Polymer Nanowire Elastic Moduli Measured with Digital Pulsed Force Mode AFM

Abstract

The mechanical bending behavior of polymer nanowires-polypyrrole and poly(3,4-ethylene dioxythiophene-co-styrene sulfonate)-produced by template molding were measured using a new innovation in atomic force microscopy (AFM). Digital pulsed force mode (DPFM) was used to image and simultaneously perform three-point bend tests along nanowires spanning microchannels in silicon. The bending profiles were analyzed for apparent elastic moduli variations along the suspended length of individually isolated nanowires and compared to classic beam deflection models for various geometric and boundary conditions. The elastic moduli calculated from these AFM data are 2-7 times that expected for bulk polymer values (approximately 1-3 GPa), demonstrating an apparent strengthening of nanostructured polymer even for diameters greater than 100 nm--the accepted boundary for nanoscience. Furthermore, detailed analysis of deflection data versus loading location demonstrates the experimental dependence on test geometry and inherent errors in relying solely on midpoint bending measurements or any single loading configuration for nanomechanical testing as well as the significant contribution of nanoindentation effects.