Structural surprises in friction-deposited films of poly(tetrafluoroethylene)

Thin films of poly(tetrafluoroethylene) (PTFE) produced by friction deposition were studied using grazing incidence X-ray diffraction as the principal tool. The structure of the deposited thin films was compared with that of the surface of the PTFE bar used for depositing the films. Both exhibited the 15/7 helix conformation characteristic of crystal PTFE phase IV. A high degree of biaxial orientation was found for the highly crystalline thin films. Whereas the unit cell of the bar surface material appeared to be single-stem hexagonal, the film displayed diffraction characteristics consistent with a larger multistem unit cell. The origin of this increase of the unit cell is attributed to a higher degree of regular packing, possibly related to alternating right- and left-handed PTFE helices-a structure which has never been verified experimentally for PTFE in the 15/7 configuration. We discuss the possibility of a continuous transition between the low-order single-stem hexagonal and the multistem high-order unit cell. The degree of chain orientation was much lower at the surface of the bar than in the thin film. A modification of the commonly accepted mechanism for the transfer of material from the bar to the substrate is proposed.