We apply fast field cycling NMR to study the dispersion of the 1H spin−lattice relaxation time T 1(ω) of linear 1,4-polybutadienes with molecular weight M (g/mol) ranging from M = 355 to 817 000. By this, the crossover from glassy dynamics through Rouse to reptation becomes accessible. Analyzing the data in the susceptibility form ω/T 1(ω) and applying frequency−temperature superposition, spectra extending over up to 8 decades in ω are obtained. Characteristic polymer spectra are revealed when the underlying glassy dynamics are accounted for. Instead of describing the unentangled melt by the full Rouse mode spectrum, the emergence of a limited number of modes is identified which saturates when entanglement sets in. A quantitative analysis yields the molecular weight of a Rouse unit M R ≅ 500, and the entanglement weight M e ≅ 2000, at which first entanglement effects are observed. Moreover, the dynamic order parameter S(M) and the behavior of the terminal time τmax(M) are obtained. Both quantities allow to identify three dynamic regimes, namely simple liquid, Rouse, and reptation dynamics. The temperature dependence of the segmental relaxation time τs(T) coincides with the corresponding dielectric relaxation times which were measured additionally, and the M dependence of the glass transition temperature T g shows distinctive kinks at M R and M e, indicating that glassy dynamics are modified by polymer dynamics.

Fast field cycling (FFC) NMR is applied to study the dispersion of the 1H spin−lattice relaxation in the low molecular weight glass-formers o-terphenyl, tristyrene, and oligomeric polybutadiene (PB, with M/gmol−1 = 355 and 466) over a broad temperature range (203−401 K). Differing from previous FFC NMR works, we analyze the relaxation data in the susceptibility form ω/T 1(ω), and applying frequency−temperature superposition, master spectra are obtained covering up to 8 decades in frequency. In all cases solely the glassy dynamics (α-process) determines the relaxation behavior, and the Rouse unit is estimated to M R ≅ 500 g/mol. The time constant τα(T) in the range 10−11−10−6 s is extracted, which agrees well with those measured at the same time by dielectric spectroscopy. For the high molecular weight PB (M/gmol−1 = 56 500, 87 000, 314 000, and 817 000) pronounced polymer effects are observed at low frequencies (ωτα ≪ 1) which are isolated from the total spectrum by subtracting the “glass spectrum” as obtained from low molecular PB. We argue that unless the underlying α-relaxation is properly accounted for, the apparent power law spectrum does not reflect the actual polymer dynamics.

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