Polytetrafluoroethylene helix

FIGURE 3.1 Schematic representation of the polytetrafluoroethylene helix. (Adapted from Bunn, C. W. and Howels, E. R., in Nature 174 (1954). With permission from Macmillan Magazines, Ltd.)... 

FIGURE 3.1 Schematic representation of the polytetrafluoroethylene helix. From Koo, G. P. in Fluoropolymers (Wall, L. A., Ed.), Wiley-Interscience, New York, p. 508, 1972. With permission.)... 

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... 

The conformation of the polytetrafluoroethylene molecule in the low temperature form (Phase II) has been determined to be 2.159 CF2 units per turn of the helix within the limits of experimental error. This conformation is slightly untwisted from the previously assigned 13/6 = 2.167 value but is substantially different from that for the 25°C form (Phase IV) in which the conformation is 15/7 = 2.143. By comparison, the planar zig-zag is 2/1 = 2.000. 

Figure 2.3 Crystalline conformation of polytetrafluoroethylene, 13/1 helix (13 CF2 units per turn of the helix).

Table 2 shows that, apart from diamond and graphite, the highest moduli are encountered with the planar zig-zag conformation. The examples are polyethylene and polyvinyl alcohol. There is a marked drop in going to the very lazy helix (15/7) of polytetrafluoroethylene, which departs only slightly from the planar zig-zag. 

As an example of a more complex polymer chain we quote the case of poly-tetrafluoroethylene -(CFijn- for which one can identify the rototranslational operator i/ (2ti/15, d/15) which describes a polymer chain made up by z = 15 CF2 units coiled in a helix with t = 7 turns. In cases such as polytetrafluoroethylene the... 

Fig. 2.2. (a) Scale drawing of the helical conformation of a polytetrafluoroethylene molecule as it occurs in the crystal, showing both side and end view. There are 13 CF, units in one turn of the helix, which is 1.68 nm in length. 

I. The simple helix, polyethylene, polytetrafluoroethylene, and a general formula. 

A general picture emerges concerning the values of chain direction moduli of polymer crystals. They tend to be high if the molecule is in the form of a planar zig-zag rather than a helix. For example, polyethylene is stiffer than polyoxymethylene or polytetrafluoroethylene which both have molecules in helical conformations (Table 4.1). The helices can be extended more easily than the polyethylene planar zig-zag. Also the presence of large side groups tend to reduce the modulus because they increase the separation of molecules in the crystal. This causes an increase in the area supported by each chain. 

Commercial polytetrafluoroethylene (PTFE) is usually the product of sintering and has its crystallinity reduced by the introduction of branched chains. PTFE (degree of crystallinity, 93-98%) has a melting point of 620 K and exhibits a helical structure which undergoes unit cell transitions at 292 and 303 Below 292 K the conformation can be described by a helix containing I3CF2... 

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