(175a) Mechanical Characterization of Polymeric Nanofibers Using An Integrated Approach

Recent advances in polymeric fiber manufacturing platforms enable integration of polymeric micro/nanofibers as 1D building blocks in a bottom-up assembly environment. These building blocks are attributed to possess superior mechanical, thermal and electrical properties compared with larger bulk counterparts, which potentially allows using them in applications of sensors, tissue scaffolds, fiber reinforced composites, drug delivery carriers, airborne structures, etc. The superior physical properties are attributed to the effect of surface tension, defect free one dimensional anistropic materials and ordered molecular chain orientation. Characterization of those fibers is essential to achive improved design and reliable performace for targetted application. The primary challengest associated with material characterization involve precise deposition of nanofibers with near exact boundary conditions in aligned configurations, availability of large number of samples and finally appropriate nanoscale characterization platform. Current techniques for repeated fabrication of polymeric nanofibers having appropriate boundary conditions for characterization involve multiple processing steps which often lead to inconsistent boundary conditions and makes experiments very tedious. Here we report an Integrated Approach for the fabrication and characterization of polymeric nanofibers with near-exact clamped-clamped boundary conditions at each end of fiber. In the proposed approach, polymer dissolved in a solvent is extruded through a glass micropipette, which in turn is moved over a commercially available Transmission Electron Microscopy (TEM) grid. Fiber formation on the individual grid patterns occurs by solvent evaporation and this technique provides flexibility in depositing aligned fibers having diameters ranging from sub-50 nm to sub- microns. The deposited fibers on the grid have clamped-clamped boundary conditions, which allows for fiber material characterization under the TEM (polymer molecular chain arrangement) and also provides a pathway for mechanical characterization using force deflection spectroscopy under the Atomic Force Microscope (AFM). This simple, straightforward and integrated approach is demonstrated for mechanical characterization of polymeric nanofibers: polystyrene (PS), poly methyl methacralate (PMMA), and carbon nanotube (CNT) embedded PS fibers. At first the fibers are deposited across commercially available TEM grids and a map of the fiber location across the TEM grid is generated under TEM, thus, providing information on the alignment of the deposited fibers and location of the fibers in the grids. The grids are then mounted under the AFM and using the lateral force microscopy (LFM), fibers are deformed till breakage. Specifically, the probe tip is then moved in the lateral direction and it makes contact with the fiber, at which point fiber starts to deflect and undergoes plastic deformation at high strains and finally breaks. This process causes the probe tip to deflect, thereby changing the voltage signal, which drops to the original signal after the fiber breaks. The magnitude of the sharp drop in voltage signal at the point of fiber breakage is converted to a force value. Finally, accurate fiber dimensions are determined using SEM.