Polytetrafluoroethylene molecular conformation

In this paper we examine electron diffraction fiber patterns of the homopolymer polytetrafluoroethylene (-CF2 CF2-)n PTFE, in which the resolution is sufficient to yield much more accurate values of layer line heights than were available from the previous x-ray diffraction experiments (1) on the crystal structure of Phase II, the phase below the 19°C transition (2). On the basis of x-ray data, the molecule was assigned the conformation 13/6 or thirteen CF2 motifs regularly spaced along six turns of the helix. This is equivalent to a 132 screw axis. The relationship between the molecular conformation and the helical symmetry has been studied by Clark and Muus (3) and is illustrated in Figure 1. The electron diffraction data of high resolution enabled us to determine if this unusual 13-fold symmetry was exact or an approximation of the true symmetry. We have also... 

For example, suppose all the hydrogen atoms are replaced by fluorine atoms with van der Waals diameter 270 pm. To accommodate the fluorine atoms a rotation around each C-C bond of about 20 is induced. This is accompanied by a slight opening of the C-C chain bond angle to about 116°. The result is that the molecular conformation of polytetrafluoroethylene (PTFE)... 

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