Length Scale and Composition Dependent Large Deformation Mechanical Behavior of Hydrogen-Bonded Polymer Nanofibers

There usually exists a trade-off between making a material ductile so that it can support large deformation and making it strong so that it can withstand large forces. Biological soft materials such as spider silk or collagen have inspired the use of a network of dynamic bonds (hydrogen bonding or metal coordination), which dissipate energy through rupture and reformation during the deformation, to simultaneously improve strength and toughness. Similarly, fabricating small nanoscale structures has also been reported to help overcome the strength-toughness trade-off. In this study, we systematically study the combined effect of structural length scale and hydrogen bond (H-bond) concentration on the mechanical behavior of polyvinylpyrrolidone (PVP) and tannic acid (TA) nanofibers. Using a unique submicron scale uniaxial mechanical testing with microelectromechanical system (MEMS), the elasto-plastic mechanical response of individual PVP-TA fibers was studied as a function of their fiber diameter and molar concentration of TA (i.e., effect of H-bond density). Our studies reveal a complex interdependence of nanofiber mechanical response on the fiber diameter and TA concentration. It was observed that elastic and plastic properties of PVP-TA nanofibers are very differently dependent on the fiber diameter and TA concentration, with the thinnest fibers being stiffer and more resilient, at lower TA concentrations, while thicker fibers show better elastic response at large TA concentrations. In contrast, the plastic deformation response of PVP-TA nanofibers was found to display a more complex, non-monotonic dependence of on fiber diameter and TA concentration. This presentation will outline these trends and infer the underlying role of the H-bonded network in controlling submicron scale mechanical behavior. The results of this study will effectively help develop novel hydrogen-bonded polymers with effective mechanical and functional responses.