Abstract:

As the temperature of a polymer melt or rubber is lowered, a point known as the glass transition temperature, Tg, is reached where polymeric materials undergo a marked change in properties associated with the virtual cessation of local molecular motion. In fact, as the thermal energy reduces, the distance between macromolecules, and consequently the specific volume of the sample, diminishes. However, this change is not abrupt, that is, the glass transition is not a rigorous phase transition from the thermodynamical point of view. Effectively, it depends not only on the polymeric structure but also on the heating rate, the measuring method, etc. Since there is a copious literature referring to traditional methods employed to measure Tg, this work will only present them as a frame to compare them with the experimental procedure we used. The determination of Tg is based on the change of the real component of the dynamic modulus with temperature. This modulus is measured at high frequencies on exciting the sample with a piezoelectric crystal attached to it. Glass transition temperatures of several glassy polymers are determined. These results are compared with values measured through other methods. Finally, the advantages of our procedure are discussed.