PTFE Molecular Conformation and Crystal Structure

PTFE is a linear chain polymer of great molecular length. The linearity is indicated by an analysis of the infra-red spectrum and by the fact that the powder as produced in the polymerisation reaction is very highly crystalline, with crystalline weight fractions of 0.90 to 0.95 being indicated by density, infra-red and X-ray diffraction measurements. Energy considerations also suggest that branching by chain transfer is unlikely.

The crystal structure and chain conformation have been discussed by Bunn and Howells  and later by others. The crystalline melting point of sintered PTFE is about 327°C (620°F) and of unsintered material 332-346°C (630-655°F) but there are two reversible first order transitions at lower temperatures,19°C and 30°C (66°F and 86°F), which together involve a 1% change in density .Three crystalline phases are observed at atmospheric pressure: phase I (< 19°C; 66°F), phase ll (19-30°C; 66- 86°F) and phase lll ( >30°C; 86°F).

Below the 19°C (66°F) transition, the chain repeat distance is 16.8 Å and the CF2 groups are equally spaced along the chain which is twisted to form a helix on which successive carbon atoms lie, thirteen carbon atoms being involved in a twist of 180°.Between 19 and 30°C (66 and 86°F) the repeat distance increases to 19.5 Å  corresponding to a twist of 15 carbon atoms in 180°. Above 30°C (86°F), further disorder sets in and although the molecular conformation prevailing at lower temperatures is maintained, the chains are displaced or rotated along their long axes by variable amounts which increase as the temperature is raised further. The reason for the helical structure is the necessity to accommodate the large fluorine atoms (van der Waals radius 1.35 Å).The rotation at each chain bond, with the slight opening up of the bond angles to 116°, relieves the overcrowding and permits the shortest F-F distance to be 2.7 Å.

Further studies by various authors have examined the effect of pressure on the room temperature transitions and the melting point. A study of pressures above-atmospheric revealed a 2% increase in density below 19°C (66°F). This fourth crystalline phase has been labelled phase lll by Weir. A triple point exists at about 70°C (158°F) and 4.5 kilobars.The heats of transition were also determined by Yasuda and Araki; dilatometric and calorimetric studies have in addition been reported by other workers.