We analyze the temperature dependence of the segmental relaxation time τ of several low-T g polymers with varying molar masses (M) as obtained from field-cycling 1 H NMR relaxometry and dielectric spectroscopy. They are compared with those of molecular liquids (ML). Time constants in the range 3 × 10 −12 s−1000 s, i.e., between T g and 413 K, are covered. Describing τ(T) by the Vogel−Fulcher−Tammann (VFT) eq a systematic difference with respect to ML is found. While VFT fails for the latter it works well for polymers. The apparent activation energy at high temperatures shows a trend toward a temperature independent value E ∞ . For polymers, its Mdependence follows that of T g (M), thus E ∞ (M) can be described by a Fox−Flory equation. Attempting to understand the difference among the two classes of liquids, we take recourse to our approach first applied to ML [J. Chem. Phys. 2013, 139, 084504]; i.e., we decompose the temperature-dependent activation energy E(T) controlling τ(T) in a constant high-temperature value E ∞ (M) and a "cooperative part" E coop (T). The latter turns out to depend exponentially on temperature, as in ML. Introducing a plot in terms of E coop (T)/E ∞ vs T/E ∞ , a master curve for each polymer series is revealed. Taking averaged parameters for all polymers a three-parameter fit well interpolates τ(T) up to highest temperatures. Describing molecular and polymer liquids within the same approach, the difference lies in the fact that the ratio E ∞ /E coop (T g ) is systematically higher for polymers; i.e., τ(T) displays an Arrhenius behavior extending over a larger temperature range.

We study a dynamically asymmetric binary glass former with the low-Tg component m-tri-cresyl phosphate (m-TCP: Tg = 206 K) and a spirobichroman derivative as a non-polymeric high-Tg component (Tg = 382 K) by means of (1)H nuclear magnetic resonance (NMR), (31)P NMR, and dielectric spectroscopy which allow component-selectively probing the dynamics. The entire concentration range is covered, and two main relaxation processes with two Tg are identified, Tg 1 and Tg 2. The slower one is attributed to the high-Tg component (α1-process), and the faster one is related to the m-TCP molecules (α2-process). Yet, there are indications that a small fraction of m-TCP is associated also with the α1-process. While the α1-relaxation only weakly broadens upon adding m-TCP, the α2-relaxation becomes extremely stretched leading to quasi-logarithmic correlation functions at low m-TCP concentrations-as probed by (31)P NMR stimulated echo experiments. Frequency-temperature superposition does not apply for the α2-process and it reflects an isotropic, liquid-like motion which is observed even below Tg 1, i.e., in the matrix of the arrested high-Tg molecules. As proven by 2D (31)P NMR, the corresponding dynamic heterogeneities are of transient nature, i.e., exchange occurs within the distribution G(lnτα 2). At Tg 1 a crossover is found for the temperature dependence of (mean) τα 2(T) from non-Arrhenius above to Arrhenius below Tg 1 which is attributed to intrinsic confinement effects. This "fragile-to-strong" transition also leads to a re-decrease of Tg 2(cm - TCP) at low concentration cm - TCP, i.e., a maximum is observed in Tg 2(cm - TCP) while Tg 1(cm - TCP) displays the well-known plasticizer effect. Although only non-polymeric components are involved, we re-discover essentially all features previously reported for polymer-plasticizer systems.

Main (α-) and secondary (β-) relaxation of the linear poly(ethylene-alt-propylene) (PEP) with different molar masses are investigated by dielectric and 2 H NMR spectroscopy. Regarding the α-process, time constants in the range 10 −11 −1 s are reported, and the correlation function is described by a Cole− Davidson function with stretching parameter β ≅ 0.34. Both methods show clear evidence of a pronounced β-process; the latter is thoroughly studied by different NMR techniques. The results are compared to toluene-d 3 and toluene-d 5 , respectively, as reference systems. The solid-echo spectra as well as the spin−lattice relaxation display typical features of spatially highly restricted angular displacements similar to those reported in other glassformers showing a β-process−like toluene. Above T g all chain segments participate. As both chain and methyl group were deuterated and as the β-process is rather fast, its angular displacement growing with temperature above T g is directly reflected in a reduction of the effective quadrupolar coupling constant; i.e., the 2 H solid-state spectra get narrower upon heating. Although the time constant τ β is the same for both segments, the angular amplitude of the methyl group bearing segment is larger than that of the other chain segments, and αand β-process merge at high temperatures. For the other segments a merging appears not to happen. In this sense, the β-process is anisotropic as is demonstrated also for toluene-d 3 for which it essentially involves a rattling around the C 2 -axisa finding, however, which does not explain the fact that the β-process is dielectrically active. Regarding PEP below T g , there are indications that only about half of the methyl group bearing segments are involved in the β-process whereas the other segments fully participate. Finally, we discuss possible implications of our findings regarding the nature of the β-process and compare time constants τ β (T) as well as τ α (T) of polymers like poly(methyl methacrylate), poly(propylene glycol), and poly(isoprene) among others.

With the availability of commercial field-cycling relaxometers together with progress of home-built instruments nuclear magnetic resonance relaxometry has gained new momentum as a method of investigating the dynamics in viscous liquids and polymer melts. The method provides the frequency dependence of the spin-lattice relaxation rate. In the case of protons, one distin-

It is a longstanding question whether universality or specificity characterize the molecular dynamics underlying the glass transition of liquids. In particular, there is an ongoing debate to what degree the shape of dynamical susceptibilities is common to various molecular glass formers. Traditionally, results from dielectric spectroscopy and light scattering have dominated the discussion. Here, we show that nuclear magnetic resonance (NMR), primarily field-cycling relaxometry, has evolved into a valuable method, which provides access to both translational and rotational motions, depending on the probe nucleus. A comparison of 1H NMR results indicates that translation is more retarded with respect to rotation for liquids with fully established hydrogen-bond networks; however, the effect is not related to the slow Debye process of, for example, monohydroxy alcohols. As for the reorientation dynamics, the NMR susceptibilities of the structural (α) relaxation usually resemble those of light scattering, while the dielectric spectra of especially polar liquids have a different broadening, likely due to contributions from cross correlations between different molecules. Moreover, NMR relaxometry confirms that the excess wing on the high-frequency flank of the α-process is a generic relaxation feature of liquids approaching the glass transition. However, the relevance of this feature generally differs between various methods, possibly because of their different sensitivities to small-amplitude motions. As a major advantage, NMR is isotope specific; hence, it enables selective studies on a particular molecular entity or a particular component of a liquid mixture. Exploiting these possibilities, we show that the characteristic Cole–Davidson shape of the α-relaxation is retained in various ionic liquids and salt solutions, but the width parameter may differ for the components. In contrast, the low-frequency flank of the α-relaxation can be notably broadened for liquids in nanoscopic confinements. This effect also occurs in liquid mixtures with a prominent dynamical disparity in their components.