F. V. Chávez, K. Saalwächter.
Time-Domain NMR Observation of
Entangled Polymer Dynamics: Universal Behavior of Flexible Homopolymers and
Applicability of the Tube Model.
Macromolecules 44, 1549–1559
(2011). DOI 10.1021/ma1025708.
Using a benchtop NMR spectrometer, we performed proton multiple-quantum NMR
experiments to study the dynamics of well-entangled linear poly(butadiene),
poly(isoprene), and poly(dimethylsiloxane) in a broad molecular weight range.
The method provides a direct time-domain measurement of the segmental
orientation autocorrelation function over many decades, based on
time-temperature superposition. The function can be directly compared to
theoretical predictions based on the tube model, and the Rouse and the
disentanglement times can be evaluated relative to the known entanglement
time.We obtain universal results for all three polymers when plotted against the
molecular weight normalized by the entanglement molecular weight, with the
familiar mass scaling exponents of 2.0±0.2 and 3.2±0.2,
respectively. However, the time scaling exponent derived for motions in the
constrained-Rouse regime II is not constant at the predicted value of -1/4 but
is always larger and also a universal function of molecular weight. This
observation was previously explained by constraint-release effects that are
active at surprisingly short times and at local scales. We further discuss
observations related to contour-length fluctuations, leading to an isotropically
mobile fraction of segments that can also be quantified by our method.