Second‐order transitions of polytetrafluoroethylene at about −30°C. Measured by several methods

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 a T 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 a T 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 T g 3 (α 1 − α 3 )/ A was about 0.04, where T g is the mean value of the two transition temperatures (absolute), α 1 and α 3 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 ε′ 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 ε′ 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.).