Glass volume changes associated with

Fig. 5 Volume changes associated with cooling of a melt Fg is the glass transition temperature and 7) is the melting point of the material.

One reason that these particular polymer systems exhibit this flexibility below their glass transition temperatures is that they do not crystallize to any appreciable extent. Thus, large volume changes, associated with crystallization, are not present and therefore there are no high stress levels at the interface brought about by these volume changes or shrinkages. 

It should be noted that Tg is expressed as the temperature corresponding to the peak of the first derivative of the heat flux (i.e., at the inflexion point of the heat flux). The volume variations associated with the glass transition, which are also simultaneously measured by scanning transitiometric measurements, are depicted in Fig. 22. In accordance with the heat flux curve, Tg increases with increasing the pressure. Above Tg, there is an increase of the slope of the variation of the specific volume versus temperature. However, the change in the slope is gradual and Tg can be determined at the point where the two lines intersect. 

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. 

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