First-order and second-order transitions of polytetrafluoroethylene in the temperature range of 80–140°C. Measured by several methods

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Transitions of polytetrafluoroethylene (PTFE) have been studied through the measurements of thermal expansion coefficient, compressive Young's modulus, dynamic modulus and loss tangent, and dielectric loss tangent and dielectric constant by a Schering bridge, in the temperature range of 50–170°C. On the curves of linear thermal expansion coefficient versus temperature, two small peaks were observed at about 80 and 100°C., and two steps at about 120 and 140°C. for several specimens of PTFE with different crystallinities. These two peaks and two steps are thought to mean two first-order transitions and two second-order transitions, respectively. Static compressive Young's modulus, dynamic shear modulus by a torsion pendulum method, and dielectric constant by a Schering bridge plotted against temperature illustrated irregular changes at about 80, 100, 120, and 140°C. These four irregular changes correspond to four transitions observed by the thermal expansion method. Plots of dynamic loss tangent and dielectric loss tangent against temperature illustrated four dispersions or irregular changes at about 80, 100, 120, and 140°C. ; specimens with higher crystallinities seem to show larger changes at about 80 and 100°C., and smaller changes at about 120 and 140°C. than those with lower crystallinities. According to these measurements, it is concluded that in PTFE there exist two first-order transitions at about 80 and 100°C. and two secondorder transitions at about 120 and 140°C. ; this is the same as concluded from the previous stress relaxation study.