Glass transition temperature measured at high frequencies by piezoelectricexcitation

As the temperature of a polymer melt or rubber is lowered, a point known as the glass transition temperature, T-g, is reached where polymeric material s undergo a marked change in properties associated with the virtual cessati on of local molecular motion. In fact, as the thermal energy reduces, the d istance 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 poi nt 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 copi ous literature referring to traditional methods employed to measure T-g, th is work will only present them as a frame to compare them with the experime ntal procedure we used. The determination of T-g 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 c rystal attached to it. Glass transition temperatures of several glassy poly mers are determined. These results are compared with values measured throug h other methods. Finally, the advantages of our procedure are discussed. (C ) 2000 Elsevier Science S.A. All rights reserved.