Rheological NMR – insight in molecular dynamics, flow pattern and transport
Ulrich Scheler
Leibniz-Institut für Polymerforschung Dresden e.V.
Rheological NMR (nuclear magnetic resonance) is the combination of various modalities of NMR with external mechanical shear. Polymer-chain dynamics directly manifests itself in NMR relaxation. The encoding of spatial and displacement information in NMR is introduced.
The aggregation of proteins triggered by shear could be demonstrated for silk fibroin, where shear leads to the formation of ß-sheet responsible for the unique mechanical properties. The application of external shear to polymer melts and solutions impacts the chain dynamics, Contrary to some expectations, shear- induced alignment is less important, the dominating effect is the loss of entanglements resulting in longer chain segments. The enhanced polymer-chain mobility exhibits in longer transverse (T2) NMR relaxation time with increasing shear. At low shear rates the polymer chains can follow without loss of entanglements. The dependence becomes clearer under oscillatory shear where both the amplitude and the frequency of shear are varied independently. Only at rate amplitudes above 1000 s-1 the polymer-chain dynamics increases.
Combining NMR imaging with pulsed-field-gradient (PFG) NMR enables measuring flow pattern, the velocity for each pixel. In a double cylinder (Searle) cell counter flow effects have been observed under oscillatory rotation after the point of reversal. If the inner cylinder is placed off the center of the outer results in a rather complex flow pattern and shear rates. Both have been confirmed qualitatively and quantitatively by computational fluid dynamics (CFD).
