Stress relaxation of polytetrafluoroethylene in the vicinity of its glass transition temperature at about 130°C.



Measurements of compressive stress relaxation for molded polytetrafluoroethylene were carried out at different temperatures over the range 60–150°C., at 5°C. increments under a small strain (less than 0.3%). By the application of the so-called Ferry's reduction method, three master curves were obtained with good overlapping from a series of stress relaxation curves obtained at different temperatures for the same specimen. This polymer has two temperatures at which the three master curves cross and the reduction method becomes entirely in applicable. Those two temperatures are about 80 and 100°C. It may be concluded that this polymer has two first-order transitions at about 80 and 100°C., and the states above and below these temperatures are quite different, as in the case of the transition observed for this polymer at about 20°C., which has been confirmed to be a first-order transition in crystal structure. The reduction factors aT at different temperatures were plotted in logarithmic scale against the reciprocals of the absolute temperatures. From the slope of those three log aT vs. 1/T curves, the diagrams of the apparent activation energy vs. temperature were obtained. There were two sharp maxima at about 120 and 130°C. in the diagram, where this polymer also has two irregular changes in other viscoelastic properties. It is conceivable that these changes at 120 and 130°C. are second-order transitions occurring in the amorphous region of the polymer.