening of T2, a loss of entanglements due to the shear results in a prolongation of T2. Rheological NMR combined external shear with NMR experiments. In a polymer melt of poly(dimethylsiloxane) under shear longer relaxation times are observed. Together with a reduction of the fraction of shorter T2 this indicates the loss of entanglements is the dominating process for high molecular weight. In a new experimental setup both the amplitude and the frequency of the deformation are varied. It demonstrates that a minimum strain rate above 1000/s is required to observe the loss of entanglements. Combining pulsed-field-gradient (PFG) NMR with NMR imaging permits to measure flow pattern. After the turning point in oscillatory shear counterflow is observed when a fraction of the liquid at the static wall still is in initial flow direction while liquid in contact with the moving bob started in the new direction. At this time the velocity gradient drastically exceeds the velocity gradient at the point of maximum velocity representing a high shear rate.