Electrospun nylon-4,6 nanofibers: solution rheology and Brill transition

Abstract

Polyelectrolyte solutions of nylon-4,6 in 99 vol.% formic acid were electrospun, and then the concentration effect on the solution spinnability was studied. The microstructure of the as-spun nanofibers was characterized by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). Based on the solution rheology, the concentration of the entangled regime and the concentrated regime (∅D) were 1 and 10 wt.%, respectively. To prepare bead-free fibers, the minimum polymer concentration used was 10 wt.%, yielding a fiber diameter of 49 ± 13 nm. The fiber diameter (df) was dependent on the solution viscosity (\( {\eta_{\mathrm{o}}} \)) or the polymer concentration (∅w) through the following simple scaling law relation: df ∼ \( \eta_{\mathrm{o}}^{0.62 } \) and df ∼ \( {{({\phi_{\mathrm{w}}}/{\phi_{\mathrm{D}}})}^{2.25 }} \). DSC heating trace on the as-spun nanofibers exhibited double-melting behavior. However, after cooling, the second heating trace showed a single melting peak. WAXD intensity profiles showed that the as-spun nanofibers possessed lamellae with small lateral dimensions, and the lattice parameter difference between a-axis and b-axis was significantly reduced due to the rapid electrospinning process. Both structural features induce the occurrence of the Brill transition of nylon-4,6 in the nanofibers at a much lower temperature of 80 °C than that in the melt-processed film, as-revealed by the temperature-variable WAXD.