Rate dependent adhesion of nanoscale polymer contacts

The adhesive interactions between polymer nanofibers strongly influence the mechanical behavior of their networks in synthetic materials and biological systems. A treatment of the adhesive interactions at polymer contacts must take into account the viscoelastic behavior of the material in the contact region and the associated energy dissipation. This study focuses on the rate-dependent adhesion of polyacrylonitrile (PAN) nanofibers which interact with strong van der Waals forces. Experimental measurements of the detachment force between PAN fibers with diameters 400 and 4000 nm showed that the apparent work of adhesion increases with the rate of unloading by a factor of two within a three orders of magnitude increase in the unloading rate. In order to obtain further insights into this rate-dependent adhesion, the Maugis-Dugdale (M-D) elastic contact model for normal detachment was extended to include a linear viscoelastic behavior of the PAN nanofibers. The extended model predicted the normal pull-off force instabilities in good agreement with the experiments, capturing well the effect of the unloading rate. The calculated viscoelastic time constants were of the order of milliseconds, suggesting fast relaxations in the contact region, which explain the instantaneous full-strength reattachment of nanoscale polymer fibers during slip-stick experiments.