Ioannis Chasiotis received his Ph.D. and M.S. degrees in Aeronautics from the California Institute of Technology in 2002 and 1998, respectively, and his Diploma in Chemical Engineering in Greece in 1996. In 2001–2004, he was an Assistant Professor in Mechanical and Aerospace Engineering at the University of Virginia. In 2005 he joined the Department of Aerospace Engineering at the University of Illinois at Urbana-Champaign where he is also part-time faculty at the Beckman Institute for Advanced Science and Technology and the Micro and Nanotechnology Laboratory. His research focuses on experimental mechanics of thin films, MEMS/NEMS, and nanostructured materials with emphasis on nanoscale deformation and failure. He is a recipient of an NSF-CAREER award in 2008, an ONR Young Investigator Award in 2007, a Xerox Award for Faculty Research in 2007, the Founder's Prize from the American Academy of Mechanics in 2000, and the Charles Babcock Memorial Award from the California Institute of Technology in 1999.

Abstract: Mechanics of Polymeric Nanofibers

Polymeric nanofibers, fabricated by electrospinning, are versatile building blocks in hierarchically structured materials, such as nanocomposites, high strength fabrics, high density filters, and scaffolds for tissue engineering. The mechanical behavior of nanoscale polymeric fibers in response to quasi-static and intermediate loading rates is yet unexplored. A novel experimental method that utilizes a MEMS-based mechanical property-testing platform was conceived to investigate the effect of strain rate during cold drawing of single electrospun polyacrylonitrile (PAN) nanofibers with 200-500 nm diameters and tens of microns in length. The mechanical strength of the PAN nanofibers at their glassy state was as high as 200 MPa while their ductility was larger than 200%. The fiber ductility was found to vary consistently with macroscale expectations, increasing with reducing strain rate. Curiously, the fiber strength did not vary monotonically with the drawing rate. At slow drawing rates (<10-4 s-1), the fiber strength increased dramatically compared to faster strain rates (<10-2 s-1), establishing a minimum at about 10-3 s-1. This seemingly conflicting behavior was the result of two different mechanisms of deformation. At slow stain rates, the fibers underwent homogeneous deformation and strain localizations were suppressed by material relaxations. This behavior permitted large fiber deformations and molecular chain alignment, and therefore large fiber strengths. At faster strain rates (>10-3 s-1), the formation of (non-propagating) periodic surface instabilities along the nanofibers allowed for large fiber stretch ratios, while maintaining the high fiber strength.

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