Transition, first-order glass

Figure 4.3b is a schematic representation of the behavior of S and V in the vicinity of T . Although both the crystal and liquid phases have the same value of G at T , this is not the case for S and V (or for the enthalpy H). Since these latter variables can be written as first derivatives of G and show discontinuities at the transition point, the fusion process is called a first-order transition. Vaporization and other familiar phase transitions are also first-order transitions. The behavior of V at Tg in Fig. 4.1 shows that the glass transition is not a first-order transition. One of the objectives of this chapter is to gain a better understanding of what else it might be. We shall return to this in Sec. 4.8. 

Phase transitions, whether first-order or second-order, are potent sources of instability of solid drugs and can usually be detected and studied by thermal methods of analysis (e.g., DSC, TGA, TMA, ODSC, DMA, DEA). In crystalline solids, typical first-order transitions are polymorphic or desolvation transitions. In amorphous solids, second-order transitions, such as glass transitions, are common. 

Predicted from the Random First Order Transition Theory of Glasses. 

When an amorphous material exists in a glassy state, it is hard and brittle. In a rubbery state, the material is soft and pliable. An amorphous material, at solid state (also referred to as glass), does not flow, but the molecules are randomly distributed as if they were in liquid state. When this "glass" is heated, it softens and eventually becomes a fluid. However, this is not a first-order transition and therefore occurs over a range of temperatures called the glass transition temperature (Tg). The state... 

DSC thermograms of the crosslinked enamels revealed onset of glass transitions (Tg) ranging from 15° to 35° for all three types of enamels. Attempts to detect first order transitions in the DSC corresponding to Tm or Ti were unsuccessful due to large exotherms starting at about 90°. An odd-even pattern was not observed. 

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. 

X. Xia and P. G. Wolynes, Fragilities of liquids predicted from the random first order transition theory of glasses. Proc. Natl. Acad. Sci. USA 97, 2990-2994 (2000). 

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