Abstract

Measurements of compressive stress relaxation, linear thermal expansion, and dielectric properties were carried out for molded polytetrafluoroethylene (PTFE) in the temperature range of −60 to 10°C. In the measurements of stress relaxation over the range −55 to 5°C., at 5°C. intervals, a master curve was obtained with good overlapping from a series of stress relaxation curves made from the same specimen at different temperatures, by application of the so-called Ferry's reduction method. The reduction factor aT at different temperatures was plotted in logarithmic scale against the reciprocals of the absolute temperatures, and a diagram of the apparent activation energy versus temperature was obtained from the slope of the log aT versus 1/T curve. There existed two sharp maxima at about −40 and −15°C. in the diagram of the apparent activation energy. Compressive relaxation moduli at a definite time after compression were plotted against temperature, and on the curve remarkable changes of Young's modulus were illustrated at about −40 and −15°C. On the curve of coefficient of linear thermal expansion versus temperature, two steps were observed at about −40 and −15°C. for several specimens of PTFE with different crystallinities. The value of Tg 3 (α1 − α3)/A was about 0.04, where Tg is the mean value of the two transition temperatures (absolute), α1 and α3 are coefficients of linear thermal expansion above and below the two transitions, and A is the amorphous fraction of each specimen. Plots of dielectric loss tangent tan δ and dielectric constant e′ measured by a Schering bridge at a frequency of 50 cycle/sec. against temperature gave two dispersions of tan δ at about −35 and −25°C., and two irregular changes of e′ at the same temperature regions. According to these measurements, it is concluded that in PTFE two second-order transitions occur at about −40 and −15°C. The mean value of the two transition temperatures in absolute scale, 245°K. (−28°C.), is nearly equal to two-thirds of the first-order transition temperature at about 363°K. (90°C.).

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