Sliding of adhesive nanoscale polymer contacts

The interaction between friction and adhesion is a defining factor of the macroscopic behavior of natural and synthetic soft materials. In this work, the interfacial normal and shear adhesion strength of contacts between nanoscale polymeric fibers were studied using novel experiments aided by micromachined devices by which the critical normal and tangential pull-off forces between individual polyacrylonitrile (PAN) nanofibers with diameters in the range 400 nm – 4 µm were measured in real-time. The work of adhesion under normal detachment, as computed using the Johnson-Kendall-Roberts (JKR) and the Maugis-Dugdale (M-D) models for elastic adhesive contact, was shown to be independent of the nanofiber diameter and comparable to twice the surface energy of bulk PAN. Under shear detachment, peeling of the contact area was calculated using the JKR and the M-D models combined with linear elastic fracture mechanics (LEFM). The M-D model combined with LEFM could predict the experimentally obtained tangential pull-off force instabilities. The interfacial shear adhesion strength is shown to be constant for a broad range of contact radii (25–140 nm) and approximately equal to the material shear stress at yielding. Thus, shear yielding is shown to be the controlling mechanism during shear detachment of individual polymer nanofibers interacting with strong van der Waals adhesion.