25-30 August 2019
Henry Ford Building
Europe/Berlin timezone

Rheological NMR for the study of polymer dynamics

30 Aug 2019, 11:05
25m
Lecture Hall D (Henry Ford Building)

Lecture Hall D

Henry Ford Building

Talk Dynamics Dynamics

Speaker

Dr Ulrich Scheler (Leibniz-Institut für Polymerforschung Dresdne e.V.)

Description

Rheo NMR has been applied to investigate the effect of external shear on the aggregation and on the chain dynamics of polymers. A Couette cell with the polymer melt or solution in the gap is applied. To get further insight, oscillating rotation in addition to continuous rotation has been applied.
An in-house built rheo NMR system using a servo motor, avoiding any vibrations has been used on a spectrometer with a microimaging accessory. For the oscillatory shear a crank mechanism has been introduced between the motor and the drive of the Couette rotor.
The combination of PFG NMR with imaging enables measuring the flow profiles In particular for low-viscosity liquids like dilute solutions or lower molecular weight polymers flow pattern deviating from the expected linear velocity gradient are observed. Around the turning point in the oscillatory shear counterflow is observed which apparently leads temporarily to a shear rate larger than the gap averaged shear rate applied when the angular velocity is at its peak value. This effect strongly depends on the viscosity.
In particular the spin-spin relaxation time T2 measured via CPMG is sensitive to the polymer chain segment motion. The expected shear-induced order in polymer melts and solutions is detected for very low molecular weight (oligomers) only. For higher molecular weight the loss or rearrangement of entanglements leading to longer chain segments between the entanglements and thus enhanced polymer dynamics manifested in longer T2 is the dominating effect.
This has been observed for both continuous and oscillatory shear at large shear angles. Varying the angle in the oscillatory shear in addition enables to study the onset of this change of the polymer chain motion and thus the onset of the rearrangement of the entangled polymers.

Primary authors

Mr Benjamin Kohn (Leibniz-Institut für Polymerforschung Dresden e.V.) Mr Vincent Körber (Leibniz-Institut für Polymerforschung Dresden e.V) Mr Enno Stündel (Leibniz-Institut für Polymerforschung Dresden e.V.) Prof. Petrik Galvosas (School of Chemical and Physical Sciences, Victoria University of Wellington) Dr Ulrich Scheler (Leibniz-Institut für Polymerforschung Dresdne e.V.)

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