Nanomechanical tests on continuous near-field electrospun PAN nanofibers reveal abnormal mechanical and morphology size effects

Abstract

Polyacrylonitrile (PAN) nanofibers have long been the subject of studies as precursor for high performance materials such as carbon nanofibers. Here, we present the first effort to study the mechanical properties of PAN nanofibers fabricated via near field electrospinning, in which bending instability was completely suppressed. The method allowed for the collection of continuous single-strand PAN nanofibers with aspect ratios exceeding 106 on a spool (rotating target). By controlling the electrospinning parameters, such as solution concentration, voltage and distance, we demonstrated that the morphology and diameter of the PAN nanofiber were well controlled. We also realized that slight changes in the distance can significantly change the shape of the cross section of the fibers, from a circular cross section to oval shaped (ribbons), which was explained in terms of solvent residues in the jet and momentum transfer between the fibers and the target, as the fibers reach the target. Our studies on individual PAN nanofibers revealed a strong mechanical size effect, in which reducing the diameter of the nanofibers from ∼290 nm to ∼200 nm, led to a large increase in modulus to as high as ∼9 GPa, among the highest in as-electrospun PAN nanofibers. The strong size effect was attributed to a loss in chain alignment in thicker electrospun nanofibers facilitated by the plasticizing effect of residual solvent. In comparison to conventional electrospinning, the NFES led to a significantly narrower diameter distribution. The demonstrated size-dependent morphology, mechanical properties of NFES PAN nanofibers provides a solid foundation for fabricating polymer nanofiber with controllable patterning and properties through NFES.