Abstract
The crystal transitions occurring in polytetrafluoroethylene at about 19°C and 30°C are investigated by x-ray diffraction. On the basis of the theoretical work of Clark and Muus (1962), it is shown that the chain molecules undergo small angular displacements about their long axes in the 19°C transition with the angular displacements becoming much larger in the 30°C transition. From nuclear-magnetic resonance measurements of polytetrafluoroethylene fibers by Hyndman and Origlio (1960), it is concluded that these displacements result from motion of the molecules. A detailed mechanism for the crystal disordering is proposed in terms of torsional oscillation of molecular segments coupled with a gradual untwisting of the chain molecules. The relationship between helical symmetry of the molecules and molecular packing in the crystal is discussed.
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