Abstract
Chain dynamics of polymer melts was investigated by field cycling and rotating-frame nuclear magnetic resonance (NMR) relaxation spectroscopy in a frequency range from 103 to 3×108 Hz. Far above the critical molecular weight, the frequency dependencies of the spin–lattice relaxation times T1 and T1ρ are characterized by a sequence of power laws ∝ν0.5, ∝ν0.25, and ∝ν0.5 occurring in ranges analogous to the Doi/Edwards limits of the anomalous time dependencies of the mean-square displacement of segments ∝t1/2, ∝t1/4, and ∝t1/2. The T1 dispersion data clearly contradict the dominance of Rouse dynamics within the Doi/Edwards tube. The ν3/4 frequency dependence predicted by de Gennes for the regime of Rouse relaxation along the tube was not observed. The spin–lattice relaxation behavior can, however, be derived from the Doi/Edwards mean-square displacement limits assuming a correlation between segment orientation and displacement direction. A corresponding formalism is presented. On the other hand, the spin–lattice relaxation in dilute polymer solutions and in melts of polymers with molecular weights below the critical value can be described perfectly by the Khazanovich NMR relaxation theory for the Rouse model.
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