Transitions and Phases of Polytetrafluoroethylene

Abstract

Behavior of transitions and the nature of high-pressure phase in polytetrafluoro ethylene are studied using methods of ultrasonic waves, thermal expansion, differential thermal analysis and X-ray diffraction. A phase diagram is obtained in pressure range from 1 atm. to 6,500 kg/cm² and in temperature range from room temperature to 165°C. Three phases are found; phase I above 30°C, phase II below 20°C, both at 1 atm. and phase III above 5,000 kg/cm². The triple point is located at 75°C and 5,000 kg/cm². Pressure dependence of transition temperatures is obtained as 15 and 9 degrees per 1,000 kg/cm² for the 20°C and 30°C transitions at low pressure, 9 degrees per 1,000 kg/cm² for both transitions at intermediate pressure range from about 3,000 to 5,000 kg/cm², -13 and 50 degrees per 1,000 kg/cm² for the II–III and the III–I transitions, respectively. It is suggested that the high-pressure phase (phase III) may be a closely packed but disordered phase.