Glass transition (Tg) describes a material moving from rubbery to glassy state. Unlike phase transitions, which are first-order phenomena, glass transition is a second-order phenomenon. Glass transition is a kinetic phenomenon where it occurs over a range of temperature and its activation energy can be evaluated and it has relevancy to time-temperature-superposition.
Thermal analysis is particularly useful for glass transition temperature (Tg) evaluations of polymers. For example, Tg can be studied using DSC, DMA and TMA analyses. We show cartoon drawings below to illustrate typical thermographs depicting glass transition using thermal analysis techniques. Depending on material type and sample geometry limitation, certain techniques are better suited than others.
Because glass transition is a kinetic phenomenon, Tg varies with heat rate in the thermal analysis testing. This is one reason that heat rate should always be specified when reporting Tg values. In general, Tg measured with a slower heat rate (such as at 1C/min) is lower than Tg measured with a higher heat rate (such as at 20 C/min). The difference in measured Tg values due to heat rate differences may be as little as a few degrees to as much as 10 deg-C.
And because different material properties are being measured depending on the techniques, the measured glass transition temperature from one technique to another should not be expected to be “identical” or “equivalent” even with the same heat rate. This is the reason that the type of thermal analysis should always be specified when reporting Tg values. Although the measured Tg values should not be expected to be identical, the variation in Tg values obtained from various techniques are typically to be less than 10 or 20 deg-C.