Rheological Aspects and Extension-Induced Phase Separation in Electrospinning of Poly(N-isopropyl acrylamide) Solutions in Dimethylformamide

Solutions of poly(N-isopropyl acrylamide) in dimethylformamide are used to study the relation between the rheological properties and electrospinning behavior. Light scattering of electrospinning jet is conducted to obtain the jet diameter at the straight jet end, from which the average extension rate of the jet is derived based on a simple approach. The extension rate is found to be higher than the terminal relaxation rate of polymer chains determined from the dynamic rheological properties. Based on this finding, the original single-phase polymer solutions may undergo extensional flow-induced phase separation in the straight jet section during electrospinning. String-like structures in the liquid jet as well as in the solid fiber are observed by OM, SEM, TEM, and AFM to validate the hypothesis. The inhomogeneity in the jet, due to the presence of dissipative string structures, may serve as the potential precursor to produce lateral perturbations of jet motion to initiate the macroscopic bending instability at the straight jet end for the jet whipping. Moreover, the formation of the beaded fiber, that is, beads-on-a-string structures, is likely relevant with the dissipative structures developed in the jet, besides the Rayleigh instability of the jet arising from surface tension.