AFM Force–Distance Curves for Perfluoropolyether Boundary Lubricant Films as a Function of Molecular Polarity

AFM force–distance curves on 10-Å films of Zdol, Z-Tetraol, and ZTMD were investigated using a carbon-modified AFM tip. The forces were non-specific dominated by adhesive and/or capillary forces. The snap-to distances for unbaked Zdol, Z-Tetraol, and ZTMD were 1.9, 1.0, and 0.8 nm, respectively, and decreased with increasing molecular polarity. ZTMD allowed the closest approach before snap-to, by approximately 1 nm. However, annealed samples showed an identical 1.6 nm snap-to distance, indicating that an identical perfluoropolyether backbone was presented to the AFM tip at the PFPE/air interface. The pull-out forces for unbaked Zdol, Z-Tetraol, and ZTMD were in the 15–40 nN range and increased with increasing molecular polarity and loading rate. After annealing, the pull-out forces for the baked Zdol, Z-Tetraol, and ZTMD were significantly reduced to 10–20 nN, indicating that the polar end groups were less involved by bonding to the underlying surface. Force–distance curves for single-chain unbinding and entropic elasticity were computed to be near 300 and <100 pN, respectively, indicating that the force–distance curves for the experimental tip-PFPE interactions originated from multiple PFPE interactions with the tip.