When any pure material goes through a change in state, its physical properties change greatly. In each phase the physical properties are relatively constant or change slowly enough with temperature that the rate of change of a property such as volume is a constant. This rate of change of volume is the thermal expansion coefficient, (∂V/V)/∂T. The thermal expansion coefficient is almost constant, experimentally, as long as the temperature range over which measurements are made does not include a phase transition. At the transition temperature, abrupt changes in volume are found as illustrated in Figure 1. Polymeric materials often show changes in physical properties not necessarily accompanied by abrupt changes in volume, even though the expansion coefficient does change. Since the expansion coefficient changes, some change in internal structure is suspected, and the name second-order transition (Tg) has been adopted. This kind of change is roughly diagrammed in Figure 2. This latter change at the second-order transition temperature can be found in every known polymer, even though many polymers possess clear, first-order, crystalline transitions as well. Hevea rubber, for example, has a crystalline melting point of 28° C, compared to its Tg about −70°. These data are shown, copied from Boyer and Spencer, as Figure 3.