synopsisThe 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 396OK. (123OC.) The change of the coefficient of linear thermal expansion at thie temperature represents a typical feature of the second-order transition or the glass transition, designated as T,. The ratio T,/Tm is equal to 0.66, in accordance with the so-called Z/,rule between the glass transition temperature (T,) and the melting point (T,) of many polymers.The coefficients of linear thermal expansion, below and above T,, designated U I and as respectively, increased with increasing amorphous fraction, A . The extrapolated values of U I and a2 a t A = 0 were both 0.6 X deg.-', and those at A = 1 were 2 X deg.-l and 3 X lo-' deg.-', respectively. The value of T,Ap/A, where AS = 3(a2 -a,), 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 a t about 400°K. is the glass transition which occurs in the amorphous region.
IntroductionIn polytetrafluoroethylene (PTFE) there are two glass transitions:These transitions one at about 400OK. and the other at about 180°K. have been studied by the thermal expansion method,' x-ray diffraction, rheological methods,2-8 and the dielectric m e t h~d .~ Some authors have claimed that the transition at about 400°K. occurs in the amorphous region,6 but others contended that it occurs in the crystalline r e g i~n .~-~J -~ In this paper, the results of thermal expansion measurements on a series of PTFE samples with different crystallinities and the influence of the crystallinity upon the thermal expansion coefficient are reported ; the nature of the glass transition at 400 OK. is also considered.
Samples and Experimental ProcedureThe nine samples of PTFE powders used in this study ( The mold charged with the powder was set in a vacuum vessel, where the pressure was reduced to about 1 mm. Hg. The piston was pushed down slowly onto the powder in the mold cavity. The preform pressure for the molding powders, samples 1, 2, 3, 4, and 5 in Table I, was 300 kg./cm2, and that for the fine powders, samples 6,7,8, and 9, was 200 kg./cm.2. The pressure was maintained for 20 min. in each case. The vacuum was intended to remove the air in the powder and to minimize the void content in the molded pieces. After removal from the mold, the resulting preformed piece was heated up to 360°C. in an air oven at the rate of 44"C./hr., sintered at this temperature for 2 hr., and then cooled gradually to room temperature. The cooling rate was about 45 "C./hr. in the temperature range 360-250 "C. , and was at a slower rate below 250 "C. The specific gravity of the molded pieces at 25 "C. was determined in accordance with the proc...
