Models of Glass Structure

The glass structure can be considered in terms of two models the crystallite and the random network models. 

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The simplest model of glass structure, Zachariasen s continuous random network model, represents glasses as being formed by random... 

Our discussion has so far been restricted to the schematic model of glass formation, which focuses on the relative flexibility of the chain backbone and side groups. The side groups in this schematic model are short linear chains (see Fig. 3b) with three united atom units, a structure inspired by many synthetic polymers in which the size of the side groups is on the order of a few... 

Figure 15-5. a) Schematic structure of solid Na20-Si02, crystal and glass. Si4+ Al3+ O 02 Na+. b) Cluster tissue model of glass. areas of increased stresses. 

Fig. 6. Model of the structure [24] of a shallow trap for etr in aqueous glasses. References pp. 221-224...

Fig. 10. Model of the structure [36] of a trap for anion radicals O in water-alkaline glasses.

Because of the analogy between (H20)n and (02Si)n, the problems associated with the structural interpretation of liquid water have their glassy state analogies in the theories of glass structure. Even Pauling s clathrate hydrate model for liquid water has its counterpart in the vitron theory of glass [750]. Both theories are discredited by the experts in the Held. 

Although glass transition is conventionally defined by the thermodynamics and kinetic properties of the structural a-relaxation, a fundamental role is played by its precursor, the Johari-Goldstein (JG) secondary relaxation. The JG relaxation time, xjg, like the dispersion of the a-relaxation, is invariant to changes in the temperature and pressure combinations while keeping xa constant in the equilibrium liquid state of a glass-former. For any fixed xa, the ratio, T/G/Ta, is exclusively determined by the dispersion of the a-relaxation or by the fractional exponent, 1 — n, of the Kohlrausch function that fits the dispersion. There is remarkable similarity in properties between the JG relaxation time and the a-relaxation time. Conventional theories and models of glass transition do not account for these nontrivial connections between the JG relaxation and the a-relaxation. For completeness, these theories and models have to be extended to address the JG relaxation and its remarkable properties. 

Figure 14.12 Two structural models of glass-like carbon heated to high temperature (a) network of ribbon stacking model (After Jenkins and Kawamura, 1971) (b) alternate model. (After, Shiraishi, 1984 ).

A number of other statements by Zachariasen have become the basis for the models for glass structures termed the Random Network Theory. These ideas will be discussed later under the topic of glass structure. It is interesting to note, however, that the term random network does not occur in the original work of Zachariasen, who referred to the glass structure as a vitreous network . Furthermore, Zachariasen specifically states that the vitreous network is not entirely random due to the restriction of a minimum value for the internuclear distances. As a result, all internuclear distances are not equally probable, and X-ray patterns of the type observed for glasses are a natural consequence of the vitreous network. 

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