Slow Motions in Glasses

The current picture of relaxation behavior of supercooled liquids is complex. Mode coupling theory introduced slow a- and fast -processes, whose existence has been confirmed experimentally in almost all known glass forming liquids. The fast -process takes place on the picosecond time scale at all 

Hollander and Prins investigated the effect of pressure on the glass-transition temperature Tg in atactic polypropylene in the pressure range up to 5000 bar. The decay rate of the deuteron-NMR quadrupole echo was used to monitor the glass transition. In further papers the same authors also studied the methyl group, segmental and chain motions of this molecular system. 

Kumar et al used spectroscopic techniques such as NMR to study the properties and structures of mixed glass systems. Rosenhahn et al. obtained insight into the structure and dynamics of the binary As-Se glass system from high temperature Se NMR studies of molten salts. Ab initio molecular orbital calculations have been carried out for sihcate, aluminosilicate and aluminate clusters to study the NMR characteristics of various types of hydroxyl that are possibly present in hydrous silicate glasses and melts. Xue and Kanzaki in particular studied the specification and dynamics of dissolved water in the silicate glasses. 

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It is not clear, whether the experimentally observed random local freezing of the deuterons in the O-D—O bonds in deuteron glasses corresponds to a true thermodynamic phase transition or whether one deals with a dynamic phenomenon which only seems static because of the finite observation time of the experimental techniques. The recently observed42 splitting between the field-cooled and zero-field dielectric susceptibilities below an instability temperature Tf seems to speak for the occurrence of an Almeida-Thouless-like thermodynamic phase transition in deuteron glasses. It is well known that ID NMR and EPR allow a direct measurement of the Edwards-Anderson order parameter qEA only on time scales of 10 3-10 8 s and 2D exchange NMR possibly seems to be a better technique for such slow motions. 

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... 

In Chapter 7, Mano and Dionisio describe how electrical methods, and particularly dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarisation current (TSDS) techniques, play a major role as tools for e2q)loring molecular mobility. DRS enables molecular relaxational processes (both slow and fast) to be studied. For example, the localized motions of glass formers in the glassy state give rise to local fluctuations of the dipole vector that are the origin of the secondary relaxation processes detected by dielectric relaxation spectroscopy, while above, but near, the glass transition, cooperative motions result in a distinguishably different relaxation process (the a-relaxation). 

NO2 is a stable paramagnetic gaseous molecule at normal temperatures. The ESR parameters of NO2 trapped in a solid matrix have been well established from single-crystal ESR measurements and have been related to the electronic structure by molecular orbital studies [39]. Thus, the NO2 molecule has potential as a spin probe for the study of molecular dynamics at the gas-solid interface by ESR. More than two decade ago temperature-dependent ESR spectra of NO2 adsorbed on porous Vycor quartz glass were observed [40] Vycor is the registered trademark of Coming, Inc. and more information is available at their website. The ESR spectral line-shapes were simulated using the slow-motional ESR theory for various rotational diffusion models developed by Freed and his collaborators [41]. The results show that the NO2 adsorbed on Vycor displays predominantly an axial symmetrical rotation about the axis parallel to the O—O inter-nuclear axis below 77 K, but above this temperature the motion becomes close to an isotropic rotation probably due to a translational diffusion mechanism. 

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