Glasses structural changes

Kokubo, T., Kushitani, H., Sakka, S., Kitsugi, T. and Yamamuro, T. (1990) Solution able to reproduce in vivo surface-structure changes in bioactive glass-ceramic A-W. Journal of Biomedical Materials Research, 24, 721-734. 

Ordered mesoporous silica have already been studied as carriers for drug delivery [1,2] recently, their use has also been proposed in bone tissue engineering [3,4], in combination with bioactive glass-ceramic scaffolds [5,6]. The kinetics of ibuprofen release in SBF [7] from MCM-41 silica with similar pore diameter has shown puzzling discontinuities [3,6,8] aim of the present work is to assess whether these anomalies may be related to structural changes in the MCM-41 mesoporous spheres under the adopted conditions. 

A glass membrane in an electrolyte solution cannot be taken to be a homogeneous system in the direction perpendicular to the surface. When the membrane is in contact with the solution, water molecules can enter it and form a 5-100 nm thick hydrated layer [319]. The formation of this hydrated layer is actually a condition for good functioning of the glass electrode. The basic characteristics of the glass structure probably do not change in the hydrated layer, but the cation mobility increases considerably compared with the compact membrane interior... 

A second-order phase transition is one in which the enthalpy and first derivatives are continuous, but the second derivatives are discontinuous. The Cp versus T curve is often shaped like the Greek letter X. Hence, these transitions are also called -transitions (Figure 2-15b Thompson and Perkins, 1981). The structure change is minor in second-order phase transitions, such as the rotation of bonds and order-disorder of some ions. Examples include melt to glass transition, X-transition in fayalite, and magnetic transitions. Second-order phase transitions often do not require nucleation and are rapid. On some characteristics, these transitions may be viewed as a homogeneous reaction or many simultaneous homogeneous reactions. 

For example, if manganese in a two valent form absorbs a photon from the U. V. portion of the spectrum, it changes to a three valent form plus an electron which becomes trapped in the glass structure, usually by ferric iron in a commercial glass. 

Its value at 25°C is 0.71 J/(g°C) (0.17 cal/(g°C)) (95,147). Discontinuities in the temperature dependence of the heat capacity have been attributed to structural changes, eg, crystallization and annealing effects, in the glass. The heat capacity varies weakly with OH content. Increasing the OH level from 0.0003 to 0.12 wt % reduces the heat capacity by approximately 0.5% at 300 K and by 1.6% at 700 K (148). The low temperature (<10 K) heat capacities of vitreous silica tend to be higher than the values predicted by the Debye model (149). 

Changes in the glass transition temperature, T , are a sensitive indicator of the structural changes occuring near the filler surrace [270]. At constant filler load-... 

The authors do not give a detailed explanation for the decreases in lifetime under paired dopings. There appear to be two possibilities, however. The first is changes in base-glass structure resulting in alterations of crystal field, and the second is resonance exchange between the ions. 

In most cases when the temperature of measurement is above the glass transition, the effect of temperature leads to very complicated spectral effects since structural changes and temperature-induced spectroscopic changes are occuring simultaneously 201,322) jn some the structural changes are well defined as in the case of polystyrene 322). 

Addition of a thin-film heater to the back of a metal sample holder allowed studies of thin polymer films by GRAS at up to 200 °C (121). In a study of an ultrathin film of polymethylmethacrylate, the two doublets near 1240/1270 and 1150/1190 cm"1 exhibited changes in relative intensity above the glass transition temperature of 100 °C, indicating that the polymer glass structure was maintained even in such thin films. 

---