A master curve for the size and strain rate dependent large deformation behavior of PS nanofibers at room temperature

Unlike the nominally brittle behavior of bulk polystyrene (PS), PS nanofibers with particular combinations of molecular weight (MW) and submicron scale diameters exhibit stable necking followed by pronounced strain-hardening, resulting in simultaneous increase in strength and ductility. The ratio, D_norm, of the fiber diameter, D, to the intrinsic macromolecular length scale described by the root-mean-square end-to-end chain distance, R_ee ∼ MW, has been shown to be an effective scaling parameter to determine the initiation and evolution of necking and strain hardening in submicron scale unoriented PS fibers. In this study, the room temperature large deformation response of individual PS nanofibers is quantified for the first time over a broad range of strain rates between 10⁻⁴–10² s⁻¹. It is shown that for all combinations of MW in the range 123,000–2,000,0000 g/mol and D in the range 200–750 nm that satisfy D_norm < 10, an increasing strain rate results in monotonic increase of the stress amplitude without any reduction in the nanofiber stretch ratio to failure. Furthermore, the experimental stress vs. stretch ratio curves for D_norm < 10 obeyed a multiplicative decomposition of stress into a shape and a rate component. This decomposition permits the construction of a normalized stress vs. strain master curve that captures well both the size effects, originating in the relative molecular and specimen length scales, and the strain rate effects on the mechanical behavior of PS nanofibers at room temperature.