Effect of Low Pressures on the Room Temperature Transitions of Polytetrafluoroethylene.

Abstract

Between approximately 20 and 30 °C at atmospheric pressure, polytetrafluoroethylene exhibits a phase (denoted as IV), which has not been observed in P-V-T measurements at pressures above 108 Nm-2 (1 Kilobar). Data are presented to resolve this phase in the temperature range 0 to 50 °C and the pressure range 0 to 0.686 × 108 Nm-2. The II-IV (20 °C) transition pressure increases with temperature according to the equation P = (-820 + 26.9t + 0.68t2) × 105 and the IV-I (30 °C) transition according to the equation P = (- 91-57.8t + 1.99t2) × 105 suggesting a possible triple point near 2.6 × 108 Nm-2 and 54 °C. However, the IV-I transition probably cannot be resolved by volume measurements at pressures above 2 × 108 Nm-2 because its volume of transition is small and the two transitions overlap. The enthalpies of the transitions are calculated for different pressures and some thermodynamic properties of phase IV are determined indirectly. It is shown that, if the effect of volume change is subtracted, random reversals of the hand of the helical conformation of the molecule can account for appreciable fractions of the enthalpy and corresponding entropy change for the combined transitions at atmospheric pressure.