Field-Cycling NMR Relaxometry Probing the Microscopic Dynamics in Polymer Melts

Field-cycling (FC) 1 H and 2 H NMR relaxometry is applied to linear polybutadiene (PB) of different molar mass ( M ) in order to test current polymer theories. Applying earth field compensation, five decades in the frequency dependence of the spin–lattice relaxation rate T 1 –1 (ν) = R 1 (ν) are accessed (200 Hz - 30 MHz), and we focus on the crossover from Rouse to entanglement dynamics. A refined evaluation is presented, which avoids application of frequency–temperature superposition as well as Fourier transformation. Instead, the power-law exponent ε in the entanglement regime is directly determined from the susceptibility representation χ NMR ″ (ω) = ω/ T 1 (ω) ∝ ω ε by a derivative method. Correspondingly, a power-law t –ε characterizes the decay in the time domain, i.e., the dipolar correlation function. For the total 1 H relaxation, comprising intra- and intermolecular relaxation, a high- M exponent ε total = 0.31 ± 0.03 is found. An isotope dilution experiment, which yields the intramolecular relaxation reflecting solely segmental reorientation, provides an exponent ε intra = 0.44 ± 0.03. It agrees with that of FC 2 H NMR (ε Q = 0.42 ± 0.03) probing only segmental reorientation. The fact that ε intra > ε total demonstrates the relevance of intermolecular relaxation in the entanglement regime (but not in the Rouse regime), and ε intra is significantly higher than predicted by the tube-reptation (TR) model (ε TR = 0.25) and, the latter being supported also by recent simulations. The ratio of inter- to intramolecular relaxation grows with decreasing frequency, again in contradiction to the TR model and results from double quantum 1 H NMR. We conclude that no clear evidence of a tube is found on the microscopic level and the so-called return-to-origin hypothesis is not confirmed. Studying the influence of chain end dynamics by FC 1 H NMR we compare differently chain end deuterated PB. For the dynamics of the central part of the polymer the exponent drops from ε intra = 0.66 ± 0.03 down to ε cent = 0.41 ± 0.03 for M = 29k which is very close to the high- M value ε intra . Thus, the protracted transition to entanglement dynamics reported before is not found when the polymer center is probed; instead full entanglement dynamics appears to set in directly with M > M e .