Transitions of polytetrafluoroethylene at about 90 and 130°C. studied by x‐ray diffraction and infrared spectra

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Abstract
By means of x‐ray diffraction, the lattice spacing of the (100) plane for molded polytetrafluoroethylene was measured at different temperatures from 25 to 190°C. In the crystalline region, the linear expansion coefficient, in the direction perpendicular to the molecular chain axis, was obtained as 1.1 × 10−4°C.−1 below 60°C., as 1.2 × 10−4°C.−1 above 90°C., and as a minimum value of some 0.2 × 10−4°C.−1 at about 80°C. As the linear expansion coefficient of the crystalline region in bulk was observed as some 0.6 × 10−4°C.−1, the expansion coefficient in the direction of molecular chain axis must be negative except in the transition region near 80°C. The variation of molecular chain axis separation with temperature showed an irregularity at about 80°C. but none near 130°C. in the crystalline region. Infrared absorbance of film samples of PTFE was measured at different temperatures of 25 to 150°C. range for 518, 627, and 639 cm.−1 bands. On absorbance–temperature curves for those b̀ands, irregularities were observed near 30, 50, 90, and 130°C. Particularly with 518 cm.−1 band, a more crystalline sample gave more distinct irregularities near 50 and 90°C. than a less crystalline sample. The change at about 90°C. in infrared spectra may correspond to that obtained by x‐ray measurements near 80°C., which was thought to occur in the crystalline region. The results obtained by x‐ray and infrared measurements support the previous results by thermal, rheological, and dielectric methods: there exist first‐order transitions in the crystalline region at about 90°C. and second‐order transitions in the amorphous region at about 130°C.