Glass transition enthalpy change

These techniques help in providing the following information specific heat, enthalpy changes, heat of transformation, crystallinity, melting behavior, evaporation, sublimation, glass transition, thermal decomposition, depolymerization, thermal stability, content analysis, chemical reactions/polymerization linear expansion, coefficient, and Young s modulus, etc. 

Figure 5 Changes in volume, V, energy, E, and enthalpy, H, during cooling or heating of the liquid, crystalline, and glassy (vitreous) forms of a substance. Tm is the melting point, and Ts is the glass transition temperature. (Adapted with permission from Ref. 14.)...

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. 

Glass transitions, both in frozen systems and in freeze-dried solids, can be difficult to detect. This may be caused by the small heat capacity change associated with the glass transition, a broad glass transition region, or both. Interpretation is made more uncertain by baseline drift or other noise. In addition, other thermal events at temperatures close to the glass transition, such as enthalpy recovery or crystallization, may disguise the heat capacity... 

Figure 5.3. Changes in volume, V, enthalpy, H, and entropy, S, around glass transition and melting. Amorphous materials can have an infinite number of glassy states which result in relaxations around glass transition.

Above this temperature, the substance retains some of the properties of a liquid, e.g., molecular mobility, and is termed rubbery. Above this temperature, the increase in molecular mobility facilitates spontaneous crystallization into the crystalline form with an exothermic enthalpy change after the glass transition. The use of amorphous forms is attractive, particularly for sparingly soluble compounds because of the enhanced solubility and dissolution rate over the crystalline state leading to increased bioavailability. However, the amorphous state is thermodynamically unstable. The glass transition temperature Tg is lowered by water or other additives, facilitating conversion... 

FIGURE 6 Schematic representation of changes in specific volume and enthalpy with temperature. The effect of annealing of a glass, and the associated molecular reorientation is also shown in addition to the melting (7 ) and Kauzmann (Tk) temperatures, the two glass transition temperatures (7gi and arc pointed out. Source From Ref. 16. 

Such measurements have proved to be of considerable value [94] in studying the ordering or disordering accompanying phase transitions in polymers and in coordination compounds. The glass transition [94] is not accompanied by an enthalpy change and appears as a discontinuity in the heat capacity of the sample, as recorded by the baseline trace of the DSC. 

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