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

摘要

AbstractBy 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.−1below 60°C., as 1.2 × 10−4°C.−1above 90°C., and as a minimum value of some 0.2 × 10−4°C.−1at 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.−1bands. On absorbance–temperature curves for those b̀ands, irregularities were observed near 30, 50, 90, and 130°C. Particularly with 518 cm.−1band, 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‐ord