Polytetrafluoroethylene extended-chain crystals

Melillio L, Wunderlich B (1972) Morphology of polytetrafluoroethylene extended chain crystals VIII. Kolloid Z Z Polym 250 417... 

Melillo, L. and Wunderlich, B. Extended chain crystals VIII. Morphology of polytetrafluoroethylene. Kolloid Z. Z. Polymere 250, 417 (1972)... 

Extended, or fully extended, chain crystallites contain straight chains at least 200 nm long and have been obtained for only a few polymers, such as polytetrafluoroethylene (PTFE), polyethylene and polychlorotrifluoro-ethylene, using special crystallisation techniques. Extended-chain polytetrafluoroethylene can be obtained by slow crystallisation from the melt the other two are obtained by crystallisation from the melt under elevated pressure. Solution crystallisation has so far not been shown to give rise to extended-chain crystals. 

Fig. 4.21 Electron micrograph of a replica of a fracture surface of melt-crystallized polytetrafluoroethylene showing chain-extended crystals. The striations define the chain-direction in each crystal (Young, Chapter 7 in Developments in Polymer Fracture edited by E. H. Andrews, Applied Science Publishers Ltd, 1979, reproduced with permission).

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. 

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