Time-Domain NMR Observation of Entangled Polymer Dynamics: Universal Behavior of Flexible Homopolymers and Applicability of the Tube Model

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 anisotropically mobile fraction of segments that can also be quantified by our method.