Self-organization of ordered structures in electrospun polymer solutions: Novel "strings" formed by flow-induced...

Liquid nitrogen is used to intermittently collect the straight segment of the electrospinning jet at 1-s intervals to minimize flow disturbance. After freeze-drying to remove the solvent, flow-induced phase-separated structures are typically observed in the as-collected jets of polystyrene, poly(N-isopropylacrylamide), poly(vinyl alcohol), and nylon-6. These structures are found alongside the jet itself, which is ∼10-50 μm in diameter. The phase-separated structures comprise various strings with a wide range of diameters, ranging from 20 to 2000 nm. The strings are universally validated in the straight jets for the four polymer solutions, which fulfill the stable cone-jet electrospinning mode. Parallel and twisted strings are found in the flowing jet, regardless of whether the polymer is crystallizable. Moreover, lateral association (or fasciation) of the long strings occurs, and the fasciated strings possess a cross-sectional width similar to the diameter of the dried fibers deposited on the grounded collector. This finding indicates that the two-dimensional assembly of fasciated strings in the solution jet serves as a precursor to the as-spun nanofibers. Notably, ribbon-like features are produced from the in-plane one-dimensional assembly of fasciated strings in the nylon-6 solution jet. After random stretching by the electric force, the ribbons may longitudinally crack randomly at weakly fasciated regions of strings, yielding net-like features comprising fine fibril threads with a diameter of ∼20 nm and empty space between them. In addition to the round fibers, the unique features of coils, ribbons, and nets observed on the grounded collector are relevant to the internal structures developed in the jet through flow-induced phase separation, a process that has been largely overlooked.