Seminar Schedule – Summer 2019
Friday, May 31, 2019
Time: 11:00am
Place: ASE 1.126
Nanoscale Polymer Contacts under Normal and Shear Forces
Ioannis Chasiotis, University of Illinois at Urbana-Champaign
Nanofibrous systems are encountered in a variety of applications, in the form of carbon nanotubes in Buckypaper, electrospun nanofibers in filtration systems, collagen fibrils in mammalian tissues, etc. In those systems, significant adhesive forces at the junctions between nanofibers can play a key role in the local and global deformation of the network, yet little is known about the mechanics of contact in soft nanofibrous systems subject to strong van der Waals interactions. This presentation will discuss the interfacial normal and shear adhesion strength of contacts between nanoscale polymeric fibers. A novel type of experiment, using micromachined devices, was carried out on individual polyacrylonitrile (PAN) nanofibers with diameters in the range 200 nm – 4 µm, to obtain the critical normal and tangential pull-off forces. The contact area under normal detachment was estimated using the Johnson-Kendall-Roberts (JKR) theory for elastic adhesive contact, whereas under shear detachment the contact area was calculated using fracture mechanics. The work of adhesion measured under normal detachment was constant in the range of fiber diameters employed in this study, however, it was found to be velocity dependent. The average interfacial shear strength, obtained for a range of contact radii (30-140 nm), was found to be independent of the applied normal force. This adhesion shear strength was approximately equal to the shear flow stress of PAN nanofibers as deduced from uniaxial tension tests. Thus, shear yielding could be the controlling mechanism of shear detachment of polymer nanofibers with strong van der Waals interactions. Such large contact forces that result in a stick-slip behavior may also give rise to large deformation of individual segments in a nanofiber network. To this effect, noteworthy aspects of the mechanical behavior of glassy polymer nanofibers subjected to large deformations will also be discussed in the context of macromolecular length scales and strain rate.
For further information, please contact Dr. Ken Liechti at kml@mail.utexas.edu or (512) 471-4164.
