Puzzles of how friction depends on intermolecular packing of the lubricant

Progress in making definitive connections between the organization of lubricant molecules at surfaces; and subsequent friction behavior, has been impeded by the paucity of methods available to explore molecular packing directly. A new nanotribometer, in which confocal Raman spectroscopy within a surface forces apparatus (SFA) is used to measure directly how molecules orient in molecularly-thin films, was presented. Polarized radiation was used to determine how sliding polydimethylsiloxane causes molecules to align in the direction of shear. Confined chains at rest were also aligned but in a direction that the system selected randomly, never the same from one experiment to the next. A second series of experiment was carried out to investigate how friction in an SFA depends on the method used to produce single crystals of atomically-smooth mica. When mica is prepared without using a hot platinum wire to cut it to size, minimal viscosity enhancement is observed except for films of thickness 1-2 molecular dimensions. The exceptions were bis-2-ethylhexylsebacate and pentamethylheptane. This is an abstract of a paper presented at the 227th ACS National Meeting (Anaheim, CA 3/28/2004-4/1/2004).