X-ray and Monte Carlo studies on the 19°C transition of poly(tetrafluoroethylene)

The 19°C transition of poly(tetrafluoroethylene) (PTFE) from the low temperature phase II to the high temperature phase IV is investigated by X-ray diffraction and Monte Carlo (MC) methods. The change in the chain packing around the transition point is examined carefully by separating the very close reflections on the equator. The conventional unit cell parameters a and γ show abrupt changes at 19°C from a = 5.61 Å and γ = 119.3° (pseudohexagonal) to a = 5.67 Å and γ = 120.0° (hexagonal), indicating a sharp first order transition. The change is ascribed to a dilatation of the spacing that occurs predominantly in one direction [1̄,1]. The rotational motion of the chains around the chain axes in phase II is examined in the vicinity of the transition point through the temperature dependence of a reflection on the second layer line. No evident increase in the rotational motion around the chain axis can be detected up to the transition point, which is again indicative of a clear first order nature of the transition. The chain packings in phases II and IV are simulated by the MC method used in our previous paper. It is shown that phases II and IV show marked differences in their chain packings. Details of the 19°C transition are discussed on the basis of these simulated structures.