Secondary relaxation process

One of the problems that has not yet been addressed adequately is how to extract meaningful parameters from the relaxation function that reflect the presence of the secondary relaxation process. Work on this question is being actively pursued. 

Of the diluents known to affect the dynamic relaxation behavior of polymers in the glassy state, water has so far received the greatest attention. Many polymers, which in the dry state are lacking any secondary relaxation process at temperatures from 77 to 273 K, e.g. poly(methyl methacrylate)135, polymethacrylamide136, cellulose and its derivatives137, collagen138, polysulfones139, poly(2,6-dimethylphenylene oxide)139, and others,... 

In the case of polycarbonates, it has been observed that by adding miscible low molecular weight additives, with specific chemical structures, it is possible to increase the yield stress of the polymer, as well as to reduce the local molecular motions that are responsible for the secondary relaxation processes... 

For the molecular origins of the secondary relaxation processes the reader is referred to McCrum, Read and Williams (1967) to start with. However, since that time considerable progress has been made in the determination of the relation between the secondary processes and molecular motions (see, e.g. Chap. 3.3 in Haward and Young, 1997). 

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.

Since a-relaxation stretching and minimum scaling can be observed up to highest temperatures, even well above the melting point, and since Tc is at a rather elevated temperature, one may conclude that MCT describes the slowing down of the dynamics in a rather dense liquid and may fail at some point due to the emergence of further slow secondary relaxation processes, which are... 

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. 

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

Secondary relaxation processes of neat molecular glasses 255... 

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. 

We show that NMR is well suited to study such highly hindered motion when the spatial resolution of the standard echo techniques is improved. The NMR results are discussed in the framework of findings from other experimental techniques so as to provide a comprehensive picture of secondary relaxation processes in glasses. 

Below Tg, structural relaxation is too slow to be observable, but secondary processes persist, which determine, e.g., the mechanical and dielectric properties of glasses. These processes have been extensively studied for polymers, where they are usually associated with polymer-specific dynamics such as side-group motion. From the point of view of glass physics, it is more interesting to investigate secondary relaxation processes in glasses comprised of rigid molecules, i.e., molecules without... 

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. 

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