Experiments and Models for the Time Dependent Mechanics of Nanoscale Polymeric Structures and Nanocrystalline Metal Films

To date, micro and nanoscale experiments have been mostly focused on the length scale dependent mechanical behavior of nanostructures and nanostructured thin films but have not been able to address their time and rate dependence. This inefficiency stems from the use of high resolution electron microscopes which are slow imaging tools, and quite often are of detrimental effect to the integrity of polymeric materials. Optical methods have been revisited in the recent years and were modified to accommodate micro and nanoscale specimens in order to obtain high resolution deformations and their time evolution at time scales varying from microseconds to days [1-4]. This research, conducted at the University of Illinois, has developed new approaches to investigate the time-dependent mechanical behavior of metallic thin films for MEMS and polymeric nanostructures in an effort to understand the important deformation processes at small scales. The extended (internal) surfaces in nanocrystalline metal films and the large surface-to-volume ratios in polymeric nanostructures favor material transport mechanisms that are not important in bulk or large grain materials because they do not result in appreciable strains. On the contrary, in nanoscale polymeric fibers for instance such processes result in large material deformations and sustained ductilities in a large range of loading rates. This presentation will summarize experimental work conducted with polymeric nanofibers and nanocrystalline metals and some early modeling efforts to rationalize the measured time- and ratedependent mechanical behavior.