Calculating Glass Transition Temperature on Polymer

The glass-to-rubber transition temperature [ Tg ], is of special interest in the development of new amorphous polymers because many properties of technological importance show a significant change in magnitude, or in temperature dependence, at this temperature. A method for calculating the Tg, of polymers from a knowledge of the chemical structure alone is therefore of great value in designing new polymers with desired properties and is of considerable theoretical interest. Many previous attempts have been reported, but the relations proposed have been limited in application, though usefully descriptive of specific polymer systems.

The Glass Transition Temperature (Tg) is one of the most important properties of any polymer and is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. As epoxies are thermosetting materials and chemically cross-link during the curing process, the final cured epoxy material does not melt or reflow when heated (unlike thermoplastic materials), but undergoes a slight softening (phase change) at elevated temperatures.

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The format of the Tg scan is similar to that of a kinetic scan except that it is performed with a cured sample. Temperature is plotted on the X axis and the heat flow response on the Y axis (see above figure).

As discussed previously, Tg is actually a temperature range, rather than a specific temperature. The convention, however, is to report a single temperature defined as the midpoint of the temperature range, bounded by the tangents to the two flat regions of the heat flow curve. For the epoxy material in the above figure, the reported Tg would be 75˚C.

Tg spans a temperature range, rather than occurring at a specific temperature, due to the cross-linked polymer chains having multiple degrees of freedom and modes of polymer chain movement in response to any applied thermal energy. The Tg value can also vary depending on its degree of cure. Generally, the reported Tg for a material is based on 100% conversion (full cure).