Secondary relaxation process, molecular glasses

Relevant dielectric results of type B glasses were already discussed in Section IV.C.2. The spectra below Tg exhibit a broad symmetric secondary relaxation peak that can be interpolated assuming a Gaussian distribution of activation enthalpies. Only recently it became clear that also NMR is able to identify secondary relaxation processes in glasses, moreover providing information on the mechanisms of molecular reorientation that is not easily accessible to most of the other methods. For detailed reports the reader is referred to the reviews by Bohmer et al. [11] and Vogel et al. [15]. Here, we summarize the major results. 

We review nuclear magnetic resonance (NMR) studies of secondary relaxation processes in glasses. The main focus is work on molecular glasses, where the secondary relaxation is usually associated with spatially highly restricted motion. 

Figure 1. A sketch of the temperature evolution of the susceptibility spectra of simple molecular liquids upon passing from low-density fluid (a) to the glass (d). We anticipate two distinguishable temperature regimes for the evolution of glassy dynamics, namely, a high-temperature regime (b) and a low-temperature regime (c), the latter characterized by the emergence of slow secondary relaxation processes.

Secondary Relaxation Processes in Molecular Glasses Studied by Nuclear Magnetic Resonance Spectroscopy... 

Secondary relaxation processes of neat molecular glasses 255... 

The main goal of this review is to report on progress of NMR work on relaxation processes in molecular glasses below Tg. We will demonstrate that applications of multidimensional NMR are not limited to studies of the a-process, but this technique is also well suited for the investigations of secondary relaxation processes. However, due to the spatial restrictions of molecular motion associated with secondary relaxation processes, the NMR techniques have to be pushed to their limits. Moreover, comparison with results from other techniques is very useful in order to portrait a coherent physical picture of the molecular dynamics. 

It may be appropriate to discuss the NMR findings on the /1-process in the context of results from other experimental methods. Unlike 2H NMR, the vast majority of experimental techniques are not capable of resolving slow reorientation about very small angles. In particular, several studies on the /1-process of molecular glasses may have overlooked the small-angle contribution of the majority of molecules and concluded that a small fraction of molecules is involved in the secondary relaxation process. In contrast, straightforward analysis of 2H NMR solid echo (cf. Section 3.2.1) and spin-lattice relaxation data (cf. Section 3.2.4 and in particular Ref. 115) clearly shows that essentially all molecules participate in the /1-process. However, the amplitude of the reorientation differs among the molecules. The mean... 

Next, we discuss NMR results on the /1-process of polymers to ascertain similarities and differences of the secondary relaxation in molecular and polymeric glasses. 

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