The linear thermal expansion of molded PTFE samples of various crystallinities was measured in the vicinity of the transition at about 400°K. In the plot of length of the specimen versus temperature, an inflection of the straight line was observed in the temperature range from 388 to 402°K., the mean value being 396°K. (123°C.) The change of the coefficient of linear thermal expansion at this temperature represents a typical feature of the second-order transition or the glass transition, designated as Tg. The ratio Tg/Tm is equal to 0.66, in accordance with the so-called ⅔-rule between the glass transition temperature (Tg) and the melting point (Tm) of many polymers. The coefficients of linear thermal expansion, below and above Tg, designated α1 and α2 respectively, increased with increasing amorphous fraction, A. The extrapolated values of α1 and α2 at A = 0 were both 0.6 × 10−4 deg.−1, and those at A = 1 were 2 × 10−4 deg.−1 and 3 × 10−4 deg.−1, respectively. The value of TgΔβ/A, where Δβ = 3(α2 – α1), was approximately 0.12 which was closely in agreement with the value of 0.08 or 0.113 predicted by the theories for amorphous high polymers. From these experimental facts, it may be concluded that the transition of PTFE at about 400°K. is the glass transition which occurs in the amorphous region.
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