Relaxation barrier distribution, amorphous

The wide bands of dielectric losses occur in the region from 10 Hz to 10 MHz for many organic species doped into a predeuterated polyethylene matrix at 4.2 K [Gilchrist, 1991]. The relaxation is caused by tunneling transitions between several equilibrium positions of OH or NH2 groups. The bandwidth is determined by the distribution of barrier parameters in amorphous matrix. 

A special feature of amorphous materials is the anomalous behaviour of several properties at very low temperatures, such as in the low temperature specific heats. Most of these anomalies are attributed to the presence of two-level states (TLS) separated by small barriers, which gives rise to tunneling excitations. These excitations are characterized by wide distribution of relaxation times and energies. Several ultrasonic and low temperature specific heat measurements have been performed to characterize the TLS but their physical nature such as their structures, etc. is far from having been understood. These are the ADWP states discussed earlier in Chapter 7 in some detail. 

An implicit assumption made in deriving the Debye equations is that of a single relaxation time. In other words, the heights of the barriers are identical for all sites. And while this may be true for some crystalline solids, it is less likely to be so for an amorphous solid such as a glass, where the random nature of the structure will likely lead to a distribution of relaxation times. 

MD simulation may be ideally suited for unraveling molecular structures in amorphous solids that may lead to instability or favor crystalhzation of one polymorphic form over another. Xiang and Anderson [24b] recently conducted an MD simulation study of amorphous IMC assemblies containing 0.6% w/w water content to explore both structural properties (e.g., IMC hydrogen bonding patterns, distribution of various IMC conformations, and the nature of water-IMC interactions) as well as dynamic processes (e.g., relaxation processes within the IMC molecule, and water diffusion). To explore the entire conformational space by MD simulation, it was necessary to reduce the torsionM barrier for (j) from 29.0 to 8.0kcal/mol in order... 

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