Glass, hole formation

Fig. 6. The hole formation energy 2ajk), the barrier to internal rotation (/de/fi) and the glass transition temperature vs. compoafdon for NaPOjijCa(PO, counterion copolymers (2 /ft) and (Aelk) calculated from the Gibbs-DiMarzio theory at 10% intervals. T, points experimental...

Radiation sources including X-rays, y-rays, and ultraviolet produce preferential light absorption at color centers in glass by formation of free electrons and holes. These are trapped at a defect such a vacancy, an interstitial atom, a multivalent impurity, or a nonbridging oxygen T The types of color center induced in siliea, soda-lime-silica, borate, and phosphate glasses are elucidated by optical and electron spin resonance studies of irradiated samples. Table 1 summarizes composition variables and reaction types that induce damage. 

The hole model discussed above is different from the hole model proposed by Ramachandrarao et al. (1977) and also used by Hirata (1979). In the latter model the hole formation energy is estimated from experimental data on the changes in specific heat and thermal expansion at the glass transition. Also in this hole model a linear relationship is found between the hole formation energy and the glass transition temperature, although the model lends itself less easily for making predictions. 

Thermodynamics of Enthalpy Relaxation and Hole Formation of Polymer Glasses... 

T Aa is seen to be a function of V only, and since, according to quite well established observations of Simha and Boyer this product is a constant for all polymers (10), eqn. (1) implies an iso-free volume glass transition state. In fact, using the empirical Simha-Boyer constant of 0.113 for T Aa, it is found that V = 0.025 at T, identical with the free volume estimated from tne WLF equation (11). The hole formation energy, E, in the Gibbs-DiMarzio treatment is a function of T alone and is directly proportional to the latter, as is seen in eqn. (2). 

The subscript "a" refers to the initial condition of the glass, which in turn depends on its mode of formation, Le., on cooling rate. Since this simplified treatment assigns every hole the same energy, the increase in specific heat at Tg due to hole formation is (see also Section II.1.5.2) ... 

Modifiers in glass are compounds that tend to donate anions to the network, whereas the cations occupy "holes" in the disordered stmcture. These conditions cause the formation of nonbridging anions, or anions that are connected to only one network-forming cation, as shown in Figure 2. Modifier compounds usually contain cations with low charge-to-radius ratios (Z/r), such as alkali or alkaline-earth ions. 

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