Growth of single crystals of polytetrafluoroethylene from the melt

The melt crystallization of dispersion‐based polytetrafluoroethylene, in amounts varying from single dispersion particles to continuous films, has been studied by electron microscopy, electron diffraction, and optical microscopy. Since the maximum temperature used was only 350°C., negligible thermal degradation of the polymer is believed to have occurred during the crystallization. Using very small amounts of the polymer, single crystals with thicknesses of about 150–900 A. having their molecular chains oriented perpendicularly to the basal plane of the crystal have been observed; this shows that the melt crystallization of polytetrafluoroethylene occurs by a chainfolding mechanism as reported previously for other high polymers. This new knowledge allows a more positive interpretation of the striated band structures previously observed on fracture surfaces of bulk specimens of both dispersion‐based and granular types of this polymer. Such bands which show striations perpendicular to their length probably represent individual crystal plates which have been fractured in planes perpendicular to their bases, while bands showing longitudinal striations result from the splitting apart of stacks of such plates, or lamellae, along fracture planes approximately parallel with their surfaces. The present observation of isolated and well defined single crystals from a polymer melt, as distinct from a solution, is believed quite novel. Although seen so far with polytetrafluoroethylene only, the growth of such single crystals from the melt might be possible with other polymers also, if these were studied in very small quantities.