The crystal structure of polytetrafluoroethylene, –(CF2)n –, is unusual in having a number of crystal forms and also possessing substantial molecular motion within the crystal well below the melting point. This article deals with the unusual first-order transition occurring at about 19°C, changing the crystal structure from form II to form I. This article also updates current literature on the crystal structures of these two forms. X-ray and electron diffraction data from forms II and IV are interpreted as helical structures. Form II is defined mathematically as a non-commensurable helix close to a 13/6 conformation of six turns per 13 (CF2) groups. Form IV is interpreted as a commensurable 15/7 helix which is consistent with space group P31. Fiber diffraction patterns of these forms are described in detail.
Посилання на статтю:
The molecular conformations of polytetrafluoroethylene: forms II and IV / E.S. Clark // Polymer. – 1999. – N 40. – P. 4659–4665.