Time and strain rate dependent mechanical behavior of individual polymeric nanofibers

In this work, the small and large strain mechanical behavior of nanofibers electrospun from glassy polymers was shown to be diameter and time-dependent. Specifically, the creep compliance of as-elecrospun polyacrylonitrile (PAN) nanofibers increased with increasing diameter, while the tangent modulus, yield stress and tensile strength followed decreasing trends, which were attributed to increased molecular orientation with reduced nanofiber diameter. Furthermore, the nanofiber capacity for energy dissipation increased dramatically with the applied strain rate, as the yield and ultimate tensile strengths increased steadily with increasing strain rate. The effect of strain rate was less significant on the ductility of PAN nanofibers, and insignificant on the ductility of polystyrene (PS) nanofibers. This outstanding mechanical response was demonstrated by homogeneously deforming PAN nanofibers at strain rates as high as 200 s−1 and by PS nanofibers exhibiting necking at local plastic strain rates as high as 27,000 s−1. The small strain time-dependent response of PAN nanofibers was modeled with a linear viscoelasticity model with diameter dependent constants, which provided a good description of the creep and strain rate behavior. The large deformation behavior was modeled via a modified rubber elasticity model which predicted quite well the overall mechanical response of PAN nanofibers.