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Dielectric spectra of a series of polydimethylsiloxanes and polystyrenes were measured covering a wide range of molecular weights M including the monomeric limit. From spectral analysis the time constants τ R (T) of the segmental dynamics are extracted, allowing determination of the glass-transition temperatures T g (M) and τ R (M) as well as the parameters D(M) and T 0 (M), respectively, ∆(M) ) T g -T 0 , of the Vogel-Fulcher-Tammann equation. In addition, we analyzed fragility m(M) including here also data on polybutadiene. It turns out that all the quantities show particularities as a function of M which are not explained by common wisdom. T g (M) as well as T 0 (M) data reveal a noncontinuous M dependence with three distinct regimes which may become plausible when comparing our results with rheological and NMR data. We attribute the three regimes to simple liquid, Rouse, and entanglement dynamics, i.e., T g (M) and T 0 (M) traces show kinks at the Rouse unit M R and entanglement molecular weight M e . Fragility m(M) shows only one kink at M e . This fact can be explained by the behavior of ∆(M), which increases with M in the simple liquid regime but saturates already at M R ; a similar behavior is observed for the exponential prefactor τ 0 (M). We conclude that polymer dynamics specifically modify glassy dynamics.

Mixtures of protonated and deuterated polybutadiene and polydimethylsiloxane are studied by means of field-cycling (FC) 1H NMR relaxometry in order to analyze the intra- and intermolecular contributions to spin–lattice relaxation. They reflect reorientational and translational dynamics, respectively. Master curves in the susceptibility representation χ″(ωτs) are constructed by employing frequency–temperature superposition with τs denoting the segmental correlation time. The intermolecular contribution is dominating at low frequencies and allows extracting the segmental mean square displacement ⟨R 2(t)⟩, which reveals two power-law regimes. The one at short times agrees with t 0.5 predicted for the free Rouse regime and at long times a lower exponent is observed in fair agreement with t 0.25 expected for the constrained Rouse regime of the tube-reptation model. Concomitantly the reorientational rank-two correlation function C 2(t/τs) is obtained from the intramolecular part. Again two power-law regimes t –ε are identified for polybutadiene. The first agrees with t –1 of free Rouse dynamics whereas at long times ε = 0.49 is obtained. The latter is corroborated by the 2H relaxation of deuterated polybutadiene, yet, it does not agree with ε = 0.25 predicted for constrained Rouse dynamics. Thus, the relation C 2(t) ∝ ⟨R 2(t)⟩–1 as assumed by the tube-reptation model is not confirmed.

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.