Abstract
The thermal properties and morphology of ultradispersed polytetrafluoroethylene, as well as those of its soluble and insoluble fractions isolated in supercritical carbon dioxide at 75°C and 30 MPa, are investigated. It is found that, in fact, the soluble fraction contains low-molecular-mass and high-molecular-mass fractions that have been previously obtained via pyrolysis in the temperature ranges 50–150 and 120–300°C, respectively. With the use of thermogravimetric analysis, differential scanning calorimetry, and powder X-ray diffraction, it is shown that the initial polymer and both fractions have semicrystalline structures and that the crystallinity of the insoluble fraction is weaker than that of the soluble fraction because of its higher molecular mass. The crystal lattice of the polymer is monoclinic with equal parameters а and с and angle β being approximately 120°. The soluble fraction does not show phase transitions during heating to the onset of mass loss. As a result of separation of the low-molecular-mass fraction, the phase-transition temperature increases from–33 to–20°C.
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