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The enhanced thermal expansion coefficient observed upon heating of a glass rod in dilatometry experiments is one of the most familiar manifestations of the glass transition (Figure 8a). The marked increase over an interval of about 50 K is rapidly followed by sample collapse because the viscosity rapidly decreases so much that the sample begins to flow under its own weight before structural relaxation is complete. As a result, the volume thermal expansion coefficient [α = 1/V (∂V/∂T)_P = 3/l (∂l/∂T)_P] may be rigorously determined from the slope of the dilatometry curve for the glass, but not for the supercooled liquid.

Figure 8 Volume effects of the glass transition. (a) Linear thermal expansion coefficient of E glass (Chapter 1.6) heated at 10 K/min; l = sample length

(Source: Data from [28]).

(b). Dependence of the volume of a glass on its fictive temperature.

In dilatometry experiments, one usually defines the glass‐transition temperature as the intersection of the tangents to the lower‐ and higher‐temperature curves. This temperature generally differs somewhat from the standard T_g simply because the glass transition depends on the particular experimental conditions of the experiment. With respect to enthalpimetry, dilatometry has the advantage of yielding absolute values of the property of interest, namely, the volume (and density). The influence of thermal history on density can thus be readily determined (which is why it was observed as early as in 1845, cf. Chapter 10.11). In contrast, the thermal expansion coefficient of glasses generally does not markedly depend on thermal history. At least above room temperature. The volumes of glasses produced at different cooling rates will then plot as a series of parallel lines (Figure 8b). To characterize the state of the glass, it suffices to know the temperature at which equilibrium was lost, which is directly given by the intersection of the glass and supercooled volumes (Figure 8b). This parameter is called the fictive temperature (), which thus represents the temperature at which the configuration of the glass would be that of the equilibrium liquid (Chapter 10.11). Knowing , it is then straightforward to determine the glass volume as a function of the fictive temperature, for example, at room temperature (Figure 8b).