Nanometer-scale tribological properties of highly oriented thin films of poly(tetrafluoroethylene) studied by lateral force microscopy

Lateral force microscopy (LFM) studies of poly(tetrafluoroethylene) (PTFE) films obtained by pressing and sliding a block of polymer on a clean heated glass surface are reported. The sample preparation process utilized, invented by Wittmann and Smith, resulted in samples of thin, well-oriented films and fibers of PTFE which were deposited on the glass. The formation of such oriented PTFE films was confirmed by dual LFM/AFM images. During LFM/AFM scanning, PTFE molecules were picked up by the nanoscope tip probe, which clearly reduced the frictional force differences measured between glass and PTFE. This observation is in line with the model of Tabor, which assumes strong interfacial tension between PTFE and glass during sliding friction. LFM experiments performed on the PTFE films at scan directions between ca. 40 and 90° with respect to the polymer main chain orientation showed a “stick−slip” type frictional motion at the molecular level. This phenomenon allowed us to obtain LFM images of a synthetic polymer with molecular resolution. Chain−chain packing distances obtained by LFM and contact-mode AFM were identical to within the experimental error and had a value of 5.78 Å at 25−30 °C imaging temperature. Dual-mode contact AFM/LFM imaging was also performed by scanning in the chain direction. Here LFM nanographs showed no measurable “stick−slip” phenomenon; i.e., basically a featureless, flat image was obtained. The contact-mode AFM images, however, exhibited clear molecular resolution with the expected chain−chain periodicity. The disappearance of the “stick” component in scans performed in the chain direction is a result of the smooth surface of PTFE on the molecular scale.